Geochemistry of Impactites - Vrije Universiteit Brussel

Geochemistry of Impactites
Suevitic polymict
breccia from the
Bosumtwi impact crater
in Ghana, showing a
variety of rock
fragments; the foamy
inclusions are impact
glass that may carry the
geochemical signature
of the impactor. Sample
is ca 10 cm wide.
Christian Koeberl1, Philippe Claeys2, Lutz Hecht3,
and Iain McDonald4
1811-5209/12/0008-0037$2.50 DOI: 10.2113/gselements.8.1.37
G
eochemical analysis is an essential tool for the confirmation and study
of impact structures and the characterization of the various rock types
involved (target rocks, impact breccias, melt rocks, etc.). Concentrations
and interelement ratios of the platinum-group elements, as well as the
osmium and chromium isotope systems, allow quantification of extraterrestrial components and the identification of impactor types in impact deposits.
In addition, chemolithostratigraphy can reveal the possible role of impacts
in environmental change throughout the geologic record. This article deals
predominantly with terrestrial impact structures.
Keywords : impacts, ejecta, geochemistry, platinum-group elements,
chromium isotopes
proximal ejecta, melt rocks, and
pseudotachylitic breccias, the
latter being dikes of melt rock at
the bottom of an impact structure;
we also consider a few examples of
distal ejecta. Impact processes
produce brecciation, shock metamorphism, and melting and
vaporization of the target rocks.
The chemical composition of
impactites provides important
information that supplements petrological data. It depends
on (1) the composition and spatial distribution of the target
lithologies; (2) impact energy, which affects the size of the
crater, the depth of material involved, and the volume of
rocks vaporized or melted; (3) the emplacement and
cooling history of impactites; (4) the admixture of projectile material; and (5) post-impact modifications by metamorphism and/or hydrous alteration (including
weathering).
INTRODUCTION
The geochemistry and cosmochemistry of impact craters
and impact processes constitute a rapidly developing
research area encompassing such wide-ranging topics as
the chemical characterization of rock types; the formation,
emplacement, and differentiation of impactites as revealed
by petrologic studies; the identification of extraterrestrial
components in impact ejecta and crater fills; the derivation
of the impactor (projectile) composition; and the determination of the causes of environmental change from analyses of samples in the stratigraphic record. One of the most
important roles of geochemistry in impact studies is the
confirmation of the impact origin for terrestrial structures
(see French and Koeberl 2010). If an impact structure is
buried, drill core samples are essential. Breccias and melt
rocks often carry unambiguous evidence for the impact
origin of a structure, such as the presence of shocked
mineral and lithic clasts or contamination from the extraterrestrial projectile (for more details see Koeberl 2007).
The chemical compositions of rock types at a crater or
distal sites can also be used to determine whether any
unusual or extraneous (noncrustal) components are present
and to determine the origin of impact glasses. Once all
rock types present at a particular impact site are analyzed,
mixing calculations allow the reconstruction of the proportions of the different rock types that combined to form
breccias or melt rocks. Such data have been used to establish scaling relationships for the impact process (for
example, the geometry of melt zones, the volume of melt
produced, and the sizes of craters). Also, these calculations
are important for defining the indigenous contents of
siderophile elements in breccias and melt rocks, which are
essential for establishing the extent of contamination by
extraterrestrial components.
FROM CHEMISTRY TO PETROLOGY
General Chemistry of Impactites
The term impactite comprises a large variety of rocks formed
by the modification of crustal rocks due to impact processes.
Here we focus on the chemistry of impactites that have
been formed or deposited at or close to the crater (i.e.
In geochemical work, proper sampling and sample preparation are crucial and depend on the analyses to be done.
Each preparation and treatment step increases the chance
of contamination or loss, and a compromise must be
reached between available sample mass and what constitutes a representative sample (details in chapter 6 of
Montanari and Koeberl 2000). In the study of impactoclastic layers (distal ejecta), this problem is even more
severe, because of the low abundance of impact-derived
debris within a large amount of local matrix.
