AN H CHONDRITE MELT CLAST IN AN LL CHONDRITE

46th Lunar and Planetary Science Conference (2015)
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AN H CHONDRITE MELT CLAST IN AN LL CHONDRITE: EVIDENCE FOR MIXING OF ORDINARY
CHONDRITE PARENT BODIES. C. M. Corrigan1, N. G. Lunning2, and K. Ziegler3. 1Smithsonian Institution,
National Museum of Natural History, MRC 119, 10th and Constitution Ave. NW, Washington DC, 20560, USA.
2
University of Tennessee, Knoxville, TN 37996, USA. 3Institute of Meteoritics, University of New Mexico, Albuquerque, NM 17131, USA. E-mail: [email protected].
Introduction: In the course of studying impact
melts in ordinary chondrites (OC) [1-4] we have identified a number of impact melt and other secondary
clasts (i.e., not chondrules) within OC impact breccias.
This is part of an ongoing search for ancient melt clasts
to gain insight into the lunar cataclysm [5-10] and
small body scattering in the early Solar System (causing the late heavy bombardment [11-13]).
Potential melt clasts: Within the OC impact breccias involved in this study, a range of types of inclusions have been identified. Some are microporphyritic
impact melt clasts (such as those found in LEW 85397,
EET 87595 and PCA 02071 [3]). Others fall along a
continuum of igneous textures ranging between
quenched glasses and fully crystalline clasts.
MET 01004 melt clast: In the brecciated Antarctic
LL5 ordinary chondrite MET 01004 we found an extremely light colored clast (Fig. 1). The clast is approximately 3 x 4 cm on the exposed face of the meteorite. Thin sections were made of this clast and analyzed using instrumentation at the SmithsonianBackscattered electron imagery and elemental mapping were completed using the FEI NanoSEM at the
Smithsonian, which is equipped with a THERMONORAN energy dispersive X-ray analytical system.
Electron microprobe analyses were conducted using
the Smithsonian’s JEOL 8900R Superprobe.
Results: Optical microscopy and SEM imaging/mapping show that the clast is composed of olivine, pyroxene and plagioclase (Figs. 2, 3), some of
which is poikilitically enclosing olivine. Very little
metal (<1 vol.%) or sulfide are present in the clast
(significantly less than the ~3-6% typical for LL chondrites [14,15]). Those metal grains that are present are
coarse, as opposed to being disseminated throughout
the clast. The clast is moderately shocked, with planar
fractures and undulose extinction in olivine, pyroxene
and plagioclase (which is not maskelynitized). The
overall texture of the clast is achondritic, lacking any
remnant chondrule features.
Mineral chemistry/Bulk Composition: Olivines and
pyroxenes were analyzed using the electron microprobe. Olivines have values of Fa19, and pyroxenes are
Fs15. Compositions are within the H chondrite range
[15], far from the LL chondrite host rock (Fa27.5-30 and
Fs23.2-25.7). When compared with bulk LL chondrites
[14], this clast is depleted in Fe and S and enriched in
Si. The H chondrite silicate bulk composition [16] is
similar to this clast (Table 1).
Oxygen isotope composition: Oxygen isotope anal-
yses were performed on 1-2 mg fragments of material
using the laser-fluorination method of [17] at the University of New Mexico. Molecular O2 was extracted in
cryogenically
and
gasa
BrF5-atmosphere,
chromatographically purified, and the isotope ratios
measured on a gas source mass spectrometer (Delta
PlusXL), with an analytical precision for Δ17O of 0.02
‰. Results are, respectively, δ17O 2.640, 2.449, and
2.485; δ18O 3.691, 3.329, and 3.410; Δ17O 0.691,
0.691, and 0.691 ‰ (reference TFL slope =
0.528; values are all linearized). Results from these
analyses place the MET 01004 clast at the low end of
the H chondrite field (Fig 4), in agreement with the
electron microprobe data.
Discussion: Given this clast’s almost complete
lack of metal and sulfides, its igneous texture, and its
shock features, we believe that this is an impact melt
clast that has been shock metamorphosed. Its oxygen
isotope composition, mineral chemistry, and bulk
composition are consistent with those of H chondrites,
rather than with its host breccia (LL chondrite).
