Regolith breccia Northwest Africa 7533: Mineralogy and petrology

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Regolith breccia Northwest Africa 7533: Mineralogy and petrology
Meteoritics & Planetary Science 52, Nr 1, 89–124 (2017)
doi: 10.1111/maps.12740
Regolith breccia Northwest Africa 7533: Mineralogy and petrology with implications
for early Mars
Roger H. HEWINS1,2*, Brigitte ZANDA1,3, Munir HUMAYUN4, Alexander NEMCHIN5,6,
Jean-Pierre LORAND7, Sylvain PONT1, Damien DELDICQUE8, Jeremy J.BELLUCCI5, Pierre BECK9,
12
€
, Eric LEWIN9,
Hugues LEROUX10, Maya MARINOVA11, Laurent REMUSAT1, Christa GOPEL
Marion GRANGE6, Allen KENNEDY6, and Martin J. WHITEHOUSE5
1
Institut de Mineralogie, de Physique des Materiaux, et de Cosmochimie (IMPMC), Sorbonne Universite, Museum National
d’Histoire Naturelle, UPMC Universite Paris 06, UMR CNRS 7590, IRD UMR 206, 75005 Paris, France
2
Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey 08854, USA
3
IMCCE, Observatoire de Paris, 77 Av. Denfert Rochereau, CNRS UMR 8028, Paris F-75014, France
4
Department of Earth, Ocean & Atmospheric Science and National High Magnetic Field Laboratory, Florida State University,
Tallahassee, Florida 32310, USA
5
Department of Geosciences, Swedish Museum of Natural History, Stockholm SE-104 05, Sweden
6
Department of Applied Geology, Curtin University, Perth, Washington 6845, Australia
7
Laboratoire de Planetologie et Geodynamique, Universite de Nantes, Nantes 44322, France
8
Laboratoire de Geologie, Ecole Normale Superieure, Paris CEDEX 5 75231, France.
9
UJF-Grenoble 1/CNRS-INSU, Institut de Planetologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble F-38041,
France
10
Unite Materiaux et Transformations, University of Lille & CNRS, Villeneuve d’Ascq F-59655, France
11
Institut Chevreul, University of Lille & CNRS, Villeneuve dAscq F-59655, France
12
Institut de Physique du Globe de Paris, Sorbonne Paris Cite, Univ Paris Diderot, UMR 7154 CNRS, Paris F-75005, France
*
Corresponding author. E-mail: [email protected]
(Received 27 November 2015; revision accepted 02 September 2016)
Abstract–Northwest Africa 7533, a polymict Martian breccia, consists of fine-grained clastladen melt particles and microcrystalline matrix. While both melt and matrix contain
medium-grained noritic-monzonitic material and crystal clasts, the matrix also contains
lithic clasts with zoned pigeonite and augite plus two feldspars, microbasaltic clasts,
vitrophyric and microcrystalline spherules, and shards. The clast-laden melt rocks contain
clump-like aggregates of orthopyroxene surrounded by aureoles of plagioclase. Some shards
of vesicular melt rocks resemble the pyroxene-plagioclase clump-aureole structures.
Submicron size matrix grains show some triple junctions, but most are irregular with high
intergranular porosity. The noritic-monzonitic rocks contain exsolved pyroxenes and
perthitic intergrowths, and cooled more slowly than rocks with zoned-pyroxene or fine grain
size. Noritic material contains orthopyroxene or inverted pigeonite, augite, calcic to
intermediate plagioclase, and chromite to Cr-bearing magnetite; monzonitic clasts contain
augite, sodic plagioclase, K feldspar, Ti-bearing magnetite, ilmenite, chlorapatite, and
zircon. These feldspathic rocks show similarities to some rocks at Gale Crater like Black
Trout, Mara, and Jake M. The most magnesian orthopyroxene clasts are close to ALH
84001 orthopyroxene in composition. All these materials are enriched in siderophile
elements, indicating impact melting and incorporation of a projectile component, except for
Ni-poor pyroxene clasts which are from pristine rocks. Clast-laden melt rocks, spherules,
shards, and siderophile element contents indicate formation of NWA 7533 as a regolith
breccia. The zircons, mainly derived from monzonitic (melt) rocks, crystallized at
4.43 0.03 Ga (Humayun et al. 2013) and a 147Sm-143Nd isochron for NWA 7034 yielding
4.42 0.07 Ga (Nyquist et al. 2016) defines the crystallization age of all its igneous
portions. The zircon from the monzonitic rocks has a higher D17O than other Martian
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© The Meteoritical Society, 2016.
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meteorites explained in part by assimilation of regolith materials enriched during surface
alteration (Nemchin et al. 2014). This record of protolith interaction with atmospherehydrosphere during regolith formation before melting demonstrates a thin atmosphere, a
wet early surface environment on Mars, and an evolved crust likely to have contaminated
younger extrusive rocks. The latest events recorded when the breccia was on Mars are
resetting of apatite, much feldspar and some zircons at 1.35–1.4 Ga (Bellucci et al. 2015),
and formation of Ni-bearing pyrite veins during or shortly after this disturbance (Lorand
et al. 2015).
INTRODUCTION
Since the Viking mission (1976), a vast amount of
information has been obtained on Mars. Its geology
and geochemistry have been studied using orbiters,
rovers, and the SNC meteorites. Orbiting visible/
infrared spectrometers, for example, OMEGA/MEx,
have led to the production of global maps showing the
distribution of olivine, pyroxene, hydrated minerals, and
ferric iron oxides (Mustard et al. 2005; Poulet et al.
2005, 2007; Ody et al. 2012). Infrared detection of
plagioclase and its variability has clarified rock types:
TES spectra suggested that the crust of Mars was not
purely basaltic (Bandfield et al. 2000; McSween et al.
2003), and CRISM spectra (Carter and Poulet 2013;
Wray et al. 2013) indicated plagioclase-rich rocks
showing that magmatic evolution must have been
complex. Oxides of ferric iron are dominant in the
spectra for the northern hemisphere while abundant
orthopyroxene is found only in the southern heavily
cratered uplands of Noachian age. Maps of secondary
minerals have given rise to the concept of changing
climate and alteration style with time (Bibring et al.
2006), with the more abundant evidence of (less acid)
water in the Noachian (phyllosian alteration), making
this period the most favorable for the development of
life. More recently, higher resolution spectral data have
given detailed pictures of local geologic history
(Murchie et al. 2009). Carter et al. (2013) reported
occurrences of 10 hydrated minerals, including the
discovery of epidote, giving evidence for hydrothermal
activity. The residual heat associated with large impact
craters is likely to have generated hydrothermal systems,
possibly accompanied by freezing of crater lakes, with
interesting biochemical possibilities (Newsom 1980;
Osinski et al. 2001, 2013; Abramov and Kring 2005).
Rovers have provided in situ bulk rock analyses on
Mars (e.g., McSween et al. 2009; Sautter et al. 2016).
Basaltic andesite was found in Ares Vallis by the
Pathfinder mission (McSween et al. 1999, 2003) and
picritic basalt at Gusev by Spirit (Gellert et al. 2004).
These rocks are usually interpreted as volcanic in
nature, but there is always the question of whether the
analyses represent pristine or altered volcanic rocks, or
volcanic debris in sedimentary rocks, or impact rocks
(Ashley and Delaney 1999; McSween et al. 2003;
Schultz and Mustard 2004). Curiosity is currently
making many fascinating observations at Gale Crater.
These include descriptions of polymict fragmentary twofeldspar-rich rocks of either sedimentary or impact
origin (Mangold et al. 2013), and analyses of a rock
with the chemistry of mugearite, that is, basaltic trachyandesite (Gellert et al. 2013; Stolper et al. 2013;
Schmidt et al. 2014). Sautter et al. (2014) have
described many feldspar-rich samples, and have
deconvolved the data using major-element ratios to
show that orthopyroxene, augite, plagioclase, and
orthoclase are present.
Martian meteorites have been subjected to very
complete analysis, particularly of isotope abundances,
which cannot be measured by rovers, and their parent
magmas yield information on the mantle of Mars (e.g.,
Debaille et al. 2007). They might constitute an inverse
ground truth for rover analyses where sample
preparation and instrument calibration may be difficult.
However, the SNC meteorites differ geochemically in
some respects from Martian surface materials analyzed
in situ (e.g., McSween et al. 2003). Matches between
shergottite and surface spectral signatures have been
reported for certain regions (Ody et al. 2015). However,
the largest group of Martian meteorites, the shergottites,
has very young radiometric ages, whereas basalts
mineralogically similar to shergottites emplaced in Gusef
Crater in the Hesperian date from ~3.65 Ga based on
crater counting (Greeley et al. 2005; McSween et al.
2006). Mineral isochrons for the shergottites give very
young ages, typically <200 Ma (Nyquist et al. 2001;
Borg et al. 2005; Bouvier et al. 2008; Niihara et al.
2012), and they may therefore come from such very
restricted regions. There has been controversy over the
formation ages of shergottites (Bouvier et al. 2008) but
young ages have recently been confirmed by dating Zrrich phases in shergottites (Niihara et al. 2012; Moser
et al. 2013). Based on crater counting on high-resolution
images, volcanic calderas on Mars vary greatly in age
(Robbins et al. 2011). Volcanism continued until a little
Martian regolith breccia Northwest Africa 7533
over a hundred million years ago in a few volcanoes like
Olympus Mons. Only the ancient orthopyroxenite ALH
84001, dated at 4.1 Ga (Bouvier et al. 2009; Lapen et al.
2010), has been a candidate to represent the Noachian of
the cratered southern uplands. Ody et al. (2015) discuss
possible occurrences of similar orthopyroxene-bearing
rocks on the Noachian surface. These cratered highlands
make up nearly half the surface of Mars, and most of
the largest impact structures occur in the oldest part of it
(Tanaka et al. 2014). It is surprising that ancient
Martian meteorites are not more common and, in view
of the extent of the early heavy bombardment on Mars,
that breccias have not been prominent among Martian
meteorites. This may be related to the efficiency of
ejection of weakly lithified material (Hartmann and
Barlow 2006).
The first Martian breccia meteorite identified was
Northwest Africa 7034 (Agee et al. 2012, 2013; Ziegler
et al. 2013; Cartwright et al. 2014) and shortly
afterward we obtained Northwest Africa 7533 (Hewins
et al. 2013a; Humayun et al. 2013), the subject of this
paper. These meteorites, and NWA 7475, NWA 7906,
NWA 7907, NWA 8114, NWA 8171 (Korotev et al.
2013; Wittmann et al. 2013a, 2015; Hofmann et al.
2014; MacArthur et al. 2015) are paired. These breccia
meteorites are very different from the SNC meteorites,
as are known Martian surface rocks (McSween et al.
2003; Stolper et al. 2013; Sautter et al. 2014) and,
because of the diverse clast suite, can potentially yield
much information on the early crust of Mars. They
form an important window on a previously unsampled
and probably major lithologic unit of Mars. A first
impression of NWA 7533, with which we concurred,
was that it was “fiendishly complex” (McSween 2013).
However, we are now inclined to agree with Simonds
et al. (1977) that “breccias aren’t so bad after all.”
NWA 7533 has yielded evidence of formation in a
regolith, of the nature of early crustal reservoirs, of
zircon crystallization at 4.428 Ga, and of atmospherehydrosphere-surface interaction before this time
(Hewins et al. 2013a, 2013b; Humayun et al. 2013;
Nemchin et al. 2014). We therefore present here an
account of the petrologic components in this polymict
breccia in the context of the new discoveries stemming
from this important find.
SAMPLES AND METHODS
A 17 g sample of NWA 7533 was cut without using
any lubricating fluids (no water) at the Museum
National d’Histoire Naturelle (MNHN). Seven polished
sections (7533-1 to -7) and one doubly polished
transparent
thick
section
(7533-LM,
≥50 lm,
unmounted) were prepared and studied using optical
91
microscopy, scanning electron microscopy (SEM), and
electron probe microanalyzer (EPMA). They are in part
serial polished sections, usually too thick for identifying
the same object in two sections. Backscattered electron
(BSE) maps and images of selected clasts were made at
MNHN using a Tescan VEGA II LSU SEM in
conventional mode (mainly 15 keV and <20 nA), and
minerals were characterized with an SD3 (Bruker) EDS
detector. A Zeiss Σigma field emission SEM with
GEMINI technology using a SmartSEMÒ interface and
an Oxford 50 mm2 detector was used at Laboratoire de
Geologie, Ecole Normale Superieure, Paris; highresolution imaging was done on polished sections 75331 and -2 at 15 keV and 8.4 nA. X-ray maps were made
on both SEM instruments. All quantitative mineral
analyses were made by wavelength-dispersive spectrometry
on the Cameca SXFive electron microprobe at the
Universite Paris VI, generally using 15 keV and 10 nA.
For analysis of Ni in silicates, we used 20 kV and
300 nA, counting for 100 s on three WDS simultaneously
(Hewins et al. 2014). San Carlos, Chassigny, and Eagle
Station olivines and Tatahouine pyroxene (Jarosewich
et al. 1980; Smith et al. 1983; Barrat et al. 1999;
El Goresy et al. 1999; De Hoog et al. 2010), were
analyzed as internal standards. The detection limit,
based on count rates on peak and background above
and below the peak, is a probability function considering
the risk of treating positive as zero, and vice versa, at
the 95% confidence level, and is 24 ppm Ni in our
pyroxene.
Powder diffraction data were collected on a Bruker
D2 Phaser diffractometer equipped with a Lynxeye
CCD detector. X-rays were generated by a copper
30 kV/10 mA) and the Ka
cathode (k = 1.54060 A
radiations isolated with a Cu Kb filter. The sample
holder rotation speed was 20 rotations/min and a
numeric filter was applied to the data set in order to
minimize the iron fluorescence contribution.
Transmission infrared spectra were measured on
7533-LM in an environmental cell with a Brucker
Hyperion FTIR spectrometer in the 500–4000 cm 1
range with a 4 cm 1 resolution. Diffuse visible and near
infrared reflectance spectra were measured using the
spectro-photo-goniometer available at IPAG.
Three fine-grained matrix areas were extracted as
focused ion beam (FIB) sections (~100 nm thick) using
an FEI Strata DB 235 at IEMN at the University of
Lille, France. They were studied by transmission
electron microscopy (TEM) with an FEI Tecnai G2-20
Twin (LaB6, 200 kV) and a Philips CM30 (LaB6,
300 kV) at the electron microscopy center of the
University of Lille, France. Both instruments use an
energy dispersive spectrometer (EDS) for elemental
microanalysis.
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R. H. Hewins et al.
D/H ratios of pyrite alteration products were
analyzed on 7533-LM on the NanoSIMS installed at
MNHN Paris by Lorand et al. (2015). A 30 pA Cs+
primary beam was used to collect H and D
secondary ions in multicollection mode on a
20 9 20 lm² surface area at a raster speed of 1 ms/pixel.
Instrumental fractionation was corrected using a terrestrial
goethite sample with a D/H isotopic composition of
150&.
One polished section 7533-3 was studied by laser
ablation inductively coupled plasma–mass spectrometry
(LA-ICP-MS) for elemental abundances. Analyses were
performed with a New Wave UP193FX (193 nm)
excimer laser ablation system coupled to a Thermo
Element XR at the Plasma Analytical Facility, FSU. A
total of 73 elements, including all major elements
(Humayun et al. 2010), S and Cl, were determined
simultaneously in low-resolution mode using methods
previously described (Humayun et al. 2007; Gaboardi
and Humayun 2009). Standardization utilized 6 MPIDING glasses, 3 USGS silicate glasses, NIST SRM 610,
SRM 1263a steel, the iron meteorites North Chile
(Filomena) and Hoba, and a pyrite (for S). Beam spots
were 50–150 lm ablated at 50 Hz, using either spot or
raster mode.