1 Department of Lithospheric Research, University of Vienna
1090 Vienna, Austria, and Natural History Museum
1010 Vienna, Austria
E-mail: [email protected]
2 Earth Systems Science, Vrije Universiteit Brussel
Pleinlaan 2, 1050 Brussels, Belgium
3 Natural History Museum Berlin
Invalidenstrasse 43, 10115 Berlin, Germany
4 School of Earth & Ocean Sciences, Cardiff University
Cardiff, CF10 3AT, UK
E lements , V ol . 8,
pp.
37–42
37
F ebruary 2012
Differentiation and Emplacement
of Impact Melt
obvious mantle component, as was later confirmed by Os
isotope studies. Early, rapidly cooled dikes of impact melt
that were emplaced into the crater floor may preserve the
composition of the initial impact melt at Sudbury (Hecht
et al. 2008a) and at the Vredefort impact structure
(South Africa).
The composition of the resulting bulk impact melt depends
on the efficiency of mixing between the individual coexisting melts. In general, the resulting impact melt is homogeneous at hand-specimen scale (Dressler and Reimold
2001), indicating that melt mixing during impact dynamics
or during post-impact convection within thicker melt
sheets is very efficient. In the case of the 200 km wide,
1.86-billion-year-old Sudbury impact structure in Canada,
however, Zieg and Marsh (2005) proposed that superheated
melts derived from siliceous and mafic target rocks were
so different in density and viscosity that they did not mix
but remained separate, forming a layered melt sheet (siliceous top layer and mafic bottom layer). The Sudbury
impact melt sheet was differentiated, resulting in the
formation of a crudely layered complex comprising granophyre and norite layers (Fig. 1), along with major Ni–Cu–
PGE deposits. Fractional crystallization and host-rock
assimilation were involved in the differentiation of the
impact melt sheet and were probably enhanced by its
cooling history, leading to significant post-impact heterogeneity of the melt body.
The heterogeneity of impact glasses is increased by incomplete melting, incomplete assimilation of rock or mineral
fragments, and rapid cooling. These factors are mostly
relevant for melt clasts in suevite and for impact melt
bodies in small craters where a melt pool large enough to
allow homogenization has not developed. Rapid disequilibrium crystallization in quenched impact melt may also
induce small-scale heterogeneity (Hecht et al. 2008b).
Post-impact Modifications of Impactites
Metamorphism and/or hydrothermal alteration are facilitated by the porosity of impact breccias and the sensitivity
of impact glass to alteration. Post-impact metamorphism
causes recrystallization but does not necessarily change
the composition of impactites. Impact-induced heating can
produce hydrothermal systems that may significantly
modify the composition of impactites (Naumov 2002).
The isotope systems Rb–Sr and Sm–Nd can be used to date
impact events, and also to confirm that impact melt rocks
were derived from near-surface crustal rocks and not from
the deep crust or mantle. For example, the Nd isotope
composition of Sudbury melt rocks shows that the target
rocks were predominantly crustal rocks without any
Need for Future Work
Our understanding of processes such as the formation and
emplacement of impact melt and the mechanisms of
mixing between impactites and meteoritic material is still
incomplete. Therefore, petrological and geochemical
studies of impact structures, combined with numerical
modeling and laboratory experiments, are very important.
Hypervelocity impact experiments may also help to better
understand the extreme, short-term dynamics of far-fromequilibrium impact processes.
K2O
> 10 km ∅
< 10 km ∅
II
SIC-G
HB
UCC
PROJECTILE IDENTIFICATION
CC
Siderophile Element Studies
Although projectile fragments rarely survive an impact
event, detectable amounts of melted and recondensed
projectile are often incorporated into impact-produced
breccias and melt rocks during crater formation. This
dispersed projectile (meteoritic) material can be conclusively identified by distinct chemical and isotopic signatures in the host rocks, thus providing reliable evidence
for a meteorite impact event.