The geologic history of this meteorite is an interesting one. Multiple possibilities for how an H chondrite
impact melt clast was emplaced onto the LL chondrite
parent body exist. One possibility is that impact melting of the H chondrite material occurred on its parent
body and this material was then ejected and incorporated into the LL body. This event or subsequent impact shock may have then shocked the clast. Another
possibility is that the H chondrite material melted as it
struck the LL chondrite body (in which case it would
have had to have cooled long before it was admixed
into the LL parent body brecciated material).
The clast found in MET 01004 shows several similarities to the melt clast found in the Peace River L6
chondrite [18]. The overall texture and mineralogy are
similar, though, the Peace River clast contains less
feldspar than that in the MET 01004 clast. Both clasts
are depleted in metal relative to bulk ordinary chondrites. The Peace River clast formed from LL chondrite material, which was later, incorporated into an L
chondrite (Peace River).
Previously reported lithic clasts (potentially impact
melts) may be additional evidence of mixing between
ordinary chondrite groups. An igneous textured clast in
Chantonnay (L6) has bulk composition consistent with
H chondrites [19]. Lithic clasts in the L6 chondrites Y75097 and Y-793241 have H chondrite oxygen isotope
compositions, but the major element chemistry of these
clasts has thermally equilibriated with the host L6 ma-
46th Lunar and Planetary Science Conference (2015)
2678.pdf
terial [20].
In any case, the impact melt clasts in MET 01004
and Peace River provide evidence for impact melting
and mixing between ordinary chondrite groups.
Age dating: Impact melt clasts from OCs are distinctly under-represented in terms of solar system ages
[15], but if successfully obtained, these dates will lead
to a better understanding of how early solar system
bombardment affected the asteroid belt. We certainly
recognize that this is a significant challenge. Age dating of the H chondrite clast and other clasts in MET
01004 will help us further understand mixing between
ordinary chondrite parent bodies.
Figure 3: BSE image of olivine chadacrysts poikilitically
enclosed by pyroxene and plagioclase. Field of view is 5 mm
(horizontally).
References: [1] Corrigan & Lunning 2013 MAPS 48,
#5256 [2] Corrigan et al., 2012, LPSC 43 #1577 [3] Corrigan
and Lunning 2012, MAPS 47, #5320 [4] Corrigan & Lunning, 2013, LPSC 44 #2615 [5] Turner et al. 1973, 4th LPSC,
1889 [6] Tera et al. 1974, EPSL 1, 19 [7] Kring & Cohen
2001 JGR 107, doi: 10.1029/2001JE001529. [8] Cohen et al.
2005, MAPS 40, 755 [9] Ryder et al. 2002, JGR 107, 6-1 [10]
Hartmann 2003, MAPS 38, 579 [11] Tsiganis et al. 2005,
Nature 435, 459 [12] Morbidelli et al. 2005, Nature 435, 462
[13] Gomes et al. 2005, Nature 435, 466 [14] Jarosewich,
1990, Mets 2, 323 [15] Gomes & Keil, 1980, Brazilian Stone
Meteorites, U of NM, ABQ [16] Mason, B. (1965) Amer Mus
Novitates 2223, 1-38 [17] Sharp (1995) Am J Sci 295, 10581076. [18] Herd et al. 2013 Canad J Earth Sci 50, 14 [19]
Yolcubal et al. (1997) JGR 102, 589 [20] Nakamura et al.
(1994) Proc. NIPR Ant. Met. 7, 125
Figure 4: Oxygen isotope composition of igneous textured
melt clast from MET 01004. Filled (red) circles correspond
to the melt clast discussed in this abstract.
Figure 1: Photograph of igneous textured melt clast in LL
breccia MET 01004. The large, light-colored clast (~3 x 4
cm) is the subject of this abstract.
Table 1: Bulk composition of the igneous clast from MET
01004 compared to ordinary chondrites.
Figure 2: XPL image of olivine chadacrysts poikilitically
enclosed by pyroxene and plagioclase.
Wt. %
Ratios
MET 01004
Igneous
Clast
Bulk LL
Chondrite
[14]
H Chondrite
Silicates
[16]
Fe/Si
0.54
0.94
0.44
Fe/Mg
0.64
1.2
0.52
Fe/Mn
34
58
38.6
S/Mg
0.01
0.12
N/A
Si/Mg
1.2
0.92
1.2