Zr-rich phases were located by X-ray mapping of
7533-4 and analyzed by SIMS. U-Pb isotope analyses of
zircon were performed on a sensitive high-resolution ion
microprobe (SHRIMP II) at Curtin University, as
described in Humayun et al. (2013). Oxygen isotope
ratios were obtained using a Cameca IMS1480 ion
microprobe at the Swedish Museum of Natural History
(Nemchin et al. 2014) using 1 nA Cs+ primary beam
with an impact energy of 20 kV and operating at a
mass resolution sufficient to resolve 17O 1 from 16O1H
(M/DM = 7500). The U-Pb apatite analyses were
performed on the same instrument with a
13 kV
molecular oxygen beam and operating at a mass
resolution (M/DM) of 5400 (Bellucci et al. 2015). The
Pb isotopic compositions of plagioclase and alkali
feldspar were determined using a 9 nA O2 primary
beam and a mass resolution of 4860 (M/DM).
PROVENANCE OF THE METEORITE
The NWA 7533 breccia has mineralogical, chemical,
and isotopic properties consistent with a Martian origin
and similar to those of NWA 7034 and NWA 7475
(Agee et al. 2013; Hewins et al. 2013a, 2013b; Humayun
et al. 2013; Nemchin et al. 2014; Wittmann et al. 2015).
The bulk oxygen isotopic composition by IR-laser
fluorination/mass
spectrometry
are
d18O = 5.92;
17
17
d O = 3.65; D O = 0.57 (all per mil) and the magnetic
susceptibility, log X = 4.45 (Gattacceca, personal
communication), are indistinguishable from NWA 7034
values (Agee et al. 2013). This material is very different
from the SNC meteorites (Agee et al. 2013; Hewins
et al. 2013a, 2013b; Humayun et al. 2013), as are many
Martian surface rocks analyzed by rovers (McSween
et al. 2009; Stolper et al. 2013; Sautter et al. 2014).
Noble gas isotope data show the presence of a
trapped gas component in NWA 7034 like the modern
Martian atmosphere (Cartwright et al. 2014) and this is
the clearest isotopic evidence for a Martian origin for
these regolith breccias. The oxygen isotope data are
complex: an orthopyroxene mineral separate exhibited
D17O of 0.33&, indistinguishable from SNC compositions
(Ziegler et al. 2013), confirming a Martian origin.
However, there is an overall trend of increasing D17O with
increasing d18O in NWA 7034 separates, toward the
composition of known Martian alteration phases, and
Nemchin et al. (2014) found D17O values in NWA 7533
zircons ranging from within error of the Mars
Fractionation Line (MFL) to +2.16 0.56&. We discuss
below the implications for the Martian atmosphere and
regolith. The FeO/MnO ratio for all pyroxenes in NWA
7533 is ~34, except for those in clast-laden melt rocks (see
below), like that of ALH 84001 pyroxene, and this is also a
Martian signature.
The bulk breccia composition is equivalent to alkali
basalt formed by 4% partial melting of primitive mantle
(Humayun et al. 2013) recycled into a suite of rocks in
this early crust. It cannot be said to be simply basaltic,
in that it contains pyroxenitic, noritic, and monzonitic
material. It is not simply related to the SNC meteorites:
they contain olivine, except for the most fractionated
shergottites, whereas olivine is almost entirely absent
from the breccia. SNC are volcanic and subvolcanic
rocks, but NWA 7533 contains material deposited on
the surface, such as melt spherules, and lithic clasts of
rocks crystallized at depths possibly up to kilometers.
Noritic clasts in the breccia contain inverted pigeonite
(whereas pigeonite is widely preserved in SNC) with
exsolution lamellae many microns thick, compared to
only hundreds of nanometers in SNC pyroxenes. The
breccia contains higher concentrations of siderophile
elements than the SNC, for the same Mg content
(Humayun et al. 2013), and its sulfides are of
hydrothermal origin (pyrite) rather than magmatic
origin (pyrrhotite) in the SNC (Lorand et al. 2015).
While the SNC are relatively young rocks (Amazonian),
this black and white breccia NWA 7533 is a major
repository of geochemical information on early Mars,
along with ALH 84001, and some of the rocks analyzed
by Curiosity at Gale Crater. As it is very diverse
lithologically, we sketch our view of the breccia and its
components in Fig. 1. We present below observations
and data first for the matrix, followed by sections on
Martian regolith breccia Northwest Africa 7533
93
Fig. 1. Schematic of components of NWA 7533 breccia;
R = clast-laden melt rock, S = spherule, B = microbasalt, sd =
sedimentary, Z = zoned-pyroxene, N = noritic, M = monzonitic,
X = crystal clast, O = orthopyroxene, A = augite, A-P = augitepigeonite, P = plagioclase, and K = K feldspar.
spherules and shards, lithic and crystal clasts. The lithic
material is described in a sequence of increasing grain
size: clast-laden melt rocks, microbasaltic clasts, clasts
with zoned pyroxenes, and noritic- monzonitic clasts.
Fig. 2. Mg Ka image of polished section NWA 7533-2. Clastladen melt rock bodies (CLMR) are darker (more Fe rich)
than breccia matrix. Feldspar-rich spherule (S1) and
sedimentary clast (SED) near top center. Microbasalt (MB),
noritic (N), and monzonitic (M) clasts are indicated. Pyroxene
clasts (and olivine in partial spherule S2) are light gray,
feldspar is black.
THE BRECCIA COMPONENTS AND MATRIX
The breccia is complex with a variety of crystal
clasts, inclusions, and crystal types, contained in breccia
matrix and in aphanitic melt rock bodies. We discuss
the matrix here and subsequently present results by
lithological type of object. A cut slab is shown in
Fig. S1 in supporting information and an Mg Ka X-ray
map of a polished section (7533-2) in Fig. 2, in which
feldspars are dark gray and pyroxenes are medium to
light gray. Both show spherules, melt rock bodies, lithic
clasts, and crystal clasts. Figure 2 shows a feldspar-rich
melt spherule 3 mm in diameter and a spherule
fragment with olivine and pyroxene dendrites, estimated
at about 6 mm in former diameter, as well as smaller
crystal and monzonitic and microbasaltic lithic clasts.
The hosts of the smaller clasts are twofold, clast-laden
aphanitic bodies and matrix, which are hard to
distinguish at this scale, although breccia matrix tends
to be brighter, that is, more Mg-rich (Fig. 2). The
aphanitic bodies, interpreted below as clast-laden melt
rocks with fine-grained igneous textures (~10 lm
crystals), and other clasts are embedded in very finegrained (~1 lm) compact crystalline breccia matrix.
The melt rock bodies, spherules, and lithic clasts
occur with crystal clasts down to at least 5 lm, all
supported by a matrix with 200 nm to 2 lm grains. This
matrix is crystalline and annealed, rather than a clastic
aggregate, and it was studied in detail by field emission
(FE) SEM in sections 7533-1 and -2. It is composed
mainly of anhedral micron-sized plagioclase feldspar,
with pyroxene grains down to a few hundred nm
surrounding and embedded in feldspar (Figs. 3a and S2
in supporting information). The pyroxene is somewhat
clustered into tiny aggregates (Fig. S2d) between
plagioclase grains. Fe-rich phases, especially magnetite
and probably maghemite, may be symplectitic or lacy,
and are irregularly distributed in the matrix, especially in
pyroxene. Microprobe analyses of minerals in the matrix
are of low quality because of the small grain size although
they are sufficient to identify pyroxenes and calcic
plagioclase similar to those in the melt rocks. Analytical
transmission electron microscope (ATEM EDS) analyses
are shown in Table S1 in supporting information. For
plagioclase, they suffer from low cation totals. Since Ca
correlates well with Si and Al, but Na does not, likely due
to migration under the TEM beam, endmembers were
calculated using Si and Al atoms per formula unit (a.f.u.)
to define An, for example, An% = 100*(Al a.f.u 1), K to
define Or, and obtaining Ab by difference. The matrix
also contains some vugs (Lorand et al. 2015; Leroux
et al. 2015) associated with pyrite veins.
94
R. H. Hewins et al.
organized along regular subgrain boundaries or
deformation bands. Fe-oxides, for which the electron
diffraction patterns are compatible with magnetite and
maghemite, are distributed throughout the matrix. The
grain size ranges from 1 lm to a few nm. Large Feoxides are located in the pyroxene-feldspar grain
boundaries, including triple junctions, and the smaller
ones may occur within the pyroxene grain interiors.
Most of the grain boundaries are unusually thick (10–
20 nm) and filled with a low-density amorphous
material very sensitive to the electron beam (Fig. S2).
The composition of the grain boundaries is close to
those of the adjacent minerals but enrichments in SiO2,
Al2O3, CaO, and Cl were frequently detected (Leroux
et al. 2015). The amorphous state could be the result of
beam damage and, because of a smectite-like
composition, the high e-beam sensitivity could be due to
the presence of water. The amorphous material may be
related to the decomposition of pyroxene into silica and
Fe-oxide that has been identified in pyroxene clasts
(Leroux et al. 2016), but another plausible explanation
is alteration in hot desert conditions (Leroux et al.
2015). HRTEM analyses of the matrix of NWA 7034
also showed minor phyllosilicates (<50 nm) on pyroxene
grain boundaries (Muttik et al. 2014a; LPSC 2763), but
the identification of phyllosilicates was not confirmed in
this study.
FINE-GRAINED CLASTS
Clast-Laden Impact Melt Rock
Fig. 3. Breccia matrix. a) BSE image showing crystal clasts in
a recrystallized matrix with interlocking submicron pyroxene
(light gray) and plagioclase grains (dark gray), the pyroxene
granules being embedded in plagioclase grain margins; and (b)
TEM bright-field image of the matrix with granoblastic
texture, 120° contacts, and porosity on grain boundaries.
The matrix was also studied by TEM on FIB
sections (Figs. 3b and S2c). It is a granoblastic
assemblage with low-Ca pyroxene and feldspar grains
with an average grain size close to 300 nm (Leroux
et al. 2015). Although some well-equilibrated triple
junctions are observed, as for NWA 7034 (Muttik et al.
2014a), most grains have preserved an irregular
morphology. Intergranular porosity is also high, in
particular, at triple junctions (Figs. 3b and S2c). The
low-Ca pyroxenes range from En64 to En74 and contain
a high density of planar defects on (100) corresponding
to thin alternating domains of clino- and orthopyroxene. Most of the feldspar grains are plagioclase
(from An14 to An57) with minor alkali feldspar. Three
grains of Cl-rich apatite >1 lm in size are present in the
FIB section and one of them contains dislocations
The largest objects (~1 cm) in NWA 7533 are
smooth or angular fine-grained melt bodies (Fig. 2),
containing lithic and crystal fragments. They include
oval and aerodynamically shaped bodies (Hewins et al.
2013a; Humayun et al. 2013), which are particles of
clast-laden melt rocks (Grieve et al. 1974; Simonds
1975). Similar material, clast-rich amoeboid vitrophyre,
is the dominant component in NWA 7475 (Wittmann
et al. 2015), and the “plumose” groundmass with clasts
described in NWA 7034 (Agee et al. 2013) is probably
the same. A croissant-shaped body in 7533-1 appears to
be sculpted by transport while partially molten
(Humayun et al. 2013). The melt rock is a fine-grained
plagioclase -pyroxene igneous rock mainly with
subophitic texture and the aspect ratio of the
plagioclase laths (mostly 1~2 lm wide) is ~10. Where
pyroxene grains are parallel to plagioclase laths, the
texture is more fasciculate. The textures of the melt
rock are shown in Figs. 4a–d and S3 in supporting
information. Finer grained melt rocks have abundant
small crystal clasts (Fig. 4a), angular to more rounded
than in breccia matrix. Coarser grain size (~30 lm long)
Martian regolith breccia Northwest Africa 7533
95
Fig. 4. Clast-laden melt rock. a) BSE image of subophitic groundmass of ~10 lm plagioclase (dark gray) and pyroxene (light
gray), containing 10–200 lm crystal clasts (x) and ~100 lm pyroxene clumps (cl). b) Sharp contact between less fine-grained melt
rock (~20 lm plagioclase) and enclosed monzonitic clast with perthitic feldspar (pe), augite (ag), and chlorapatite (ap). c)
Orthopyroxene clump with calcite in cracks and dendrites in plagioclase aureole perpendicular to clump surface. High-Z phases
include magnetite and pyrite. d) Si Ka image of clumps and aureoles in melt rock in NWA 7533-2 (lower left edge of Fig. 2). Up
to about 30 aureoles can be distinguished. See also Figs. S3 and S4 in supporting information.
is associated with fewer crystal clasts and the presence
of lithic clasts (Fig. 4b). Angular lithic clasts in the melt
rock are up to ~1 mm long.
The finer grained melt rocks are remarkable in
containing ellipsoidal aggregates or clusters of
orthopyroxene granules or dendrites (with embedded
magnetite, chromite, ilmenite, or pyrite) surrounded by
rings of plagioclase laths which radiate outward from
the pyroxene cluster (Figs. 4a and 4c). We have
described examples of this pyroxene-plagioclase
association as clot-aureole structures (Hewins et al.
2013a; Humayun et al. 2013). The most obvious
ellipsoidal objects are around 100 lm in size, but many
smaller ones (down to ~30 lm) are recognized in X-ray
maps (Figs. 4c, S3c, and S3d), especially Si Ka and
RGB composite images (Fig. S4a) extend into the
plagioclase aureoles, in which plagioclase laths extend
into the groundmass. The pyroxene dendrites may be
attached to the pyroxene of the groundmass. A Ca Ka
map demonstrates that all the fractures in the clumps
(dark in Figs. 4c and 4d) are filled with calcite
(Fig. S4c). Figures S3c, S3d, and S4b also show
chemical heterogeneity in the groundmass, with Mgversus Al-rich zones or schlieren. Zones with few
clumps are richer in plagioclase and poorer in pyroxene,
that is, more Al-rich, than zones with many clumps,
which are Mg-rich.
Microprobe analysis of minerals in the groundmass
of the melt rock is difficult because of the small grain
size. Good analyses of pyroxene, however, are
96
R. H. Hewins et al.
consistently magnesian and show moderate Al2O3 (2–3
wt%) and TiO2 (up to 1 wt%) contents. The results
are shown in the pyroxene quadrilateral (Fig. 5a)
and Table S2 in supporting information. The analyses for
12 melt bodies in 4 sections are very similar and have a
nearly constant Mg number of 69 2. The continuity in
Ca concentrations (Wo1.8–47.4) suggests mixtures of
orthopyroxene and augite, which is confirmed by BSE
and X-ray images. However, the clumps are dominantly
orthopyroxene En68.1 1.3Fs29.5 1.3Wo2.4 0.3, whereas
the groundmass pyroxene is on average more Ca-rich
(En52.3 6.1Fs23.3 4.5Wo24.4 12.9).
The
clump
orthopyroxene is close in major-element composition to
ALH 84001 orthopyroxene, which we show in this
(Fig. 5a) and subsequent pyroxene quadrilaterals to
emphasize similarities (and differences in the case of the
more fractionated rocks). However, it is more Al-rich
(2.9 0.9 wt% Al2O3) than ALH 84001 pyroxene and
other orthopyroxenes in the breccia, and also extremely
Cr-poor (0.03 wt% Cr2O3). Furthermore, whereas
pyroxene in the breccia generally has an FeO/MnO ratio
of ~34, with scattered high Mn exceptions, pyroxene in
the clast-laden melt rocks is consistently more Mn-rich
than this typically Martian value, with an FeO/MnO
ratio of ~25 (Fig. 5b). The plagioclase is mainly
andesine, An56–32 (An41 7), and not significantly
different in the aureoles surrounding the clumps than in
the groundmass (Fig. 5c, Table S3 in supporting
information). It is similar in composition to that in the
less fractionated lithic clasts.