BR
EG
RK
WP
RS
IM-avg
PG
MK
VF
SIC-QD
SIC
CX
During impact, original projectile material is diluted by
mixing with a volume of vaporized, melted, and fragmented target rock that may be orders of magnitude larger
than the volume of the projectile. As a result, the actual
amount of projectile material incorporated into impactcrater rocks is generally small, typically <1 wt%. Siderophile
elements, such as Ni, Co, and the platinum-group elements
(PGEs, i.e. Pt, Pd, Os, Ru, Rh, Ir), occur at significantly
higher concentrations in meteorites than in average crust.
They also show interelement ratios distinct from those of
crustal rocks and mantle melts. With target that have low
siderophile element contents, it is possible to measure meteoritic contributions down to 0.1% using the PGEs (Huber
et al. 2001; Simonson et al. 2009). Distinctly higher siderophile element contents in impact melts, compared to targetrock abundances, can be indicative of the presence of either
a chondritic or an iron projectile. Achondritic projectiles
are much more difficult to discern because they have
significantly lower abundances of the key siderophile
elements, and it is necessary to sample all possible target
rocks to determine the so-called indigenous component
(i.e. the siderophile element content of the impact melt
rocks contributed by the target) and thus ascertain that no
MS
MC
SIC-N
MgO
CaO
K 2O–MgO–CaO plot showing average compositions of
impact melt rocks from craters of different diameters
in comparison with the compositions of bulk and upper continental
crust (data mostly from Dressler and Reimold 2001; Hecht et al.
2008a, b; and references therein). With some exceptions there is a
general trend from smaller towards larger craters with their impact
melts changing from more upper crust (UCC) towards bulk crust
composition (CC). Il = Ilyinets, BR = Brent, HB = Henbury, RK =
Roter Kamm, EG = El’gygytgyn, WP = Wanapitei, RS = Ries, PG =
Popigai, MC = Manicouagan, MK = Morokweng, MS = Mistastin,
SIC = average of Sudbury Igneous Complex, SIC-G = average of SIC
granophyre, SIC-N = average of SIC norite, SIC-QD=average of
quartz diorite dikes at Sudbury, VF = Vredefort, CX = Chicxulub,
IM-avg = average of impact melt rocks
Figure 1
E lements
38
F ebruary 2012
possibly siderophile element–rich mantle-derived target
rock has remained undetected. So far, meteoritic components have been identified in about 45 out of the ca 180
currently known impact structures on Earth (cf Koeberl
2007).
al. (2006), who discovered a 25 cm diameter fragment of
an ordinary chondrite (LL type) in the Morokweng melt
sheet (Fig. 2a) that confirmed an earlier projectile characterization by McDonald et al. (2001) based entirely on
geochemistry.
A detailed study of the abundances of siderophile elements
in the various target rocks is necessary so that mixing
calculations can be used to constrain the relative proportions of the target rock types involved in the production
of a breccia or a melt–rock mixture. From this the indigenous concentrations can be determined and subtracted
from the abundances found in the impact melt, thereby
yielding “pure” meteoritic abundance ratios. In reality, it
is difficult to identify all target rocks involved in forming
impact melt rocks or breccia (for example, because of the
loss of some rock types due to erosion or because of lack
of exposure). Such identification can also be made difficult
by very low or highly variable indigenous siderophile
concentrations.
The best-documented, impact-induced, geochemical signature is the large positive iridium anomaly in the clay layer
marking the Cretaceous–Tertiary (K–T, now called
Cretaceous–Paleogene, K–Pg) boundary. Today more than
120 K–Pg boundary locations worldwide displaying the
famous iridium anomaly have been found—from deep
oceanic settings to continental lacustrine environments
(Schulte et al. 2010). Iridium concentrations range from a
few hundred picograms/gram to almost 100 nanograms/
gram. There is no real concentration trend, except for a
dilution factor at very proximal K–Pg sites around the Gulf
of Mexico. This dilution resulted from the shear volume
of sediment and the high-energy deposition associated
with the formation of the Chicxulub crater. The reason
why, in some cases, only the Ir concentrations were
measured was that, until recently, the contents of this
element could be determined to lower concentrations and
with more ease than any of the other PGEs. Thus, Ir acts
as a marker for the other PGEs, which, if an Ir anomaly is
found, may then also be determined in a small subset of
samples. In general, however, isolated Ir data, without
complementary petrographic and geochemical information, are difficult to interpret and thus practically useless.
The PGEs are probably the most valuable elements for
projectile characterization. Their variable abundances in
different meteorite groups allow the fingerprinting of
impactors (e.g. Palme 2008). When the contents of individual PGEs are plotted against each other (for example,
as in figure 2a, using data from the Morokweng impact
structure, South Africa), the trend follows a mixing line
between the composition of the average target rocks (a few
parts per billion at Morokweng) and the impactor (whose
composition is considered to be similar to that of a chondrite clast found in the Morokweng melt rock (Fig. 2a). The
slope of the mixing line, determined by regression analysis,
represents the PGE ratio (e.g. Ru/Ir, Pt/Ir, etc.) of the
impactor, without the need to make a correction for indigenous PGEs (McDonald et al. 2001; Tagle and Hecht 2006).
PGE ratios and associated uncertainties can then be
compared with those of different types of chondrites to
determine the most likely impactor. Figure 2b shows examples of this for impact melt rocks at the Morokweng (80 km
diameter, 145 Ma) and Popigai, Russia (100 km, 35 Ma)
impact structures and early Archean impact spherules in
the Barberton greenstone belt of South Africa. The validity
of the regression technique was demonstrated by Maier et
At the K–Pg boundary, the concentrations of all the PGEs
are significantly enriched compared to background. At
most sites, the enrichment displays a sharp increase at the
boundary, followed upwards by a progressive decrease in
the lowermost Paleocene rocks. The shape of the peak is
conditioned by local sedimentological factors, such as the
intensity of bioturbation and diagenesis, the type of sediments, the chemistry of percolating fluids, and possible
reworking. In general, Ir, Ru, and Rh are the least remobilized elements. The abundances of Ni, Cr, and Co also
increase in the K–Pg boundary layer; however, their
concentrations are less diagnostic than those of the highly
siderophile PGEs.
A
490
B
480
0.40
Chondrite clastt
CI
470
≈
CV
H
60
50
Rh/Ir
Pt (ppb)
70
CM
0.36
Slope (Pt/Ir) = 2.08 ± 0.15
0.32
LL
L
EH
0.28
40
EL
Morokweng
0.24
30
Popigai
20
10
0
0.20
3.6
Intercept = 2.4 ± 2.2 ppb Pt
0
10
20
30
≈
225
230
4.4
4.8
5.2
5.6
6.0
Ru/Rh
235
(B) Rh/Ir versus Ru/Rh ratio plot of the Morokweng, Popigai crater
rocks, and the Barberton S3 impact spherule layer, compared with
those of various chondrites. The PGE ratios were determined by
regression (data from Reimold et al. 2000; McDonald et al. 2001;
Tagle and Berlin 2008; and references therein). Morokweng and
Popigai formed from the impact of ordinary chondrites, whereas
the S3 layer involved a carbonaceous chondrite. Error bars are
two sigma.
Ir (ppb)
(A) Regression between Pt and Ir contents at
Morokweng, South Africa (McDonald et al. 2001),
recalculated using ISOPLOT 3.0. A clast of LL ordinary chondrite
discovered in the Morokweng M3 borehole (Maier et al. 2006)
plots close to the regression line projected through the impact
melt rocks.
Figure 2
E lements
Barberton S3
4.0
39
F ebruary 2012
Osmium Isotopes
So far, the Cr isotope method has been used to confirm
that several mid-size to large impact structures and Late
Archean impact spherule deposits were formed by ordinary
chondrite projectiles (Koeberl et al. 2007; Simonson et al.
2009). In addition, Cr isotope data from a globally distributed late Eocene impact layer (likely related to the Popigai
impact structure, Russia) also indicate an ordinary chondrite projectile (Kyte et al. 2011). In contrast, for some
much larger impact events, such as Chicxulub, Mexico (i.e.
the K–Pg boundary), and some early Archean spherule
layers in South Africa and Australia, carbonaceous chondrite impactors are indicated. The Cr isotope method has
revealed that about 80% of the Cr in samples from the
K–Pg boundary originated from a CM2 carbonaceous chondrite projectile (Trinquier et al. 2006; Quitté et al. 2007).