Shards and Melt Spherules
Orthopyroxene-rich objects probably related to the
melt rock clumps occur as clasts in the meteorite
matrix, and tend to have an irregular or polygonal
form. Most have concave surfaces bounded by
plagioclase with the orthopyroxene dendrites in the
interior (Figs. 6a and S5a in supporting information).
These appear to be shards of fine-grained vesicular melt
rock or devitrified glass. Other shard-like clasts have a
foliated or spherulitic appearance. Rare small spherules
resembling droplets of the clump material are described
below. The mineral compositions of the shards are like
those of the clumps, and some are included in Tables
S2, S3, and Fig. 5.
Large melt spherules are prominent on the sawn
surfaces of this breccia (e.g., Figs. 2 and S1) and of
paired meteorites (e.g., Agee et al. [2012] fig. 1;
Wittmann et al. 2015 fig. 1). The spherules can be
grouped into three types: vitrophyric, crystalline, and
altered. The largest spherules (diameters of several mm)
are vitrophyric, although their groundmass is not glassy
but finely crystalline (grain size 1–5 lm) with pyroxene
Fig. 5. Mineral compositions for the groundmass and the
clump-aureole structures of the clast-laden melt rocks: (a)
pyroxene quadrilateral, (b) FeO-MnO of pyroxene compared
to pyroxene elsewhere in the breccia, and (c) feldspar.
Orthopyroxene and augite in ALH 84001 are shown for
comparison.
and feldspar laths in fine spherulitic to variolitic textures
(e.g., Figs. 6b–d). The spherule 7533-2-2 in Figs. 6b and
6c is 3 mm in diameter, with a groundmass speckled with
submicron Fe-rich phase(s), including Cr-bearing
magnetite and chromite, especially just under its surface,
and it develops coarsely crystalline patches near its
borders. It has no large dendrites and is traversed by
Martian regolith breccia Northwest Africa 7533
97
Fig. 6. BSE images. a) Shard with peripheral plagioclase resembling pyroxene clumps. Concave margins indicate vesicle walls. b)
Spherule 7533-2-1 with feldspar veins and magnetite-rich border. c) Close-up of (b) showing igneous texture and hyalophanebearing veins. d) Detail of olivine-bearing spherule 7533-2-2 in Fig. 2 (bottom right). Olivine dendrite with plagioclase inclusions
and overgrowth of orthopyroxene on both sides of dendrites are continuous with orthopyroxene intergrown with plagioclase
laths in the groundmass of the spherule.
veins of plagioclase and hyalophane (Figs. 6b and 6c)
consistent with hydrothermal activity. Another veined
feldspar-rich spherule (7533-7-sph1; Fig. S5b) has a Nadepleted signature consistent with melting of an altered,
clay-bearing protolith (Humayun et al. 2014c). Oval
objects with a patchy texture, in some cases with a
vitrophyric interior region, have more coarsely crystalline
dendritic/spherulitic borders with pigeonite and subcalcic
augite (Fig. S5b), in turn coated with probable
accretionary rims. Similarly, spherules in NWA 7475
tend to preserve quenched mesostasis in their interiors
(Wittmann et al. 2015). Sampling of nonequatorial
sections of larger spherules and differences in alteration
may account for variations in the appearance of
spherules, which require detailed study.
Olivine occurs in NWA 7533 only in one melt
spherule 74533-2-2 (Fig. 2) as long chain dendrites
(Fig. 6d) along with long pyroxene dendrites. In NWA
7034, olivine dendrites occur in a large partial spherule,
but also as one grain in a microbasalt, probably in
reaction relationship with pyroxene (Santos et al.
2015a). Plagioclase occurs as small insets in the olivine
dendrites. The olivine dendrites are bordered by
orthopyroxene that is continuous with pyroxene
alternating with plagioclase in the spherulitic
groundmass (Fig. 6d). A similar object from NWA
7034, a vitrophyre with olivine dendrites (Agee et al.
2013; Udry et al. 2014), has a high Na/K ratio and
resembles the more basaltic components of the breccia.
The derivation of the spherules from both clay-rich and
98
R. H. Hewins et al.
basaltic sources suggests an analogy with S-type and
I-type granites. Smaller spherules (a few hundred lm in
diameter) are more coarsely crystalline. In texture and
mineralogy, these show resemblances to aspects of clastladen melt rocks, for example, clumps and groundmass
(Figs. S5c and S5d).
Mineral compositions for spherules are shown in
Fig. 7, and Tables S4 and S5 in supporting information.
For olivine-bearing spherules, olivine (Fo74–65) and
pyroxene (En69Wo2-En63Wo5) compositions are shown
in Fig. 7a, along with selected data for the similar
material in NWA 7034 (Agee et al. 2013; Udry et al.
2014). The pyroxene in the latter is similar (En73Wo2En49Wo9) but more zoned, and the olivine is more
ferroan (Fo65–53; Udry et al. 2014) than that in 7533-22. Pyroxenes from a small finely crystalline spherule,
7533-1-ENS (see Fig. 10c), are also shown: they are
orthopyroxene En71.9Fs25.6Wo2.5 and augite En35.1Fs18.0
Wo46.9. As shown in Fig. 7a, the orthopyroxene in the
small spherule is similar to that in the vitrophyric
spherules in 7533-2 and -7, as is its plagioclase, but
augite has also crystallized. The orthopyroxene is in all
cases quite close in composition to that of ALH 84001.
For the 3 mm extremely fine-grained spherule in section
2, we have only vein feldspar analyses, in part mixtures
of plagioclase An41.8Ab53.2Or5.0Cn0.3 and hyalophane
An4.5Ab20.4Or63.7Cn11.4. These are shown in Figs. 7b
and 7c, along with plagioclase from four other
spherules, none of which have Ba-rich feldspar. The
vein feldspar in 7533-7-1 is mainly plagioclase similar in
composition to all the other plagioclase, for example,
the spherule with olivine dendrites (An33.8Ab62.6Or3.6)
and the small 7533-1 ENS plagioclase An39.6Ab58.5Or1.9.
The latter also contains chromite Sp19.1Chr56.8
Usp1.4Mgt22.4. Analyses of oxide minerals for spherules
and all other clast types are shown in Table S6 in
supporting information.
Fig. 7. Mineral compositions for olivine vitrophyre and
feldspathic spherules. Feldspars mainly in veins for 2-1 and 71. a) Pyroxene quadrilateral with olivine below the baseline. b)
Feldspars Ba- (Ca+Na)-K-Cn. c) Feldspars Ca-Na-K.
Fine-Grained Igneous Clasts
NWA 7533 contains basaltic clasts with grains
~50 lm to ~100 lm (Fig. 8) and subophitic to
granoblastic textures, which we have called microbasalts
(Hewins et al. 2013a) and analyzed samples plot in the
basalt field of an SiO2 Na2O+K2O plot (Humayun
et al. 2013). Similar clasts in NWA 7034 were called
igneous clasts by Santos et al. (2015a) who classified
them as basaltic, andesitic, etc., based on their bulk
compositions. They contain pyroxenes, plagioclase,
oxide minerals, chlorapatite, and in some cases K
feldspar. They are coarser grained than the groundmass
of the clast-laden melt rocks, and, according to Santos
et al. (2015a, 2015b) have grains typically 200 lm or
less in their longest dimension. Crystal clasts are rare in
this material, but their occasional presence (Fig. S6a in
supporting information) suggests that the microbasalt is
clast-poor impact melt, and both of them have similar
basaltic and Ni-Ir-rich bulk compositions (Humayun
et al. 2013). The microbasalt is not observed as clasts
within the clast-laden melt rock. The subophitic texture
is illustrated in Figs. 8a and 8b, where pyroxene grains
are molded around euhedral plagioclase laths.
Plagioclase laths are irregular in size and distribution,
with trachytic patches where they are subparallel, in
which case a pyroxene grain (pigeonite in Fig. 8b) may
range from poikilitic to apparently interstitial. There are
also clasts with metamorphic (granoblastic) textures
with anhedral plagioclase and chains of pyroxene
crystals as shown in Fig. 8c.
Martian regolith breccia Northwest Africa 7533
99
Fig. 8. BSE images showing textures of microbasalts: (a) subophitic 7533-2-cent3; (b) poikilitic-intersertal 7533-4-L; (c)
granoblastic 7533-1-obj2. d) Mg Ka image of layered sedimentary clast containing pyroxene (light gray) and feldspar (dark gray)
with mudstone-size and clay-size layers.
The microbasalts, like the clast-laden melt rocks, may
contain magnesian orthopyroxene, but pigeonite is also
identified with certainty (Table S7 in supporting
information). The presence of orthopyroxene versus
pigeonite does not correspond to metamorphic versus
igneous texture, and the two phases have a similar range
of Fe/Mg ratios. All microbasalt pyroxenes are plotted in
Fig. 9a showing that their most magnesian compositions
are close to those of ALH 84001, and of melt rock
pyroxenes. Pyroxene compositions for individual clasts in
six polished sections are shown and discussed in Fig. S7
in supporting information. The rocks tend to have either
orthopyroxene or pigeonite, with only one clast augiterich. Orthopyroxene extends to more Mg-rich
compositions than pigeonite but otherwise the
composition ranges are similar. One clast, 7533-2-84, has
both zoned pigeonite and zoned augite, like coarser
grained rocks described below and shergottites.
Plagioclase has a composition range of An55–23 in 15
clasts of microbasalt from 5 sections, with little variation
between clasts or in %Or (Fig. 9b; Table S8 in
supporting information). A sixteenth clast, 7533-2-84, the
one with zoned pigeonite and augite; however, it has a
similar range of Ca/Na ratios (An50–24) but is enriched in
K, up to Or22 (Fig. 9b), probably due to small grains or
exsolution blebs of K feldspar in andesine. A seventeenth
clast (7533-7-MH4; Fig. S6b) contains discrete K feldspar
(Or84; Fig. 9b); its pyroxene is also fractionated although
it is orthopyroxene. Such clasts may have basaltic
andesite major-element compositions (Santos et al.
2015a, 2015b) and they have mineralogical similarities to
the medium-grained rocks with zoned pyroxenes
discussed below. Fe-rich oxide compositions are shown in
Fig. 9c and Table S6. The spinel is Fe-rich, but contains
significant Cr and Ti, with a composition range Chr1–13
and Usp1–29 with an outlier at Usp63. Coexisting
100
R. H. Hewins et al.
Sedimentary Clasts
Fine-grained clasts in NWA 7533 with layers a few
hundred microns in thickness are interpreted as
sedimentary in origin. They have clastic textures unlike
the granoblastic texture of the fine matrix, with the
smallest pyroxene grains independent and not molded
around anhedral plagioclase grains. Variations in grain
size and mineral abundance (pyroxene versus feldspar)
are responsible for this lamination or bedding. One such
clast is observed in 7533-2 (Fig. 1b) immediately below
and to the left of the 3 mm spherule. A more detailed
image, also an Mg Ka map, is given in Fig. 8d, which
shows layers of clay-size particles (<4 lm) probably
transported in suspension alternating with layers of very
fine siltstone- or mudstone-size particles (4–64 lm). The
latter are very poorly sorted with both clay-size and
larger particles, mainly crystal clasts, up to 100 lm.
Indeed, the two flat igneous “pebbles” (2-1521 and 2-84)
appear to be part of the sedimentary sequence, as
shown in the BSE image Fig. S8 in supporting
information. Both BSE and Mg Ka show that there are
alternating pyroxene- and plagioclase-rich layers. Similar
lithic clasts, with ~100 lm layers, have been observed in
NWA 7475 (Wittmann et al. 2015; fig. 17c). The finest
grains are similar in size to observed atmospheric dust
grains (Lemmon et al. 2004) and we concur with
Wittmann et al. (2015) that these may have been eolian
dust deposits.
MEDIUM-GRAINED LITHIC CLASTS
Fig. 9. Mineral compositions of NWA 7533 microbasalts. a)
All data plus ALH 84001, to show that different phases
(orthopyroxene, pigeonite, and augite) can be present. b)
Feldspar compositions for microbasalts for 15 clasts, and for
for the clast 7533-2-84, with pigeonite and augite, and K
feldspar-bearing 7533-7-MH4 with orthopyroxene. c) Crbearing spinel compositions for microbasalts.
magnetite and ilmenite are present and, as for other
ilmenite-rich
clast
types
(zoned-pyroxene
and
monzonitic), indicate fO2 2–3 log units above the FMQ
buffer (Table S9 in supporting information).
A major distinction in clast types in the breccia is
between fine-grained type described above and those
with 1–2 mm grains. We have no barometry for these
rocks but they have coarser exsolution features in
pyroxene and alkali feldspar than the subvolcanic/
hypabyssal SNC meteorites. This suggests slow cooling
and we have chosen to use plutonic rock names: they
are mainly noritic to monzonitic fragments (Hewins
et al. 2013a; Humayun et al. 2013). Plutonic rocks are
normally classified by abundance of minerals but here
typical crystals and crystal fragments are ~1 mm long
and small clasts may be made up of only a few of the
possible phases. We have used a combination of phase
compositions and associations to make a classification,
and use adjectives rather than names to indicate
uncertainty in modal abundances. For example, as we
are not sure of the K feldspar abundance, some
monzonitic clasts could be syenites or diorites. Ca-poor
pyroxene is the key mineral for noritic clasts and K
feldspar for monzonitic clasts, and both may contain
augite and plagioclase. In the absence of the key
minerals, plagioclase composition (more calcic in noritic
Martian regolith breccia Northwest Africa 7533
101
clasts, more sodic in monzonitic clasts) and spinel
composition (Cr-bearing in noritic clasts and Ti-bearing
in monzonitic clasts) can be used. Crystal clasts also
give clues to lithologic types, and the presence of
magnesian orthopyroxene suggests a source from
orthopyroxenites (see below). However, in addition to
plutonic material, there is a minority of clasts whose
strongly zoned pyroxenes suggest a near-surface origin,
and we discuss these first.
Clasts with Zoned Pyroxenes
Four clasts contain zoned pyroxenes, pigeonite and
augite with no exsolution (Table S10 in supporting
information), like those in shergottites and in
microbasalt 7533-2-84, plus two feldspar phases and
abundant chlorapatite (analyses of phosphates from all
clasts types are given in Table S19 in supporting
information). NWA 7533-4-K and 7533-1-12 shown in
Figs. 10a and 10b, respectively, contain up to 1 mm
pyroxene and 500 lm chlorapatite. Accessory phases
include Ti-magnetite, ilmenite, a TiO2 phase, plus late
hydrothermal pyrite, and terrestrial alteration in the
form of Fe oxyhydroxides and veins with calcite. Clast
1–12 is similar to but has less augite than 4-K.
The feldspars in the medium-grained clasts
(Table S11 in supporting information) are very sodic
plagioclase An35–18, and homogeneous orthoclase or
sanidine Or69–88 (Fig. 11a). The plagioclase in
microbasaltic clast 7533-2-84 is also shown: with An50–24
it is more calcic but overlapping the plagioclase in the
clasts with zoned pyroxenes. Discrete orthoclase was
not recognized in the microbasalt but spikes in the K
content of “plagioclase” (Or22) indicate overlaps of the
two phases. Oxide phases in the medium-grained clasts
(Tables S6 and S9) are Ti magnetite notably poor
in Cr, Sp3–0Chr0.5–0.1Usp23–12Mgt62–86, and ilmenite
Ilm80–91Hem18–8. Spinel compositions are plotted in
Fig. 11b.