The isotope 187Os (one of seven stable isotopes of Os) forms
by ß - -decay of 187Re (half-life = 42.3 ± 1.3 billion years).
Meteorites have Os concentrations several orders of magnitude higher than terrestrial crustal rocks and Re/Os ratios
less than or equal to 0.1 (e.g. Carlson et al. 2008). In
contrast, the Re/Os ratio of terrestrial crustal rocks (which
have much lower Re and Os abundances) is usually greater
than 10. As is the practice for conventional isotope systems,
the abundance of the radiogenic isotope 187Os is normalized to the abundance of a nonradiogenic isotope (188Os).
As a result of the high Re and low Os concentrations in
old crustal rocks, their 187Os/188Os ratio increases rapidly
with time (average upper-crustal 187Os/188Os = 1–1.2). In
contrast, meteorites have low 187Os/188Os ratios of about
0.11 to 0.18, and, due to the low Re/Os ratio, only small
changes in the meteoritic 187Os/188Os ratio occur with time.
Despite its selectivity, the Cr isotope method is complicated
and time-consuming, and a significant proportion of the
Cr in an impactite, compared to the abundance in the
target, has to be of extraterrestrial origin for it to be
detected. The detection limit is a function of the Cr content
in the target rocks involved in the formation of the impact
breccias or melt rocks. For example, with a Cr concentration in the target rocks of ~185 ppm (the average Cr concentration in the bulk continental crust), an extraterrestrial
component in the impactite of more than 1.2 wt% can be
detected (Fig. 3). Nevertheless, the Cr isotope method holds
great potential for further projectile identifications at
impact structures where abundance data yield ambiguous
results.
Due to the relatively high meteoritic Os abundances, the
addition of even a small amount of meteoritic matter to
the crustal target rocks can lead to a significant change in
the Os isotope signature of the resulting impactites. The
addition of achondritic meteoritic matter requires a much
higher percentage of meteoritic contribution due to the
much lower PGE abundances in achondrites compared to
chondritic and iron meteorites. A caveat is the present-day
187Os/188Os ratio of mantle rocks of about 0.13, which is
similar to meteoritic values; however, PGE abundances in
typical mantle rocks are at least two orders of magnitude
lower than those in chondritic and iron meteorites. Due
to these differences in abundance, a mantle contribution
needs to be about one hundred times larger than a meteoritic component, and such a significant mantle component would be easily discernable petrographically and
geochemically. Compared to PGE elemental abundances
and ratios, the Os isotope method is superior with respect
to detection limits and selectivity. Several impact craters
have been confirmed using the Os isotope method, which
also revealed a clear extraterrestrial signal at the K–Pg
boundary (references in Koeberl 2007).
Tungsten Isotopes
Another isotope recently proposed as a tracer for meteoritic
components in terrestrial material is 182W. This isotope was
produced by the decay of now-extinct 182Hf (half-life = 8.9
million years). Meteorites and the terrestrial crust have
distinct W isotope compositions. 182W has been used to
identify the impactor at the K–Pg boundary by analyzing
the sediments and Ni-rich spinels (cf Quitté et al. 2007).
Moynier et al. (2009) analyzed W isotopes in a variety of
well-defined impactites and ejecta from four different
impact structures and two K–Pg boundary-layer locations,
but in all these samples, the isotopic composition of W is
identical, within analytical error, to that of the Earth’s
continental crust, and no 182W anomalies are present, even
in the samples containing a significant (percent-level)
meteoritic component. Thus, earlier suggestions that W
isotope analyses indicate a meteoritic component in early
Archean (3.8 Ga) metasedimentary rocks are unfounded,
and W isotopes are not suitable for the identification of
meteoritic components in terrestrial rocks (Fig. 3).
Chromium Isotopes
Identifying an extraterrestrial component in impactites
requires meticulous geochemical analyses; moreover,
element abundances can, in some cases, yield ambiguous
results. The Os and Cr isotope methods potentially remove
much of this ambiguity, and the Cr isotope method, in
particular, can help identify the type of meteorite involved
in an impact event. The method is based on the determination of the relative abundances of 53Cr, which is the
daughter product of the extinct radionuclide 53Mn (half-life
= 3.7 million years). 53Cr relative abundances are measured
as the deviations of the 53Cr/52Cr ratio in a sample relative
to the standard terrestrial 53Cr/52Cr ratio. This is done using
high-precision thermal ionization mass spectrometry, and
the results are given in e units (1 e = 1 part in 104, or 0.01%).
Terrestrial rocks do not show any variation in the 53Cr/52Cr
ratio, because differentiation of the Earth was completed
long after all 53Mn had decayed. In contrast, data for most
meteorite groups, such as carbonaceous, ordinary, and
enstatite chondrites, primitive achondrites, and other
differentiated meteorites, show a variable excess of 53Cr
relative to terrestrial samples. For the various meteorite
types, the range is about +0.1 to +1.3 e. Only the carbonaceous chondrites show a deficit in 53Cr, of about –0.4 e.
The presence of 54Cr excesses in bulk carbonaceous chondrites distinguishes these meteorites clearly from the other
meteorite classes.
E lements
STABLE AND OTHER ISOTOPES
Stable Isotopes
The most commonly measured stable isotopes in impactrelated materials are those of carbon, oxygen, and sulfur.
Such analyses have three main goals. (1) If the isotopic
compositions of impactites themselves are measured,
source rocks can be determined and alteration processes
can be studied. For example, oxygen isotope measurements
have shed light on the sources of tektites. (2) The geochemistry of carbon in impactites can best be determined from
isotopic studies of carbon components, such as impactderived diamonds (e.g. Gilmour 1998). (3) Lithological or
paleoenvironmental effects of impact events in the geological record can be studied. In addition, some so-called
“nonconventional stable isotopes” have been analyzed in
tektites, which are glasses thought to have formed very
early in the impact process and been subjected to short-
40
F ebruary 2012
1.1
0.9
source rocks for tektites. 10Be has a short half-life (1.4 million
years) and thus can be studied only in correspondingly
young impact structures and glasses. The average content
of 10 Be in Australasian tektites is comparable to those
measured in near-surface source materials, such as soils
(terrestrial) or sediments (marine and terrestrial), and
within the Australasian strewn field there is a correlation
between tektite type and 10Be concentration (Ma et al.
2004). Aerodynamically shaped tektites found distally from
the presumed impact site in Southeast Asia have higher
10 Be contents than more proximal, layered (Muong Nong–
type) tektites. This agrees with the Cu and Zn isotope fractionation pattern for tektites, which shows that the greater
the distance from the impact site, the greater is the fractionation of the isotope systems (Moynier et al. 2010).
Serefiddin et al. (2007) found 10Be in Ivory Coast tektites
and in the much older Central European tektites,
confirming that tektites are early, high-temperature and
high-velocity, distal impact ejecta that formed before the
main crater was excavated.
Os/188 Os
ε53 Cr
Ir/Pd
ε182 W
187
typical ε53 Cr 2σ
typical Ir/Pd 2σ
typical ε182 W 2σ
Isotope Ratio
0.7
0.5
0.3
0.1
-0.1
-0.3
0
1
2
3
4
5
6
7
8
9
10
Other Geochemical Indicators
Meteoritic fraction (%)
Other geochemical indicators, which cannot be discussed
here in detail, include the Ni and Cr spinels whose compositions are unique indicators of an extraterrestrial origin.
Studies of such indicators contributed significantly to the
interpretation that during the Middle Ordovician an
L-chondrite parent body was disrupted, leading to a
dramatically enhanced flux of extraterrestrial matter to
Earth (Schmitz et al. 2003). Extraterrestrial chromite grains
dispersed in mid-Ordovician sediments contain abundant
neon implanted by solar wind; this demonstrates that such
chromite grains originated from micrometeorites that
decomposed on the seafloor (Heck et al. 2008).