The pyroxene compositions of these clasts are
shown all together in Fig. 11c, again with
microbasaltic clast 7533-2-84 that has similar mineral
compositions despite its texture, and separately in Fig. S9
in supporting information. In clast 4-K, the pyroxenes are
pigeonite En60–36Fs31–50Wo8–14 and augite En48–23
Fs21–39Wo28–38 (Figs. 11c and S7), like those of
shergottites. The unusual microbasalt clast 7533-2-84,
with a range of pigeonite compositions, is seen in Fig. S9
to have pigeonite and augite compositions more
magnesian on average than, but overlapping with, the
compositions of pyroxenes in the four zoned-pyroxene
clasts. Some noritic-monzonitic clasts containing both
unzoned pigeonite and K feldspar may be related to the
zoned-pyroxene rocks.
Fig. 10. BSE images of medium-grained clasts with zoned
pyroxenes. a) NWA 7533-4-K K (clast 1 of Bellucci et al.
2015). b) NWA 7533-1-12. PI = pigeonite, AG = augite,
PF = plagioclase, AF = alkali feldspar, AP = chlorapatite,
Mt = magnetite, Il = ilmenite, Pt = pyrite.
Noritic-Monzonitic Clasts
Clasts with orthopyroxene or inverted pigeonite,
and plagioclase (An 50–30), plus augite, Cr-Timagnetite, and rarely alkali feldspar, are classed as
noritic; those with alkali feldspar as well as
plagioclase (An< 30) and augite, plus rare inverted
pigeonite, abundant chlorapatite, Ti-bearing magnetite,
ilmenite, and zircon, as monzonitic. Orthopyroxene
including inverted pigeonite is more abundant in the
noritic clasts and augite in the monzonitic clasts, but
there is considerable overlap in pyroxene compositions
102
R. H. Hewins et al.
Fig. 11. Mineral compositions for four zoned-pyroxene clasts
and for 7533-2-84, a slightly fractionated microbasaltic clast;
(a) feldspars, (b) Ti-bearing spinels, (c) pigeonite and augite.
between the two types, with pyroxene in monzonite
only slightly more ferroan on average. Concentrations
of siderophile elements in these clasts indicate
crystallization from impact melts (Humayun et al.
2013).
Noritic clasts apparently representing rocks with
hypidiomorphic granular textures (Turner and
Verhoogen 1951) are illustrated in Figs. 12a, 12b, and
S10 in supporting information. The Ca-poor pyroxene
may be homogeneous orthopyroxene (Figs. 12b, S10a,
and S10b), but inverted pigeonite (Fig. 12a) and rarely
pigeonite also occurs (Fig. S10c). Fresher orthopyroxene
is more ferroan, and dusty speckled orthopyroxene is
more magnesian (OF and OM in Fig. S10a). Augite can
be single phase (Fig. S10a), or exsolved (Fig. S10b).
Most of the augite grains coexisting with orthopyroxene
in other clasts equilibrated at temperatures (Lindsley
and Andersen 1983) of 800–900 °C (Fig. S11 in
supporting
information).
Chromite-magnetite
is
common, and accessories include zircon (Fig. S10a) and
chlorapatite (in rare cases overgrown on merrillite or
containing lacy monazite, as in Fig. S10c, Table S19).
Monzonitic clasts contain either two homogeneous
feldspar phases (Fig. 12d) or plagioclase plus
intergrowths (perthitic or antiperthitic; Figs. 12c and
12e). The minority phase occurs as irregularly
distributed ragged lamellae or flames. As perthitic
feldspar is found only in monzonitic clasts, a crystal
clast of perthitic feldspar is automatically monzonitic
(Fig. 12c). Most clasts contain subhedral to anhedral
augite, partly molded around feldspar grains, but there
is also peculiar stringy augite interstitial or subophitic
to alkali feldspar laths (Fig. 12e). Euhedral chlorapatite
is abundant. Zircon, although present in some noritic
clasts, is more abundant in the monzonitic ones
(Figs. 12c–e), as is baddeleyite (Fig. 22 discussed
below).
Individual mineral analyses are given in Tables S12,
S13, S6, and S9, and Fig. S12 in supporting information.
Average mineral compositions for lithic clasts are shown
in Fig. 13a (feldspars), 13b (pyroxenes), and 13c (spinels).
The separation into two groups is seen well in the
feldspar triangle (Fig. 13a), where there is one exception
of K feldspar coexisting with calcic plagioclase. Another
case of intermediate properties is shown where low
calcium pyroxene coexists with sodic plagioclase. Oxide
minerals, if present, also serve to distinguish the rock
types. Spinel in noritic rocks is Cr-rich or Cr-bearing,
whereas those in monzonitic rocks are Ti-bearing
(Fig. 13c). Spinels are also more Al-rich in noritic clasts,
up to Sp24. There is a small overlap of the two spinel
trends near the magnetite pole.
The monzonitic fragments are enriched in K, REE,
P, Ti, Cl, and Zr and contain zircon, and occasionally
baddeleyite (Humayun et al. 2013) that have provided a
crystallization age for the noritic-monzonitic rocks, as
discussed below. Compositions of minerals associated
Martian regolith breccia Northwest Africa 7533
103
Fig. 12. BSE images of noritic and monzonitic clasts: (a) noritic clast 7533-5-B with pigeonite and andesine, (b) norite 7533-1-5,
(c) antiperthite clast with zircon, Z2, (d) monzonitic 7533-4-Z1, (e) monzonitic clast with zircons Z5,6. Opx, OF,
OM = orthopyroxene; aug = A augite; P = plag; PF = plagioclase; KF = K feldspar; Cr = chromite; Mgt = magnetite;
IL = ilmenite; AP = chlorapatite; PT = pyrite; Z = zircon.
with zircons are included in Tables S12, S13, and S6.
Zircon Z1 (Fig. S10a) is almost completely included by
a 300 lm clast rich in pyroxene that is dominantly a
fractured ferroan orthopyroxene grain (En45.3Fs52.1
Wo2.6). The clast also contains two small grains of
plagioclase (An39.5Ab57.8Or2.6) and Cr-bearing magnetite
Sp3.59Cr1.40Usp10.14Mgt79.54.
This
assemblage
is
characteristic of the noritic clasts in NWA7533. Zircon
Z2 (Fig. S12c) is a skeletal, hopper-shaped crystal
(Humayun et al. 2013; Fig. S2) inside a shard of
antiperthite. Its shape suggests rapid growth, possibly
due to supercooling of the melt. The antiperthite has a
bulk composition of An2.8Ab69.6Or28.6, with host
An2.0Ab96.0Or2.0 and lamellae An1.7Ab14.2Or84.1. Similar
feldspars are found in monzonitic clasts in NWA7533
and their exsolution suggests formation at depth,
although the patchiness of the coarse lamellae may
indicate hydrothermal or deuteric modification, making
them unsuitable for cooling rate determination.
Figure 12e shows a third monzonitic clast with two
analyzed zircons Z5 and Z6, and a fourth baddelyitebearing monzonitic clast (discussed below in Fig. 22)
yields age information from four phases (Humayun
et al. 2013; Bellucci et al. 2015).
104
R. H. Hewins et al.
orthoclase. Tie-lines for exsolved phases in perthite and
antiperthite are also shown. Two unusual clasts are
shown separately. One (7533-5 C) contains both
Ca- and Na-rich plagioclase, suggesting alteration.
Fe-Ti-P-Rich Clasts (FTP)
The second uncommon clast type containing
abundant ilmenite and chlorapatite blades, an Fe-, Ti-,
and P-rich FTP (Agee et al. 2013; Santos et al. 2015a,
2015b; Nyquist et al. 2016), has only albite-rich feldspar
(Fig. 14). However, the analyses of three grains of this
anorthoclase feldspar extend between the most sodic
plagioclase and bulk antiperthite compositions,
resembling a segment of the continuous crystallization
trend from plagioclase to alkali feldspar found in some
alkali basalts (e.g., Keil et al. 1972). The FTP clasts in
our sections are too small and too rare for detailed study.
Crystal Clasts and Rock Types
Fig. 13. Average mineral compositions for noritic and
monzonitic lithic clasts: (a) feldspars; (b) pyroxenes, with ALH
84001 for reference; (c) spinels, both Cr-bearing and Tibearing.
Individual
single-phase
feldspar
analyses
(Table S13) are shown in Fig. 14, along with
representative tie-lines for coexisting plagioclase and
Crystal clasts are angular anhedral grains distinct
from the igneous or metamorphic textured material
containing them. The bigger clasts (up to ~2 mm) are
dominantly crystal clasts (pyroxenes and feldspars, as
well as magnetite-chromite and chlorapatite). Those
analyzed are mainly >100 lm, so there are very few
derived from the finer grained materials. The
compositions of crystal clasts of orthopyroxene,
pigeonite, and augite (Table S14) are shown in the
pyroxene quadrilateral (Fig. 15). Their average FeO/
MnO ratio is 34.5 6. Note that most of the
compositions are like those of the noritic-monzonitic
rocks, but that the orthopyroxene extends to more
magnesian compositions. Orthopyroxene with En80–74
has not been seen attached to plagioclase, although it
may be associated with augite and/or chromite. We
assume such orthopyroxene-dominated clasts represent
orthopyroxenites, and treat them below with crystal
clasts.
ALH
84001
(Mittlefehldt
1994)
has
orthopyroxene compositions falling close to the range of
the breccia pyroxenes not known to coexist with
plagioclase. The orthopyroxene minor element
concentrations of orthopyroxenite ALH 84001 fall in
the center of the clouds for NWA 7533 crystal clast
orthopyroxene, while its augite on the other hand is at
the extremity of the breccia augite range (Fig. S13 in
supporting information), probably because augite is a
minor phase in ALH 84001 and thus contributions of
augite from similar or more magnesian orthopyroxenites
to the polymict breccia would be negligible.
To investigate the question of clast provenance, and
the possibility of pristine clasts, for example, of
orthopyroxenite, we identified the carriers of Ni in the
Martian regolith breccia Northwest Africa 7533
105
Fig. 14. Coexisting feldspars in noritic and monzonitic clasts.
Fig. 15. Major-element compositions of pyroxene crystal
clasts in the breccia.
breccia. The most Ni-rich phase is pyrite with up to 4.5
wt% Ni (Lorand et al. 2015), but this occurs as an
accessory mineral. Spinel (chromite-magnetite) is more
abundant, and is found in the matrix and most clasts.
The more Fe-rich spinels contain ~0.3 wt% NiO
(Hewins et al. 2013a, 2013b), and Cr-rich spinels have
lower (<1000 ppm) Ni contents (Fig. 16a), approaching
those for chromite in ALH 84001 (Gleason et al. 1997).
Ni analyses for silicates are given in Table S15 in
supporting information. The Ni contents of silicate
standards (Jarosewich et al. 1980; Smith et al. 1983;
Barrat et al. 1999; El Goresy et al. 1999; De Hoog et al.
2010) were easily measured at 20 KeV with error bars
(SD of replicates) bracketing literature values for those
with the lowest concentrations, olivine in Eagle Station
and orthopyroxene in Tatahouine (Fig. 16b). The
average compositions of 11 orthopyroxene crystal clasts
(plus one augite and two pigeonites), plus pyroxenes in
lithic clasts, are shown in Fig. 16c. Excluding two high
values (possibly from noritic rocks), the crystal clasts,
mostly
magnesian
orthopyroxene,
have
Ni
concentrations (mean 21 ppm, SD 13) close to the
detection limit and to the Ni content of orthopyroxene
from ALH 84001, consistent with an origin in pristine
crustal rocks. The pyroxenes in plagioclase-bearing
lithic clasts have between 100 and 800 ppm Ni, mean
299, SD 184 (excluding 2 clasts with low values), like
those in the groundmass of clast-laden melt rock.
Inverted pigeonite is common as crystal clasts, but
augite with exsolved Ca-poor pyroxene is less abundant
(Hewins et al. 2013a, 2013b; Humayun et al. 2013).
Lamellae are generally up to about 8 lm wide
(Figs. S14a and S14b in supporting information).
Exceptionally they are ~50 lm wide (Fig. S14c).
Analyses of hosts and lamellae are given in Table S16 in
supporting information. Composition profiles across the
inverted pigeonite and augite of Figs. S14a and S14b
are shown in Figs. 17a and 17b. These are top-hat
profiles, with no concentration gradients. Compositions
of coexisting phases are shown in the pyroxene
quadrilateral and of augite in the Lindsley-Andersen
geothermometer plot (Figs. S15a and S15b in
supporting information). Despite the equilibrium
suggested by the top-hat profiles, the estimated
temperatures range from 700 to 900 °C, possibly due to
excavation of hot rocks.
106
R. H. Hewins et al.
Fig. 17. Exsolution lamellae in pyroxene crystal clasts, NWA
7533-1-9 and -10. a) Augite in inverted pigeonite. b)
Orthopyroxene in augite. See also Fig. S2 and Table S16.
Fig. 16. EMP analyses of Ni-bearing phases. a) Wt% NiO
versus mole% chromite in spinel in matrix and lithic clasts in
NWA 7533. Magnetite-rich spinels are Ni-rich, whereas
chromites approach the low Ni contents of chromite in ALH
84001 (Gleason et al. 1997). b) Analyses of Ni in ppm versus
mole% Fo (or En) match well known olivine and
orthopyroxene (Jarosewich et al. 1980; Smith et al. 1983;
Barrat et al. 1999; El Goresy et al. 1999; De Hoog et al.
2010). c) Many orthopyroxenes have low Ni contents
consistent with a source in pristine rocks like ALH 84001.
Other pyroxenes, including orthopyroxene-augite pairs found
in noritic and monzonitic rocks, have high Ni contents
consistent with crystallization from impact melt.
Feldspar clasts (Table S17 in supporting
information) include plagioclase An54–31, anorthoclase,
K feldspar, plus perthite and antiperthite (Table S18 in
supporting information) and their compositions are
plotted in Fig. S15c. It is striking that the sodic
plagioclase (An<30) of the monzonitic clasts is not
represented in the clasts analyzed. Perhaps there is a
sampling bias because such compositions are
inconspicuous relative to the brighter (higher Z) more
calcic plagioclase and the two-tone perthites. However,
the most calcic plagioclases of the noritic clasts are also
missing, with the modes of the two distributions similar
(Ab50–60), so this may just reflect the low number of
analyses of clast feldspars.
ALTERATION AND VEINS
Pyroxene Alteration
In some pyroxene clasts, in many cases those
showing exsolution, magnetite is observed as small
grains in conjunction with a silica phase. In some cases,
magnetite is abundant, but silica cannot be located (e.g.,
Fig. S14c). This grain shows extremely coarse lamellae,
indicating very slow cooling, and has somewhat kinked
cleavage, fracturing, and very irregular exsolution
lamellae, suggestive of annealing.
Martian regolith breccia Northwest Africa 7533
107
In lithic clasts, a pyroxene phase may show a
striking bimodal composition distribution indicating
disequilibrium in the pyroxene assemblage, as illustrated
in Fig. S16 in supporting information (Table S16). For
example, the orthopyroxene crystals of clast 7533-4 Z1
in Fig. S10a are both high-Z (OF) and low-Z (OM),
respectively OF (En45) and OM (En65) in the figure.
Augite in this clast is speckled with Fe-rich material,
and gives both stoichiometric and nonstoichiometric
(near the large concordant zircon) analyses. Similarly
7533-1-obj D with a distribution of pigeonite
compositions like those in zoned-pyroxene lithic clasts,
shows wispy patches of magnesian pigeonite in ferroan
pigeonite (not zoning). Lithic clast 7533-2-2229 contains
augite, which is more magnesian than expected for the
coexisting orthopyroxene. In 7533-4 clast Z4-Z12 augite
En13Wo40 contains a patch of salitic augite En42Wo49
(Fig. S16). The magnesian pyroxenes are often patchy
or vein-like and appear secondary, possibly as a result
of oxidation causing the Fe-rich speckling. They are
dark and tend to follow cleavage (Fig. S4). They show
low Fe/Mn consistent with oxidation. Similar alteration
of pyroxene is described by Santos et al. (2015a,
fig. 17), and Lorand et al. (2015); and Wittmann et al.
(2015, fig. 14a) describe a discordant vein of salitic
augite En37Fs15Wo48 in orthopyroxene.
Pyrite Veins and Oxyhydroxide
None of the rock types sampled by NWA 7533
contains magmatic sulfides, except for very rare grains
of pyrrhotite totally enclosed in plagioclase. Pyrite is
present, however, and was fractured and subsequently
altered to oxyhydroxide (Fig. 18a). Some crystals of
pyrite that contain mineral inclusions must have grown
when the breccia was incompletely lithified. Finegrained matrix, clast-laden impact melt rocks, melt
spherules, microbasalts, medium grained-lithic clasts,
and crystal clasts are all cut by fractures or veins
containing euhedral pyrite (Lorand et al. 2015).
Euhedral pyrite also occurs as a late mineral in vugs
(Fig. 18b.) S-rich hydrothermal fluids were injected
during lithification of the breccia and after the breccia
became brittle, generating vein systems by hydraulic
fracturing (Lorand et al. 2015). Pyrite veins or fractures
are of late origin but predate an impact event that
shocked pyrite in NWA 7533 (see below).
Pyrite displays mixed signatures, preserving
refractory metals (PGE, Re) as rare micronuggets
mostly inherited from repeated meteorite bombardment
while less resistant refractory metals (Mo and Re) were
redistributed by late alteration (Lorand et al. 2015,
2016). It contains up to 4.5% Ni that, in association
with magnetite, indicates a temperature up to 500 °C,
Fig. 18. BSE images showing (a) replacement of pyrite by
terrestrial Fe-oxyhydroxide, which has spread from shock
planar fractures, and (b) euhedral pyrite in vug in NWA 7533-3.
fO2 > FMQ+2, and near-neutral solutions (Lorand
et al. 2015). Os-Ir-rich particles (200–700 nm) were
found in altered pyrite, converted from the parental
meteoritic phases into As-S rich phases by hydrothermal
sulfurization reactions (Lorand et al. 2015, 2016).
The fracture systems interpreted as a penultimate
event on Mars would also have allowed the passage of
fluids on Earth, which we argue below caused the
partial to complete replacement of pyrite by Fe
oxyhydroxides. There are micro- and nano-scale Fe-rich
phases in the breccia, difficult to identify because of
grain size, although many have been identified by
different techniques. We identified maghemite by XRD,
as did Agee et al. (2013) for NWA 7034, which was
108
R. H. Hewins et al.
confirmed by FTIR (Muttik et al. 2014b). Goethite was
identified in NWA 7034 by M€
ossbauer spectroscopy
(Gattacceca et al. 2014). Beck et al. (2015) identified
ferrihydrite in NWA 7533 by IR spectroscopy. EMP
analyses of the pyrite pseudomorphs show that these
have compositions similar to that of goethite, but
containing ~4% SiO2 (Table S20 in supporting
information). The silica is seen in TEM images as
nanolayers (Lorand et al. 2015). Electron diffraction
shows that the structure of the replacement phase is
compatible with hematite, but other iron-oxides or
oxyhydroxides cannot be excluded (Lorand et al. 2015).
Muttik et al. (2014b) showed the banded structure and
identified the phase as maghemite from HRTEM,
although maghemite normally has low water contents.
If the main phase is anhydrous oxide, the low EMP
totals must be explained by porosity and/or a hydrated
phase, possibly silica as a gel. NanoSIMS imaging of
the Fe oxyhydroxides indicates a terrestrial dD value of
10 85& with respect to SMOW, similar to values for
NWA 7034 (Liu et al. 2014), requiring alteration by
terrestrial water, which explains gradients in pyrite
replacement from fusion crust to the interior of the
meteorite (Lorand et al. 2015). Alteration of pyrite in
the desert would have yielded acidic pore fluid that
could have caused alteration along grain boundaries in
the fine-grained matrix, possibly producing the
amorphous material filling them.
Feldspar and Calcite Veins
The large olivine-free melt spherules are cut by
veins (Figs. 6b and 6c and S5b) but they do not
continue into the material outside the spherules, unlike
those cutting the pyroxene-plagioclase cluster-aureole
structures in clast-laden melt rocks. This alteration
probably occurred before the consolidation of the
breccia. The veins contain two feldspars, intermediate
plagioclase like that in the spherules and Ba-rich K
feldspar (hyalophane) in one case (0–14 mole% Cn,
Fig. 7) and Ba-poor K feldspar in another (0–3 mole%
Cn), in addition to several pyrite crystals. Terrestrial
hyalophane is dominantly a hydrothermal mineral
(McSwiggen et al. 1994).
Calcite veins cut all components of the breccia,
and in some cases calcite was deposited in sulfidefilled fractures, where it is in contact with the
oxyhydroxide replacing pyrite (Lorand et al. 2015).
We show examples in clump-aureole structures and in
an inverted pigeonite clast (Figs. S4 and S14c).
Calcite in NWA 7034 is inferred to have a terrestrial
oxygen isotopic composition (Agee et al. 2013)
although the fractures in the pyroxene clumps appear
to predate the fall to Earth.
Shock Effects
Despite a probable major reheating at 1.35–1.4 Ga,
and the later removal of the breccia from the surface of
Mars, NWA 7533 shows only weak shock effects (after
the impact melting events). These are primarily intense
fracturing of the crystals, particularly of plagioclase.
NWA 7533 lacks two of the most useful shock indicator
phases, olivine, and quartz. Feldspars in some cases
have a crackled appearance resembling perlitic
fracturing, and may be deficient in Na cations.
Maskelynite has not been observed. A TEM
investigation of augite showed (100) mechanical twins
which have accommodated the plastic deformation, and
shear of the exsolution lamellae crossed by the twins,
which can be activated at relatively low shock intensity
(Leroux et al. 2016). Shock markers in pyroxene such as
twinning on (001), mosaicism, planar deformation
features were not observed, although Wittmann et al.
(2015) reported undulatory extinction and mechanical
twinning in some orthopyroxene clasts in NWA 7475.
The most strongly shocked clasts they observed had
reduced birefringence and shock melt veins. The
generally low shock intensity in the breccias is in strong
contrast to that of SNC meteorites (e.g., El Goresy
et al. 2013).
The most distinctive sign of shock is found in
pyrite. This fragile mineral shows planar elements
(fig. 18a in Lorand et al. 2015) resembling planar
deformation features (PDF). Pyrite also shows fracture
networks, and disruption into subgrains. Pyrite
occurring in veins or fractures postdates all rock types
in the breccia, and other late phases appear to be
terrestrial. Pyrite appears to be the last phase to
crystallize in the breccia on Mars, so that the shock
creating its planar fractures was a very late event,
possibly related to the excavation event (Lorand et al.
2015).
DISCUSSION
Classification of the Breccia
There are well-developed classification schemes for
breccias from the Earth and the Moon (St€
offler et al.
1980; French 1998; and particularly St€
offler and Grieve
2007). NWA 7533 contains crystal and lithic clasts
derived from pyroxenitic to monzonitic source rocks,
making the breccia distinctly polymict. The dark melt
bodies up to ~1 cm embedded in its fine matrix are its
most prominent feature (Fig. 2) and have the distinctive
textures of clast-laden impact melt rocks (Hewins et al.
2013a). Similar objects in NWA 7475 were described as
clast-rich amoeboid vitrophyres (Wittmann et al. 2015).
Martian regolith breccia Northwest Africa 7533
Such rocks were recognized in relation to both lunar
and terrestrial cratering shortly after the Apollo
program (Grieve et al. 1974; Simonds 1975; St€
offler and
Grieve 1994) and are widespread in both lunar and
other meteorites (Warren et al. 2005; Hudgins et al.
2007; Schmieder et al. 2016). Broadly speaking, NWA
7533 is a polymict breccia containing melt particles, that
is, compared to terrestrial rocks it is suevite-like: it
consists of fine matrix with melt rock bodies in it
(Hewins et al. 2013b). However, it contains both clastrich and clast-free impact melt rocks, with the latter
sometimes enclosed in the former (e.g., Fig. 4d), as in
“cored bombs.” Thus, the monzonitic and noritic
xenoliths are an older generation, indicating at least two
impact melting events, the former at ~4.43 Ga. This
evidence is consistent with an origin for NWA 7533 as a
regolith breccia (St€
offler and Grieve 2007).
In regolith breccias, surface fines are important and
the matrix in NWA 7533 (Fig. 3) is finely crystalline,
containing submicron grains (Hewins et al. 2013a;
Muttik et al. 2014a; Leroux et al. 2015). Although dust
grains could be part of impact ejecta (Wittmann et al.
2015), Humayun et al. (2013) also proposed on
geochemical grounds that this breccia is derived in part
from wind-blown dust. The presence of vesicular shards
(Fig. 6a) is also consistent with a surficial origin.
Several kinds of melt spherules up to 6 mm in apparent
diameter are present (Figs. 2 and 6), and they are
typical of a surface origin as in regolith breccias
(St€
offler et al. 1980; French 1998), but can be formed
by volcanic fire fountains or deposited as ejecta from
large craters (French 1998). Siderophile element
concentrations in NWA 7533 components require ~5%
chondritic component and confirm an impact origin for
the melt rocks, including the older monzonitic-noritic
clasts, and for at least some of the spherules (Humayun
et al. 2013; Udry et al. 2014). The similarity to Gusev
soils (Humayun et al. 2013) is also consistent with an
origin as a regolith breccia. Rather than a simple
regolith breccia, however, NWA 7034, 7475, and 7533
may be a suevitic breccia formed by recycling of
regolith and bedrock during a large event depositing the
clast-laden melt rock splashes (Wittmann et al. 2015).
Despite previous lack of evidence of impact melts in
SNC meteorites, except as shock veins, inevitably they
must exist on Mars, given the scale of cratering. Impact
melt flows can be recognized at very young craters, like
Pangboche on Olympus Mons (Mouginis-Mark 2015).
Many Mars surface features formed since the late
Hesperian, such as dark dunes, dark layers in crater
floor deposits, and dark streaks, can be interpreted in
terms of impact melt breccias and glassy impact debris
(Schultz and Mustard 2004). For large complex craters,
the long-lived heat particularly associated with impact
109
melt sheets can give rise to hydrothermal systems
(Newsom 1980; Osinski et al. 2001, 2013). Impact
structures are associated with 55% of the occurrences of
hydrous minerals prevalent in the phyllosian alteration
of the PreNoachian (Carter et al. 2013). Although
hydrous minerals are not abundant in the breccia
(Muttik et al. 2014b), there is petrological evidence of
hydrous precursors for spherules and melt rocks
(Humayun et al. 2014a, 2014b, 2014c) and oxygen
isotope evidence of melt-fluid-atmosphere interaction in
the zircons of noritic and monzonitic melt rocks
(Nemchin et al. 2014). The high D17O of the zircons
indicates mass-independent fractionation in the
atmosphere, and due to weathering or hydrothermal
alteration, this signature was incorporated into the
protoliths of the melt rocks which crystallized the
zircons at 4.43 Ga.
Mars regolith breccias should differ from lunar and
meteorite regolith breccias, because of a lack of
gardening by micrometeorite impacts (St€
offler and
Grieve 2007), and of implanted solar wind (Bogard
et al. 1973; Wieler 1998). Indeed, the absence of
agglutinates in NWA 7533 is explained by the filtering
effect of the Martian atmosphere on small projectiles.
Similarly, the Martian surface is shielded from the solar
wind by the Martian atmosphere, so that the absence of
solar wind and the presence of Martian atmospheric
gases in this regolith breccia (Cartwright et al. 2014) is
clear evidence of a Martian origin. There is evidence of
multiple events, spherule deposition, and the
accumulation of a meteoritic component on the surface,
as for the Moon, but also interaction with the
hydrosphere and atmosphere (Humayun et al. 2013;
Nemchin et al. 2014). Assuming NWA 7533 is
representative of the southern hemisphere, it represents
a new kind of regolith breccia unique to early terrestrial
planetary surfaces, providing clues to the nature of
Hadean surfaces on Earth.
Formation of Clast-Laden Melt Rocks
The NWA 7533 breccia was not formed
instantaneously: it is an accumulation of melt rock
bodies and matrix, somewhat younger than the 4.43 Ga
monzonitic clasts they both contain. Abundances of Ir
and other siderophile elements indicate the presence of
~5% projectile component in most clasts, as in lunar
breccias (Humayun et al. 2013). This contamination
includes the monzonitic and microbasaltic fragments
with normal igneous textures, except clasts from pristine
orthopyroxenite (Hewins et al. 2014). The pervasive
siderophile enrichment even in the melt rocks with
pyroxene exsolution apparently formed at depth
suggests a thick sequence of impact-generated rocks
110
R. H. Hewins et al.
including mature regolith. Only the fine-grained clastladen rock bodies have a texture characteristic of
impact melting and some of them have sculpted forms
as in suevitic breccias (Hewins et al. 2013a; Humayun
et al. 2013). The presence of schlieren in impact glasses
shows that melts formed in different locations can be
intimately mixed during excavation (See et al. 1998),
and heterogeneity in the groundmass of the melt rocks
(Figs. S3 and S4) is consistent with this process. The
presence of aligned ellipsoidal structures in a breccia
clast in NWA 7034 section 3A,3 (Santos et al. 2015a;
Figs. 8a and 8b), similar to the clump-aureole structures
described above in clast-laden melt rocks, but deformed,
suggests flow in melt rock before the end of
solidification.
Several origins have been discussed for clast-laden
melt rocks. For coarser grained impact melt rocks,
superheated melt mixes with fractured target rock at the
contact between the melt and the crater footwall. Melt
jetted from the crater overtakes colder fragmental
debris, the incorporation of which partially quenches it
(Grieve et al. 1974; Simonds 1975). Such rocks were
recognized in relation to both lunar and terrestrial
cratering shortly after the Apollo program (Grieve et al.
1974; Simonds 1975; St€
offler and Grieve 1994) and are
widespread in both lunar and other meteorites (Warren
et al. 2005; Hudgins et al. 2007; Schmieder et al. 2016).
Recent detailed investigations of terrestrial craters
(Grieve et al. 2010; St€
offler et al. 2013) have indicated
that suevites are not necessarily fallback or ejecta from
the
initial
plume
but
can
be
generated
phreomagmatically when hydrated rocks and/or water
fall into impact melt pools. Vaporization at depth can
cause explosive fragmentation of melt and mixing of
melt splashes with solid debris. The presence of shards
with concave surfaces (Fig. 6a) indicates vesiculation in
the melt rocks. There are ample indications of the action
of water on the precursors to the melt materials
incorporated into NWA 7533. Thus, these early impacts
were probably on a wet Pre-Noachian terrain with
phyllosian weathering effects. Formation of clast-laden
melt rocks may therefore be due to late contamination
and phreatic eruptions of an impact melt pool. The large
volume of Onaping breccias above the Sudbury Igneous
Complex suggests that seawater reacted with the impact
melt in the crater (Grieve et al. 2010). Evidence of wide
spread occurrence of breccias like NWA 7533 in the
Noachian would have interesting implications for water
abundance, if the suevite analogy is correct.