Comparison of the detection limits of the Cr, Os, and
W isotope and PGE (as Ir/Pd ratio) methods as tracers
of meteoritic components in terrestrial impact rocks for a carbonaceous chondritic component. Each curve represents the evolution
of either the element ratio (Ir/Pd) or the isotopic composition
(ε53Cr, ε182W, and 187Os/188Os) of the impactite as a function of the
fraction of meteoritic component. Typical error bars for the
different systems are plotted directly onto the graph, with the
exception of the Os isotopes, where the error is within the thickness
of the curve (after Moynier et al. 2009).
Figure 3
Caveats
lived, very-high-temperature conditions. Because volatilization can potentially fractionate isotopes, comparing the
isotopic compositions of volatile elements in tektites with
those of their source rocks may improve understanding of
the physical conditions during tektite formation.
Interestingly, volatile chalcophile elements (e.g. Cu, Zn,
and Cd) are the only elements for which isotopic fractionation has been established in tektites (e.g. Moynier et al.
2010).
Geochemical data have at times been used in inappropriate
ways, have not been properly verified, or have been overinterpreted. Claims of the presumed impact origin of some
spherule beds had to be rejected once appropriate chemical
data were obtained. The claimed presence of supposedly
extraterrestrial fullerene molecules in impactites (e.g. at
Sudbury or at the Permo-Triassic boundary) has never been
independently confirmed. Diamonds of supposed meteoritic origin and allegedly related to the Younger Dryas event
were found to be carbon forms of terrestrial origin.
Suggestions that geochemical evidence (e.g. W isotopes, Ir
abundances) exists for the Late Heavy Bombardment were
based on incomplete data taken out of context. Similarly
to other aspects of impact studies, geochemistry is vulnerable to overinterpretation and wishful thinking. It is
imperative that data be carefully obtained and verified,
using independent methods and multiple laboratories, and
that they be calibrated with the appropriate methods and
standard reference materials (French and Koeberl 2010).
Helium-3
The helium-3 isotope is extremely rare in terrestrial crustal
rocks, but it is relatively abundant in interplanetary dust
particles because of reactions with cosmic rays. A significant enrichment of 3He was found in two Late Eocene
impactoclastic layers that are correlated with the Popigai
and Chesapeake Bay impact events, which also had a
climatic influence. As 3He is a proxy for the influx of extraterrestrial dust, the late Eocene 3He enrichment was interpreted to indicate a time of enhanced dust activity in the
inner Solar System; this dust was due to an increased flux
of comets, which probably resulted in a higher impact rate
than usual. More recently a 3He signal in deep-sea sediments was correlated with an asteroidal breakup event in
the mid-Miocene; this may resolve the discrepancy with
other geochemical proxies (Cr isotopes, PGEs) that suggest
Eocene impactors were asteroids by indicating that particles
derived from both cometary and asteroidal sources carry
a 3He signal (see Farley 2009 and references therein).
CONCLUSIONS
Geochemistry is an extremely versatile tool for studying
impact events. Studies can range from simple major and
trace element characterization of impact breccias, melt
rocks, glasses, and target rocks to elaborate isotope investigations and oxidation state determinations. Geochemistry
can also be used to search for extraterrestrial components
and to identify projectiles in impactites and ejecta, or to
determine noble gas abundances in minute minerals and
the clay mineral composition of fracture fillings in impact
breccias. Geochemical analyses are of crucial importance
for establishing the impact origin of suspicious geological
structures or stratigraphic units, and they contribute
invaluable information about every part of the impact
Beryllium-10
The cosmogenic radionuclide 10Be forms by the interaction
of cosmic rays with nitrogen in the atmosphere and is
concentrated at the top of any sediment column. 10Be can
be used to constrain the location and characteristics of the
E lements
41
F ebruary 2012
ACKNOWLEDGMENTS
process. From nano- and microscale features to global
processes, our understanding of impact as a geological
phenomenon would be incomplete and lack quantification
without geochemical data.
REFERENCES
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Dressler BO, Reimold WU (2001)
Terrestrial impact melt rocks and
glasses. Earth-Science Reviews 56:
205-284
Farley KA (2009) Late Eocene and late
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