Clump-Aureole Structures in Clast-Laden Melt Rocks
Although the clast-laden melt rocks are similar in
many ways to those known on other planetary bodies
(Grieve et al. 1974; Simonds 1975; St€
offler and Grieve
1994; Warren et al. 2005; Hudgins et al. 2007; Osinski
et al. 2008), with textures characteristic of melts
formed by impact, their oval somewhat spherulite-like
clump-aureole structures (Figs. 4–5 and S2) are
unique. Other phases in the orthopyroxene clusters
may include chromite (early-forming), fine Fe-oxide
dust, and (late-forming) pyrite. Even in low
magnification, BSE images (Figs. S3f and S4a; Santos
et al. 2015a, fig. 8) these melt rocks may be recognized by
the presence of the dark plagioclase aureoles. Fractures
partially filled with calcite (Figs. 4 and S4c) emanate
from and cross in the clumps, and are at their thickest in
the centers of the clumps suggesting pull apart by
shrinkage as in septarian nodules. The groundmass to the
clump-aureole structures contains plagioclase and more
augite than orthopyroxene. The dominance of plagioclase
laths in the texture of the melt rocks and local subophitic
texture suggests that plagioclase is the equilibrium
liquidus phase. We considered several possibilities for the
origin of orthopyroxene clumps.
Inversion of clasts of a high-pressure phase such as
majorite to pyroxene would explain its concentration,
but in this case textural evidence of expansion rather
than contraction should be present.
Thin reaction coronas of pyroxene grains may be
observed around olivine and quartz clasts in melt rock
and they may also contain plagioclase laths (Hewins
1984) or glass depleted in Fe and Mg (Grieve 1975).
The clumps in NWA 7533 are unlikely to represent
coronas from totally transformed relicts, as olivine is
rare and quartz has not been observed. However, the
plagioclase aureoles in NWA 7533 probably formed in
similar boundary layer liquids depleted by pyroxene
crystallization.
Glass spherules are often encountered in proximal
and distal impact ejecta deposits (French 1998;
Wittmann et al. 2013b). Devitrified glass spherules are
often surrounded by inward-growing fibro-radial
feldspar (Simonson 2003). However, the clumps in
NWA 7533 melt rocks are of different composition and
the feldspar forms laths, not fibers, which like the
pyroxene extend outwards into the melt groundmass
(Figs. 4b and 4c). Aggregates of pyroxene like those in
melt rock clumps occur in shards in the breccia, often
with concave surfaces (Fig. 6a), consistent with
vesiculated melts. The textural similarity suggests a
similar crystallization process in both vesicle-rich and
vesicle-free melt rocks.
In some cases, a large chromite grain in the center
of the clump constitutes a possible nucleation site for
orthopyroxene in a highly supercooled liquid, if
nucleation of plagioclase were inhibited. However, in
many other cases chromite is absent from the plane
Martian regolith breccia Northwest Africa 7533
exposed. Nucleation on unknown sites similar to the
formation of spherulites remains a possibility. The
similarity to textures in irregular fragments, concave
fragments, and the pyroxene-rich vesicular shards
supports this idea. Schlieren in impact glasses show
mixing of melts formed in different locations (See et al.
1998). The heterogeneity in the groundmass (Fig. 5) and
the similarity in pyroxene composition to that in ALH
84001 suggest the presence of melted orthopyroxenite.
Rapid crystallization of these magnesian schlieren,
supercooled by mixing with cold clasts, would be
natural before the growth of plagioclase in the more
aluminous groundmass. This chemical heterogeneity
suggests assimilation by the impact melt with the
clumps forming on unresorbed orthopyroxene relicts,
although overgrowths rather than clusters of grains
would be expected.
Some small aggregates of micron-sized pyroxene are
observed in breccia matrix (Fig. S2d) and some are
traversed by cracks. On early Mars, alteration produced
phyllosilicates including Fe-Mg clays (Bibring et al.
2006) and the phreomagmatic model of suevitic impact
melt rocks (Grieve et al. 2010; St€
offler et al. 2013)
involves addition of water or hydrated material into
melts. Dehydration of altered mafic dust pellets might
have led to shrinkage on heating before or after
incorporation in the melt, and subsequent deposition of
calcite in the fractures. Dust balls of pyroxene or
altered pyroxene might survive better in the impact melt
if dehydration occurred before incorporation. Thus, the
clump-aureole structures may have formed by
spherulitic growth on remnants of pyroxene or hydrated
dust pellets.
Clast Bulk Compositions and Mars Surface
Measurements
We have described clast-laden melt rocks,
microbasalts generally free of clasts, and mediumgrained noritic-monzonitic clasts. The siderophile
element
concentrations
of
all
three
indicate
incorporation of a projectile component (Humayun
et al. 2013). The high Ni in the pyroxenes of the noriticmonzonitic clasts of NWA 7533 is consistent with
crystallization from impact melts, as proposed by
Humayun et al. (2013). Ni from projectiles is not
fractionated out of the silicate, due to high fO2 and low
fS2 in the melt, indicated by the presence of magnetite
and absence of magmatic sulfides (Lorand et al. 2015).
Santos et al. (2015a) have shown that the bulk
compositions of “microbasaltic” clasts in NWA 7034
are mostly basaltic but extend to mugearitic. Majorelement ICP-MS data were acquired on NWA 7533
clasts (Humayun et al. 2013) (Table S1) and we show a
111
diagram of silica vs. total alkalis (Fig. 19a). The finegrained materials plot in the basaltic field, where many
Gusev soils fall and where Agee et al. (2013) showed
that the bulk composition of NWA 7034 (measured on
fine-grained groundmass) plots.
There is partial overlap in composition between our
fine-grained samples, but variation in Al/Si shows that
microbasalts are closer to SNC meteorites than matrix
and clast-laden melt rocks (fig. S4 in Humayun et al.
2013). This is probably due to mixing with feldspar
clasts from the medium-grained noritic-monzonitic
rocks, or similar rocks. The REE data (Fig. 19b)
emphasize the overlap between our generally clast-free
microbasalt and the clast-laden melt rocks, with the
difference between the two being due to the absence or
presence (surviving or digested) of clasts. The
monzonitic rocks are distinctly higher in alkali
(Fig. 19a) and REE (Fig. 19b) contents. In particular,
one monzonitic sample (clast II) falls in the middle of
the mugearite (basaltic trachy-andesite) field, and has a
composition resembling that of sample Jake M. at Gale
Crater (Stolper et al. 2013). Such rocks appear to be
absent at Gusev.
Though the Curiosity rover is especially concerned
with sedimentary rocks at Gale Crater, many rocks
analyzed by ChemCam are highly feldspathic like lithic
clasts in NWA 7533. Of course the rover has
encountered a more diverse suite of rocks, including
olivine-bearing and quartz(?)-bearing rocks not seen in
the breccia, as well as the sedimentary rocks. Here, we
examine the similarities between our monzonitic and
noritic clasts and anhydrous samples analyzed by
ChemCam with H lacking in the laser-induced
breakdown spectra (LIBS). While individual grains can
be imaged and identified by physical properties, laser
data are generally of overlapping grains. The mixing
lines in ratio diagrams can be used to extrapolate
endmember single mineral compositions (Sautter et al.
2014, 2016). In Fig. 20a, we compare monzonitic clast
II with Black Trout sample from Gale Crater (Sautter
et al. 2014). Ideal augite, orthopyroxene, and olivine
plot at 0.5, 1.0, and 2.0 on the (Fe+Mg)/Si axis;
anorthite CaAl2Si2O8 plots at 1.0 and both albite
NaAlSi3O8 and orthoclase KAlSi3O8 plot at 0.33 on the
Al/Si axis. Our electron probe analyses of pyroxenes
and feldspars plot on or near the axes, and can be
identified by stoichiometry. The laser spot analyses, by
ICP-MS (Humayun et al. 2013) and LIBS/ChemCam
(Sautter et al. 2014) are generally mixtures of phases.
For NWA 7533 clast II we also have a laser bulk
analysis performed by rastering. The LIBS data for
Black Trout clearly require the presence of two
feldspars (similar to the intermediate-sodic plagioclase
and alkali feldspar of the breccia) and also suggest the
112
R. H. Hewins et al.
Fig. 19. a) SiO2 – alkalis diagram to show compositions of
clast-laden melt rock, microbasalt, and monzonitic clasts in
NWA 7533. b) REE concentrations of these clasts. Data from
Humayun et al. (2013).
presence of a silica-rich phase such as quartz. In Fig. 20b,
we compare analyses of noritic clasts in the breccia with
sample Mara (Sautter et al. 2014) from Gale Crater.
Here, laser spots can be explained as mixtures of calcic
plagioclase, augite, and orthopyroxene, possibly with a
higher augite/orthopyroxene ratio in Mara than in the
breccia norite. Matrix, melt rock, and microbasalt in
NWA 7533, not shown here, are similarly noritic, but
there the data points are pulled across the plagioclaseorthopyroxene tie-line by the presence of Fe-rich phases,
especially chromite and magnetite.
The zoned-pyroxene clasts resemble enriched
shergottites mineralogically. Although the clasts have
the same composition ranges for pigeonite and augite as
Shergotty, they differ from shergottites (even Los
Angeles) in having abundant K feldspar. The nakhlites
and chassignites are even more distinct in mineralogy.
In addition the pyroxene in 7533-1-12 was analyzed for
Ni (Table S15), and is Ni-rich with 486 ppm Ni, SD
Fig. 20. Comparison of chemical data for rocks at Gale
Crater (Sautter et al. 2014) with breccia noritic and
monzonitic clasts.
229, the pigeonite having more Ni (648 ppm, SD
5 ppm) than the late subcalcic augite. Based on these
data, the zoned-pyroxene rocks are likely to have
incorporated projectile material, and to represent impact
melts, like the other lithic clast types analyzed by LAICPMS (Humayun et al. 2013). Bellucci et al. (2015)
found a U–Pb age for chlorapatite in clast 7533-4-K
(their clast 1) of 1.357 81 Ga. They also used Pb
isotope measurements to show a model age near
4.43 Ga for one plagioclase and near 1.35 Ga for most
feldspar in the same zoned-pyroxene lithic clast. Ody
et al. (2015) report four locations with spectral
signatures (strongly influenced by pyroxenes) matching
basaltic shergottites in Sirenum Planum. Because of the
Noachian age of this terrain, another possible
interpretation of these spectra would be impact melt
rocks rather than basaltic shergottites.
Along with the 4.43 Ga noritic-monzonitic impact
melt clasts, NWA 7533 contains orthopyroxenes with
low Ni contents. Many of these orthopyroxene clasts
Martian regolith breccia Northwest Africa 7533
are Mg-rich with compositions not associated with
plagioclase in lithic clasts. This suggests a pristine
orthopyroxenite crustal component, which resembles
ALH 84001 in major and minor element abundances
(Figs. 15 and S13). Orthopyroxene in NWA 7033 also
has d17O and d18O very close to those of ALH 84001
(Franchi et al. 1999; Ziegler et al. 2013). Pyroxene clasts
are not yet individually dated, although pyroxene
separates in NWA 7034 have a crystallization age close
to that of the zircon (Nyquist et al. 2016). ALH 84001
has an age of 4.1 Ga (Bouvier et al. 2009; Lapen et al.
2010), both indicating early Noachian terrain and
suggesting an origin in the southern hemisphere. Ody
et al. (2015) have examined orthopyroxene-rich terrains
and found that the IR spectra indicate the presence of
plagioclase, that is, noritic rocks, or an association of
orthopyroxenite and norite, as in the breccia. Based on
the clasts in NWA 7533, the noritic rocks might equally
well be melt rocks or breccia as pristine norite.
The History of the Breccia
To understand the earliest history of Mars we must
find the relative ages of major processes, such as the
crystallization of earliest magmatic rocks and the
transition from primary to secondary atmospheres. The
timing of crust formation, the extent of the early heavy
bombardment, and the onset of aqueous alteration have
been uncertain (Carr and Head 2010; Fassett and Head
2011). There are over 40 impact basins with diameters
>400 km in the Noachian (Hartmann 2005). Craters
>150 km in diameter have been dated by crater
counting and about 73% of them formed in the Early
Noachian, that is, as a result of the early heavy
bombardment completing accretion (Tanaka et al.
2014). Most of the formative events in Mars’ history
occurred before the limit where crater counting becomes
a viable dating tool, and has been consigned to the
PreNoachian, that is, buried under the crater-saturated
surface (Frey et al. 2003; Carr and Head 2010; Fassett
and Head 2011; Tanaka and Hartmann 2012). Clearly
impact breccias must be important in this story and the
dating of NWA 7533 and NWA 7034 (Humayun et al.
2013; Nyquist et al. 2016) give us a unique glimpse of
geological processes in this earliest time period.
Crystallization Absolute Ages
The monzonitic fragments are enriched in K, REE,
P, Ti, Cl, and Zr; and contain zircon, and occasionally
baddeleyite, which have provided a crystallization age
for the noritic-monzonitic rocks (Humayun et al. 2013).
Zircon and baddeleyite in one noritic and four
monzonitic fragments, and others occurring as clasts or
xenocrysts in fine-grained matrix and clast-laden melt
113
rocks, were analyzed by SHRIMP. Two dated zircons
Z2 and Z1 are shown in their respective noritic and
monzonitic host in Figs. 12c and S10a, and described
by Humayun et al. (2013, SOM). The morphology and
oscillatory zoning of many such zircons (e.g., Z14, a
matrix grain with a concordant age, Fig. S10d) suggest
that such grains have crystallized during solidification of
these rocks.
The crystallization age of the zircon in noriticmonzonitic clasts in NWA 7533 is 4.428 0.025 Ga
(Humayun et al. 2013) and zircons in NWA 7034 have
similar ages (Tartese et al. 2014; Yin et al. 2014).
Similar crystallization ages were inferred for pyroxene
and plagioclase based on Sm-Nd dating for NWA
7034 (Nyquist et al. 2016) and a common-Pb model
age for plagioclase (Bellucci et al. 2015). For NWA
7533, the hosts to zircon and baddeleyite are matrix
and noritic-monzonitic melt rocks. The least radiogenic
Pb isotopic compositions measured in plagioclase and
K feldspar lie within error of the 4.428 Ga Geochron,
and hosts include monzonitic melt rock and mediumgrained zoned-pyroxene melt rock. We conclude that
the crystallization of medium-grained rocks was at
~4.4 Ga.
Relative Ages of Melt Rocks
The history of the breccia as well as it can currently
be deciphered is outlined in Fig. 21. While an early age
of crystallization is firmly established for the oldest
components at ~4.35–4.43 Ga, the timing of assembly,
and lithification or compaction age of the breccia is not
well understood, at present. Lithification may be related
to heating by initially hot components, compaction by
impacts, and cementation by circulating fluids. We
consider two possible models here.
(1) The lithification age is ~4.4 Ga. One could
expect that the impact record of Mars should include a
broad range of ages between 4.4–3.8 Ga, related to the
late, heavy bombardment, as observed for the lunar
breccias (Grange et al. 2013; Joy et al. 2014). The
number of zircon grains dated is still relatively small, but
no upper intercept age younger than 4.35 Ga has been
observed (Humayun et al. 2013; Tartese et al. 2014; Yin
et al. 2014). One interpretation of this absence of post4.35 Ga zircons is that the breccia was compacted at
~4.3 Ga, and that all subsequent disturbances recorded
by zircons reflect later impacts or other geological events
that affected the assembled breccia. This model implies
rapid assembly and closure within the Pre-Noachian
period. Certain aspects of this model are unsatisfactory,
including that the abundant Fe-oxides are expected to
form in the “siderikian” (Bibring et al. 2006), a time
(<3.5 Ga) overlapping the Amazonian period (<3.0 Ga)
of Martian history. However, the absence of excesses of
114
R. H. Hewins et al.
Fig. 21. Schematic development of the breccia, showing two
impact events slightly separated in time. 1) Regolith (reg) on
primitive crust with felsic rocks (FLC) and orthopyroxenites
(pxt). 2) Noritic-monzonitic impact melt rocks (NO-MO). 3)
Microbasaltic and clast-laden melt rocks (MR).
Cl, S, and Zn in the breccia (Humayun et al. 2013) may
be consistent with a pre-theiikian age (>4.0 Ga) of
lithification or at least alteration.
(2) The lithification age is ~1.4 Ga. Yin et al. (2014)
argued that some of the zircon analyses at the lower
intercept were actually concordant grains that
crystallized from melts at ~1.4 Ga. This led them to
interpret the lithification age of the breccia as ~1.4 Ga.
This model of the assembly time cannot explain the
absence of zircons between 4.35–1.4 Ga, but is more
permissive to sampling a very wide range of Martian
history (~3 Ga), even if not recorded by zircon-forming
events. Resolving these two models is essential to fully
reading the record of ancient Mars from this breccia.
The ages of components such as clast-laden melt
rocks, microbasaltic melt rocks, and spherules are not
strongly constrained like those of the monzonitic melt
rocks.
However,
the
zoned-pyroxene
rocks,
microbasalts, and clast-laden melt rocks differ from
each other and noritic-monzonitic rocks mainly in clast
abundance, grain size, and extent of fractionation. The
microbasalts in part resemble zoned-pyroxene clasts in
mineral compositions (Fig. 11) and the clast-laden rocks
in containing clasts in a few cases (Fig. S6a).
Furthermore, these components appear to have the
same zircon and plagioclase crystallization ages as the
monzonitic clasts (Nemchin et al. 2014; Yin et al. 2014;
Bellucci et al. 2015; Santos et al. 2015a). Specifically, a
clast in NWA 7034 with basaltic trachyandesite
composition (Santos et al. 2015a), which forms part of
the microbasaltic group of clasts, contains concordant
and
discordant
zircons
(4333 38 Ma
and
1434 65 Ma; Yin et al. 2014); zoned-pyroxene clast
7533-4-K has a plagioclase model age near 4.5 Ga with
apatite and K feldspar reset (Bellucci et al. 2015).
However, the fine-grained melt rocks must have formed
in a later impact event than the source rocks of the
noritic and monzonitic melt rock clasts they contain
(Fig. 4b), and the orthopyroxene clasts they contain,
although not necessarily much later. Wittmann et al.
(2015) suggested that the melt rock (“amoeboid clastrich vitrophyres”), spherules, and pulverized target rock
were deposited in a single event, possibly at ~1.4 Ga.
The zircon and feldspar age data cited above suggest
that this event is followed closely after the
crystallization of the medium-grained melt rocks. The
occurrence of both igneous and annealed textures in
microbasaltic clasts shows that some of them were
metamorphosed before incorporation in the breccia.
This suggests burial by hot debris and close formation
in time in a series of impact deposits, the last of which
formed the breccia. The new Sm-Nd dating of bulk
breccia chips and mineral separates (Nyquist et al.
2016) yields an age of 4.42 07 Ga, indistinguishable
from the zircon crystallization ages and showing that all
the rock types making up the breccia formed within a
very short lapse of time.
Martian regolith breccia Northwest Africa 7533
Late Events
Despite the low level of shock in the breccia, there
is evidence of late disturbances of the crystallization
ages of the melt rocks. Imaging of some zircon grains
(Z1 and Z2) reveals variable later degradation of
internal structures as a result of damage induced by
radioactive decay, consistent with discordant ages and
annealing at ~1.7–1.4 Ga (Humayun et al. 2013). Some
zircon U-Pb data for monzonitic clasts fall on a
discordia line with a lower intercept of 1.712 85 Myr
indicating a major subsequent resetting, presumably by
a later impact (Humayun et al. 2013). Re-examination
of the lower intercept for NWA 7533 zircons shows that
of the four zircons that plot near this intercept, the PbPb ages of at least two of these zircon analyses (Z2,
Fig. 21; Z3) yield lower intercepts of ~1.4–1.5 Ga. For
NWA 7034, Yin et al. (2014) and Tartese et al. (2014)
also report zircon resetting at ~1.4–1.5 Ga. The U-Pb
isotopic compositions measured for the chlorapatite in
NWA 7533 (Bellucci et al. 2015) yield a 1.357 81 Ga
age and in NWA 7034 (Yin et al. 2014) a
1.345 47 Ga age. Feldspar Pb-Pb dating by Bellucci
et al. (2015) also shows that some Pb-Pb model ages for
plagioclase in lithic clasts are within error of 4.4 Ga,
but K feldspar and plagioclase also have reset ages close
to the 1.35 Ga Geochron.
Some monzonitic clasts and one zoned-pyroxene
melt rock clast (NWA 7533-4 K) show both preserved
crystallization ages and reset ages in different phases,
demonstrated by data in figs. 3 and 4 of Bellucci et al.
(2015). As an example of this, in Fig. 22 we show a
monzonitic clast with baddeleyite ≥4.298 0.020 Ga,
plagioclase ~4.4 Ga, chlorapatite 1.357 0.081, and K
feldspar ~1.35 Ga ages (Humayun et al. 2013; Bellucci
et al. 2015). This clast 7533-4-Z4 is referred to as clast 2
in Bellucci et al. (2015). Bellucci et al. (2015) proposed
that this strong disturbance indicated by the isotope
data was a thermal event related to impact in the
Amazonian toward 1.35. The partially sintered sub-lm
granoblastic texture (Muttik et al. 2014a; Leroux et al.
2015) is consistent with a moderate annealing intensity,
corresponding to brief heating possibly during the
1.35 Ga event. The role of other disturbances in the
isotopic record, and the ages of specific clast types, may
need to be clarified, but the 1.35 Ga event probably
represents the lithification of the breccia.
The latest breccia event before excavation, during
and after annealing of the matrix, is hydrothermal
activity depositing pyrite in vugs in matrix and in
fractures cutting all facies of the breccia (Lorand et al.
2015). Pyrite precipitation in these oxidized rocks
required T up to 400–500 °C and near-neutral rather
than acidic solutions, possible because of the absence of
magmatic sulfides in the breccia components and/or
115
Fig. 22. Monzonitic clast 7533-4-Z4 (BSE) showing two distinct
U-Pb and Pb-Pb ages recorded in four phases, baddeleyite (B),
plagioclase (P), K feldspar (K), and chlorapatite (C). Data from
Humayun et al. (2013) and (as “clast 2”) from Bellucci et al.
(2015).
their protoliths (Lorand et al. 2015). The Martian origin
of the pyrite is demonstrated by its fracturing caused by
shock (Lorand et al. 2015), possibly during the launch
event. Wittmann et al. (2015) using Fe and Th
abundances, and the ~5 Ma exposure age of Cartwright
et al. (2014), suggest one possible launch site for the
breccias, the young rayed crater Gratteri. This crater
has also been suggested as the source of ALH 84001
(Tornabene et al. 2006). NanoSIMS imaging of pyrite
grains shows that Fe oxyhydroxides replacing them
along their fractures exhibit a homogeneous terrestrial
D/H ratio (Lorand et al. 2015). The average dD value
(10 85&) indistinguishable from the terrestrial values,
and calcite veins with terrestrial oxygen isotopic
compositions (Agee et al. 2013), record the breccia’s
relatively brief residence (typically thousands of years
for desert meteorites) on Earth.
Implications for Conditions on Mars
Radiometric dating has yielded crystallization ages
of 4.43–4.35 Ga for all rock types in the breccia
(Humayun et al. 2013; Nyquist et al. 2016). Thus, a
crustal reservoir containing debris from pristine igneous
rocks heavily contaminated by projectiles was in place
on Mars at 4.43 Ga, a little over 100 Ma after its
116
R. H. Hewins et al.
accretion. Debaille et al. (2007) showed that combined
142
Nd-143Nd isotope evidence in shergottites implies
mixing between two distinct source reservoirs formed at
4.535 Gyr and 4.457 Gyr ago and argued that the
younger reservoir was residual melt from a magma
ocean, rather than ancient crust. However, the
solidification of any magma ocean is thought to have
occurred within 5 to 10 Ma after Martian accretion
(Elkins-Tanton 2012), a much shorter time frame than
the 30–110 Ma required by Debaille et al. (2007). The
early crust rather than a late magma ocean reservoir is
available to contaminate and enrich shergottite magmas
emplaced through it. The Martian Nd isotope
systematics (Debaille et al. 2007) can be explained by
formation of this crust and depleted mantle, without the
need for a magma ocean (Agee et al. 2013; Humayun
et al. 2013). In that case, H2O and CO2 would have
been outgassed continuously during early crust
formation to form the (Pre-) Noachian atmosphere,
allowing clement surface conditions (Nemchin et al.
2014). In this period of the most intense basin
formation, hydrothermal activity would have been
important, and could have promoted an extensive
aquifer even if surface material was frozen (Pope et al.
2006; Abramov and Mojzsis 2014).
Oxygen Isotope Data
An orthopyroxene mineral separate (Ziegler et al.
2013), likely to be Ni-poor orthopyroxene (Hewins
et al. 2014) from pristine orthopyroxenites like ALH
84001 (Mittlefehldt 1994), exhibited D17O of 0.33&,
indistinguishable from SNC compositions, and reflecting
mantle isotopic composition. There is an overall trend
of increasing D17O with increasing d18O in NWA 7034
separates, toward the composition of known Martian
alteration phases. These D17O values were attributed to
photochemically induced isotope fractionation and
transfer of the atmospheric signal to rocks by surface
alteration (Farquhar and Johnston 2008). The Amazonian
and younger Martian atmosphere has been proposed to
have had a high D17O value of >0.6& (Farquhar and
Thiemens 2000; Ziegler et al. 2013; Shaheen et al. 2015).
The similar O isotopic compositions of ALH 84001 and
SNC alteration phases suggest that exchange with the
atmospheric reservoir occurred through most of Martian
history (Farquhar and Thiemens 2000).
The zircon crystals in NWA 7533 dated by the UPb method were subsequently analyzed for O isotopes
(Nemchin et al. 2014). Different zircon grains show a
difference in D17O values, many higher than the SNC
values defining the Mars fractionation line. Some are
also higher than bulk NWA 7034 and magnetite
separate data (Ziegler et al. 2013), and include the
largest D17O reported from a Martian meteorite
(Nemchin et al. 2014). The zircons probably crystallized
from melts that had incorporated surface material with
the
atmosphere-hydrosphere
isotopic
signature,
although are also partly altered (the most extreme D17O
values being found in metamict portions of the zircons
with an age of ~1.4 Ga). The excess of noritemonzonite zircon D17O values over SNC values
(Nemchin et al. 2014) implies that the atmosphere was
thin enough to create reactive 17O species before impact
melting and zircon crystallization, and hydrated crust
was already present at 4.43 Ga (Humayun et al. 2013;
Nemchin et al. 2014).
Early Aqueous Alteration
All crystallization ages for clasts in NWA 7533 date
from the earliest Noachian (PreNoachian) characterized
by phyllosilicate alteration in remote sensing. Newsom
(1980) argued for the production of clays as well as
ferric hydroxides by hydrothermal alteration associated
with Martian impact melt sheets.
Geochemical evidence that the precursors of the
monzonites and some spherules were altered and clayrich (Humayun et al. 2013, 2014a, 2014b, 2014c) is
consistent with the zircon record of melt-hydrosphereatmosphere interaction (Nemchin et al. 2014). Humayun
et al. (2014c) have shown that spherules, although
themselves little altered, have alkali element ratios
(enrichments of K, Rb, Cs, and Tl and depletion in Na)
characteristic of terrestrial clay minerals and consistent
with a weathered source. Vesicular shards (Fig. 6a) also
indicate the presence of volatiles. The bulk compositions
(Humayun et al. 2013) also indicate degassing or
leaching of S from the precursors and the melt rocks
lack magmatic sulfides in consequence (Lorand et al.
2015). Thus, phyllosian conditions (Bibring et al. 2006)
are indicated before 4.43 Ga. However, there is no clear
evidence that phyllosilicates formed in the assembled
breccia in the Noachian, for although NWA 7034 does
contain clays, they occur as nanogranoblastic crystals in
the annealed matrix (Muttik et al. 2014a). Their
Martian origin is based on the observation that water
from NWA 7034 is Martian in oxygen isotope
composition (Agee et al. 2013). Possibly these are
annealed Pre-Noachian clay dust grains, but their time
of formation is not well defined.
Fractional crystallization of a Martian basalt
magma like those in Fig. 19 does not yield evolved
rocks like the zircon-bearing noritic-monzonitic clasts.
The various clast types in the breccia may be derived by
melting different Pre-Noachian protoliths with different
degrees of alteration or sedimentary geochemical
characteristics, rather than by fractionation of thick
melt sheets. If all the iron is ferrous, olivine will
crystallize instead of orthopyroxene and magnetite. The
Martian regolith breccia Northwest Africa 7533
magnetite-ilmenite assemblages in these rocks indicate
oxygen fugacities at FMQ+2–3 log units (see below and
Table S9) and show that the melt and therefore the
protolith were highly oxidized. This oxidation,
accomplished by weathering or alteration of olivine in
precursors to serpentine + magnetite, explains the
crystallization of pyroxene and magnetite in its stead in
the noritic-monzonitic melt rocks (Humayun et al.
2014a, 2014b), although the high alkalis and high K/Na
of the monzonitic rocks remain a problem in modeling.
NWA 7533 could possibly record global climate
changes during the Noachian, Hesperian, and
Amazonian periods with different weathering styles
(phyllosian, theiikian, and siderikian; Bibring et al. 2006).
However, the breccia does not contain the weathering
sequence phyllosilicate/sulfate/anhydrous oxide and this
suggests that local environmental effects might have
countered the global-scale patterns. Murchie et al. (2009)
describe both phyllosilicate-rich and silica-, sulfatebearing layered deposits deposited on ancient cratered
terrain. These deposits may both have formed from
volcanic and impact glasses, but the phyllosilicates would
have formed by abundant neutral fluid alteration, and the
sulfates from more acid solutions. Thus, the general
climatic transition may depend on local environmental
conditions, due to variations in volcanic emissions and
availability of S and/or water (Murchie et al. 2009).
Either magmatic sulfide was virtually absent in NWA
7533 (Lorand et al. 2015) or sufficient water was present
to remove S along with Cl and Zn in solution (Humayun
et al. 2013). In addition, there is no record of the
Hesperian (theiikian alteration style) in the dating. Fluid
compositions and alteration phases may vary locally, but
as similar breccias are probably widespread in the
southern hemisphere this may also be a regional effect.
Oxidation and Magnetics
The magnetite-ilmenite temperature assemblage in
lithic clasts in the breccia indicates a high fO2 of IW+2
to IW+3 log units at ~750 °C (Table S9). Pyroxene has
been oxidized to magnetite plus silica (Leroux et al.
2016). Alteration of NWA 7533 lithic clasts includes
disequilibrium pyroxene assemblages probably related
to oxidation, and pyroxene decomposition into Mg-rich
pyroxene, silica, and Fe-oxide (Leroux et al. 2015). This
reaction requires a high temperature and a high oxygen
fugacity, at least 2–3 log units above FMQ, consistent
with a water-rich environment. Late-forming pyrite
veins also formed at a minimum log fO2 of >FMQ+2 at
400–500 °C (Lorand et al. 2016) from near neutral
(6 < pH<10),
H2S-HS-rich
fluids.
Hydrothermal
alteration under oxidizing conditions appears to have
given rise to fine-grained Fe-rich phases influencing the
magnetic properties of the breccia.
117
Alteration of spherules and melt rock clasts consists
of a fine speckling with high-Z Fe-rich material, which is
difficult to characterize in situ, and different techniques
have identified different phases. The micro- and nanoscale Fe-rich phases indicated include maghemite,
hematite, goethite, and ferrihydrite, a complication being
the terrestrial alteration of pyrite. A poorly crystalline
Fe–OH bearing mineral gives the best match to the IR
spectrum of the breccia. IR and XANES measurements
of NWA 7533 show that a strong 3 lm absorption band
is due to Fe3+-OH, and corresponds to one observed in
Mars surface spectra particularly of the dark areas (Beck
et al. 2015). Although there are also terrestrial
oxyhydroxides in the breccia these are confined to
specific areas corresponding to former pyrite crystals
and there must be a widespread occurrence of a phase
like ferrihydrite on Mars. Abundant fine-grained
magnetite and maghemite explain the breccia’s magnetic
properties especially the high saturation remanence
(Gattacceca et al. 2014).
Parts of the Noachian terrain display anomalies in
crustal remanent magnetization reported by Acu~
na
et al. (1999). This could be explained if the crust were
constructed of certain SNC meteorites, but implausible
thicknesses would be required, whereas only ~1 km of
breccia could cause the observed remanent magnetism
(Gattacceca et al. 2014). This is consistent with
formation of the NWA 7533 breccia in the southern
hemisphere while the Martian dynamo was active, that
is, in the Noachian (Rochette et al. 2013; Gattacceca
et al. 2014). Magnetic anomalies are restricted spatially
but the 3 lm absorption indicating dust rich in Fe3+ is
widespread on Mars. The source of the hydrated and
oxidized dust covering may be the ancient breccias in
the heavily cratered southern highlands eroded and
transported by wind (Beck et al. 2015). The similar
compositions of fine-grained materials in NWA 7533
and the soils of Gusev Crater (Humayun et al. 2013) is
in accord with this idea.
CONCLUSIONS
NWA 7533 is a polymict Martian breccia paired
with NWA 7034, 7475, 7906, 7907, 8114, and 8171. The
abundance of fine-grained clast-laden melt rock bodies,
spherules and shards, and their high siderophile element
contents is consistent with classification as a regolith
breccia. NWA 7533 is essentially a deposit of cogenetic
melt particles differing texturally and chemically as a
function of clast abundance. In addition, the breccia
contains a (slightly) earlier generation of mediumgrained noritic and monzonitic melt rocks. There is a
paucity of material that can be interpreted as pristine
crustal rock, although the low-Ni orthopyroxene clasts
118
R. H. Hewins et al.
are interpreted as pristine orthopyroxenite of a deepcrustal origin similar to ALH 84001, a conclusion
consistent with their low siderophile element
abundances. The crystallization of the monzoniticnoritic, zoned-pyroxene, and microbasaltic melt rocks
occurred
at
~4.43 Ga.
Then
orthopyroxenites
crystallized in the Martian crust from before 4.43 Ga at
least until 4.09 Ga (Lapen et al. 2010). The noritic and
monzonitic clasts are essentially two-pyroxene and twofeldspar rocks, respectively, but can also be
distinguished by plagioclase and spinel compositions.
Exsolution in this clast suite pyroxene is coarser than in
SNC meteorites, suggesting a thick sequence of impact
melt rocks. The nanocrystalline matrix of the breccia
was partly annealed, possibly in the ~1.35 Ga event
disturbing K feldspar, chlorapatite, and zircon ages, but
it preserves some porosity. Matrix and clasts are cut by
pyrite veins, probably caused by this heating event.
Oxygen isotopic analyses of zircon grains and the
geochemistry of the fine-grained rocks show that the
protoliths interacted with atmosphere-hydrosphere
before melting. These breccias document the formation
of evolved crust on Mars with a wet surface
environment before 4.43 Ga.
Acknowledgments—We are indebted to L. Labenne, J.
Gattacceca, and A. Jambon, for sample and insights,
oxygen isotope measurements, and Fe/Mn ratios,
respectively; to A. Wittmann for discussions; to D. W.
Mittlefehldt, J. G. Spray, and associate editor G. K.
Benedix for thorough reviews which led to much
improvement of the manuscript; to M. Fialin for help
with the electron probe and particularly with Ni
analyses; to M. J. Carr for IGPET; to L. Lepage for
ILMAT; and to D. Troadec for the preparation of FIB
sections. We are grateful for funding from NASA
Cosmochemistry grant NNX13AI06G (M. H.), CNESINSU grant 2013-PNP (B. Z.) and grant 2014-PNP (J.P. L.). The National NanoSIMS Facility at the
Museum National d’Histoire Naturelle was established
by funds from the CNRS, Region ^Ile de France,
Ministere delegue a l’Enseignement superieur et a la
Recherche, and the Museum itself.
Editorial Handling—Dr. Gretchen Benedix
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SUPPORTING INFORMATION
Additional supporting information may be found in
the online version of this article:
Fig. S1: Sawed surface of a 2.5 cm wide slab of
NWA 7533 with two melt spherules (black) and dark
bodies of clast-laden melt rock; feldspathic clasts are
light colored.
Fig. S2: Breccia matrix. a) Low magnification BSE
view, with many crystal clasts (plagioclase dark gray,
pyroxene medium gray). b) High magnification BSE
image, with light orthopyroxene embedded in dark
plagioclase. c) TEM image showing granoblastic texture
with porosity on grain boundaries. d) BSE. Micronsized medium gray orthopyroxene grains in a cluster
like a dustball.
Fig. S3: Clast-laden melt rock BSE images. a)
somewhat rounded 40 lm clasts in subophitic finegrained groundmass; (b) less fine- grained, fewer clasts;
(c, d) Mg and Al Ka X-ray maps complementary to
Fig. 4c showing pyroxene and plagioclase, light/dark and
dark/light, respectively. These images show the clusters
and aureoles very clearly, because of the enhanced
contrast between pyroxene and plagioclase relative to
BSE. In Mg Ka, the top half is seen to be an Mg-rich
groundmass with radial orthopyroxene in the clumps. In
the Al image, the groundmass laths are seen to be
skeletal-dendritic and the aureole plagioclase is radial; the
plagioclase lath abundance defines the schlieren. e)
Orthopyroxene granules/dendrites in clump and (arrows)
fractures partly filled with calcite (f) elongated and large
irregular orthopyroxene clumps in 7533-5, suggesting
flow. The large amoeboid clump has an aureole that is
discontinuous on its borders and partially in its interior,
suggesting fragmentation. Compare to Santos et al.
(2015a, 2015b; Figs. 8a and 8b).
Fig. S4: a) Detail of NWA 7533-1 showing crescentshaped clast-laden melt rock body (MR), with crystal
clasts, several clump-aureole structures (small dark
ellipses), and also microbasalt (MB) and spherule (S). b)
Red-green-blue coded X-ray (Mg, Al, Si, Ka) image of the
clump-aureole structures (orthopyroxene clusters with
plagioclase borders) in melt rock in Fig. 4c: orthopyroxene
purple, augite dark purple (with orthopyroxene rims),
plagioclase light blue; Fe-oxides and crack-filling calcite
are black. Plagioclase-rich and pyroxene-rich schlieren are
evident. c) Ca Ka map shows white calcite in fractures.
123
Ziegler K., Sharp Z. D., and Agee C. B. 2013. The unique
NWA 7034 Martian meteorite: Evidence for multiple
oxygen isotope reservoirs (abstract). 44th Lunar and
Planetary Science Conference. CD-ROM.
Fig. S5: BSE images of shards and spherules. a)
Triangular shard with orthopyroxene surrounded by
plagioclase in breccia matrix. b) Detail of large aphanitic
spherule in NWA 7533-7 with feldspar veins and coarse
crystals near the periphery. c, d) Small spherules
containing pyroxene, plagioclase, chromite, and magnetite.
Spherules (c) in 7533-LM and (d) in 7533-5 resemble
orthopyroxene clumps.
Fig. S6: a) Microbasaltic clast in 7533-6, with two
small lithic clasts near its center. b) Microbasaltic clast
7533-7-4, containing potash feldspar (K), as well as
orthopyroxene (O), augite (A), and plagioclase.
Fig. S7: a–f) Pyroxene compositions for microbasalt.
a) One clast E in 7533-4 with primary orthopyroxene only,
and one with pigeonite only, 7533-4-L. Both phases have
compositions extending to more Fe-rich values
(En69–55Wo~3 and En66–49Wo~10) than for clast-laden melt
rocks. b) In granoblastic clast 7533-1-obj2 (Fig. 8c) fine
mixtures of orthopyroxene and augite are present with little
variation in Fe/Mg ratio. Magnesian orthopyroxene
En63.70.6Fs32.00.7Wo4.30.2 is less abundant than more
ferroan
pyroxene
whose
average
composition
En52.72.5Fs38.63.5Wo8.85.4 indicates that primary
pigeonite was originally present. Fe-Mg fractionation is
recorded in the orthopyroxene, En67–53Wo~3. c, d) A
crystallization sequence of orthopyroxene followed by
pigeonite, and/or augite. e) The fine-grained clast 2-84
(which sits at the bottom of the sedimentary clast in
Fig. 8d) has zoned pigeonite and augite En66.4–51.9
Fs28.8–37.8Wo6.3–14.2 and augite En49.3–40.5Fs17.8–22.2
Wo28.1–32.0, and is discussed below along with similar
medium-grained rocks, that also have zoned pyroxenes like
those of shergottites.
Fig. S8: BSE image of sedimentary clast in 7533-2.
Border (white) established by locating the characteristic
texture of breccia matrix using a 1.2 Gpix FEG SEM image
of true dimensions ~6 9 9 m. Noritic clast 2-1521 at top
and microbasaltic clast 2–84 at bottom are attached to the
sedimentary layers.
Fig. S9: Individual pyroxene quadrilaterals for four
small zoned-pyroxene lithic clasts and one microbasalt
(7533-2:84), showing pigeonite with or without augite or
subcalcic
augite
(clast
size
effect).
Fig. S10: BSE images. a) Noritic clast 7533-4-C with
magnesian (OM) and ferroan (OF) orthopyroxene, augite
(A), plagioclase (P), and Cr-bearing magnetite (M). Z is
zircon Z1, which partially retains a concordant U-Pb age
124
R. H. Hewins et al.
despite dark speckled pyroxene alteration in the clast. b)
Noritic clast 7533-4-C containing orthopyroxene (opx),
augite (aug, with opx lamellae), plagioclase, and Cr-bearing
magnetite. c) Noritic clast 2-nor7479X; Merrillite (Me)
overgrown by chlorapatite (Ap); similar two-phosphate
clasts are found in breccia matrix. Lacy monazite (Mo) in
chlorapatite. Pigeonite (pig) with augite (exsolved and as
alteration, associated with pyrite) and chromite. d)
Igneously zoned zircon clast Z14 in breccia matrix (black)
has concordant U-Pb age (Humayun et al. 2013).
Fig. S11: a) The coexisting pyroxenes of several noritic
clasts (Table S14), including those in Figs. 12b, S10a, and
S10b, are plotted. b) The augites after recalculation with
IGPET for the Lindsley and Andersen (1983)
geothermometer. We show separately clast NWA 7533-4-C
(Fig. S10a) where the large augite, unlike the small
orthopyroxene granules, is unmixed to more calcic augite
and orthopyroxene lamellae. This thermometer is not
rigorously applicable for moderate amounts of
nonquadrilateral components and there is a risk of
underestimating the Ca content of thin augite lamellae.
Nevertheless clast C shows the expected behavior where
igneous equilibration is about 300 °C higher than
equilibration within the unmixed augite. However, most of
the augite grains coexisting with orthopyroxene in the other
clasts equilibrated at temperatures of 800–900 °C, little
different from that of the augite lamellae in clast C.
Exsolution in pyroxene crystal clasts is discussed below.
Fig. S12: Compositions of individual analyses of (a)
feldspars and (b) pyroxenes in noritic and monzonitic
clasts. Compositions for clasts of intermediate properties
are
not
shown,
for
clarity.
Fig. S13: Minor elements in pyroxene crystal clasts in
NWA 7533.
Fig. S14: Clasts with exsolution. a) Orthopyroxene
with augite lamellae (BSE), 7533-1 -10. b) Augite with
orthopyroxene lamellae (BSE), 7533-1-9. c) Coarsely
exsolved pyroxene 7533-7-MH3. X-ray map Mg red, Fe
blue, Ca green. Note tiny magnetite granules in host
orthopyroxene (salmon), coarser magnetite (blue), irregular
augite lamellae (greenish), and calcite veins (green). The
dark
lines are
relics of LA-ICPMS
work.
Fig. S15: Crystal clasts; (a) compositions of host and
lamellae in pyroxenes, with (b) temperature estimates after
Lindsley and Andersen (1983), and (c) compositions of
feldspar crystal clasts.
Fig. S16. Examples of disequilibrium pyroxene
assemblages due to oxidative alteration forming magnesian
pyroxene. These are the clasts from 7533-4 shown in
Fig. s10a and Fig. 22, plus 1-objD and 2-2229.
Table S1: ATEM analyses (atom%) of pyroxenes
and feldspars in fine-grained breccia matrix.
Table S2: Analyses of pyroxenes in clumps and
groundmass of clast-laden melt rocks, and shards.
Table S3: Analyses of feldspars in aureoles to clumps
and groundmass of clast-laden melt rocks, and shards.
Table S4: Analyses of olivine and pyroxene in spherules.
Table S5: Analyses of feldspars in spherules and veins in
spherules.
Table S6: Analyses of spinel, ilmenite, and maghemite
in lithic and crystal clasts.
Table S7: Analyses of pyroxenes in microbasaltic (melt
rock) clasts.
Table S8: Analyses of feldpars in microbasaltic (melt
rock) clasts.
Table S9: Magnetite-ilmenite geothermobarometry.
Table S10: Analyses of pigeonite and augite in lithic
clasts with zoned pyroxenes.
Table S11: Analyses of feldspars in lithic clasts with
zoned pyroxenes.
Table S12: Analyses of pyroxenes in noritic,
monzonitic, and intermediate lithic clasts.
Table S13: Analyses of feldspars in noritic, monzonitic,
and intermediate lithic clasts.
Table S14: Analyses of pyroxene crystal clasts.
Table S15: Pyroxenes and controls analyzed for Ni by
EMP at 20 kV and 300 nA.
Table S16: Coexisting pyroxenes in equilibrated twopyroxene rocks, in exsolved crystals, and in disequilibrium
assemblages.
Table S17: Analyses of feldspar crystal clasts.
Table S18: Analyses of perthitic feldspar crystal clasts.
Table S19: Analyses of phosphates in lithic and crystal
clasts.
Table S20: Analyses of oxyhydroxide in lithic and
crystal clasts.

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