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Title: Ancient porosity preserved in ordinary chondrites: Examining shock and compaction on young asteroids

Authors:
; ; ; ; ; ;  [1];  [2];  [2];  [2]
  1. (UCLA)
  2. (
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
Sponsoring Org.:
NSFDOE - BASIC ENERGY SCIENCESNASA
OSTI Identifier:
1140029
Resource Type:
Journal Article
Resource Relation:
Journal Name: Meteorit. Planet Sci.; Journal Volume: 49; Journal Issue: (7) ; 07, 2014
Country of Publication:
United States
Language:
ENGLISH

Citation Formats

Friedrich, J.M., Rubin, A.E., Beard, S.P., Swindle, T.D., Isachsen, C.E., Rivers, M.L., Macke, R.J., Ariz), UC), and Fordham). Ancient porosity preserved in ordinary chondrites: Examining shock and compaction on young asteroids. United States: N. p., 2014. Web. doi:10.1111/maps.12328.
Friedrich, J.M., Rubin, A.E., Beard, S.P., Swindle, T.D., Isachsen, C.E., Rivers, M.L., Macke, R.J., Ariz), UC), & Fordham). Ancient porosity preserved in ordinary chondrites: Examining shock and compaction on young asteroids. United States. doi:10.1111/maps.12328.
Friedrich, J.M., Rubin, A.E., Beard, S.P., Swindle, T.D., Isachsen, C.E., Rivers, M.L., Macke, R.J., Ariz), UC), and Fordham). Thu . "Ancient porosity preserved in ordinary chondrites: Examining shock and compaction on young asteroids". United States. doi:10.1111/maps.12328.
@article{osti_1140029,
title = {Ancient porosity preserved in ordinary chondrites: Examining shock and compaction on young asteroids},
author = {Friedrich, J.M. and Rubin, A.E. and Beard, S.P. and Swindle, T.D. and Isachsen, C.E. and Rivers, M.L. and Macke, R.J. and Ariz) and UC) and Fordham)},
abstractNote = {},
doi = {10.1111/maps.12328},
journal = {Meteorit. Planet Sci.},
number = (7) ; 07, 2014,
volume = 49,
place = {United States},
year = {Thu Jul 10 00:00:00 EDT 2014},
month = {Thu Jul 10 00:00:00 EDT 2014}
}
  • Collisions and attendant shock compaction must have been important for the accretion and lithification of planetesimals, including the parent bodies of chondrites, but the conditions under which these occurred are not well constrained. A simple model for the compaction of chondrites predicts that shock intensity as recorded by shock stage should be related to porosity and grain fabric. To test this model, we studied sixteen ordinary chondrites of different groups (H, L, LL) using X-ray computed microtomography (μCT) to measure porosity and metal fabric, ideal gas pycnometry and 3D laser scanning to determine porosity, and optical microscopy (OM) to determinemore » shock stage. These included a subsample of six chondrites previously studied using transmission electron microscopy (TEM) to characterize microstructures in olivine. Combining with previous data, results support the simple model in general, but not for chondrites with low shock-porosity-foliation (low-SPF chondrites). These include Kernouvé (H6), Portales Valley (H6/7), Butsura (H6), Park (L6), GRO 85209 (L6), Estacado (H6), MIL 99301 (LL6), Spade (H6), and Queen’s Mercy (H6), among others. The data for these meteorites are best explained by high ambient heat during or after shock. Low-SPF chondrites tend to have older 40Ar/39Ar ages (~4435–4526 Ma) than other, non-low-SPF type 6 chondrites in this study. We conclude that the H, L, and LL asteroids all were shock-compacted at an early stage while warm, with collisions occurring during metamorphic heating of the parent bodies. Results ultimately bear on whether chondrite parent bodies have internal structures more akin to a metamorphosed onion shell or metamorphosed rubble pile, and on the nature of accretion and lithification processes for planetesimals.« less
  • Silicate darkening in ordinary chondrites (OC) is caused by tiny grains of metallic Fe-Ni and troilite occurring mainly within curvilinear trails that traverse silicate interiors and decorate or, in some cases, cut across silicate grain boundaries. Highly shocked OC tend to have greater degrees of silicate darkening than lightly shocked OC; this indicates that silicate darkening is probably a result of shock metamorphism. The low Fe-FeS eutectic temperature (988C) renders metal and troilite susceptible to melting and mobilization during shock heating. Unshocked OC tend to have plagioclase with uniform compositions; shocked OC tend to have plagioclase with more variable (albeitmore » still stoichiometric) compositions. The low impedance of plagioclase to shock compression makes it particularly susceptible to melting and mobilization; this is consistent with the molten appearance of plagioclase in highly shocked OC (e.g., Rose City and Paragould). CK chondrites also have compositionally variable plagioclase. The common association of silicate darkening with compositionally variable plagioclase is consistent with the hypothesis that both are products of shock metamorphism. Some CK and OC chondrites exhibit light shock effects in olivine that are consistent with equilibrium peak shock pressures that are too low to account for the silicate darkening or opaque shock veins in these meteorites. Therefore, the olivine in these chondrites may have been annealed after intense shock produced these effects. A few CK chondrites that contain olivine with undulose or mosaic extinction (e.g., LEW87009 and EET83311) may have been shocked again, after annealing.« less
  • The induced thermoluminescene (TL) properties of 121 equilibrated ordinary chondrites have been measured. The samples were 74 H and 47 L chondrites, of which 33 H and 32 L were from Antarctica. The distribution of TL sensitivities for non-Antarctic L chondrites differs from that of non-Antarctic H chondrites, consistent with a greater proportion of the former class suffering post-metamorphic shock. Data on the effect of laboratory annealing on TL sensitivity, and step-wise Ar release measurements, enable the meteorites to be sorted into three shock-related temperature groups (<800{degree}C, 800-1000{degree}C, >1000{degree}C). The distribution of TL sensitivities of Antarctic meteorites suggests that onlymore » a few of the present samples have suffered intense shock. Antarctic H chondrites have TL sensitivities typically one third those of non-Antarctic H chondrites; this may reflect a greater proportion of meteorites which have suffered mild shock levels or greater degrees of weathering. On a diagram of TL leak temperature against peak width, L chondrites produce tight clusters with minimal overlap between Antarctic and non-Antarctic meteorites. Antarctic H chondrites also produce a tight cluster, but non-Antarctic chondrites plot in a band in which peak temperature increases with peak width, and there is little or no overlap between the Antarctic and non-Antarctic meteorites. Because TL peak temperature and width reflect the thermal history of the feldspar, and changes in these TL parameters can be produced by laboratory annealing experiments, this implies significant differences in the thermal (probably metamorphic) history of the Antarctic and non-Antarctic ordinary chondrites.« less
  • Cited by 2
  • To examine the role of impacts in the evolution of asteroids as seen through their chondritic offspring, we have performed a quantitative three dimensional (3D) study of metal grains in a suite of increasingly shocked L chondrites with synchrotron X-ray microtomography (XMT). Our data allow rigorous quantification of size-number distributions and collective morphology of Fe(Ni) metal phases in chondritic meteorites. At the resolution of our XMT measurements (8.4--17.9 {mu}m/voxel), the number of metal particles increase with higher degrees of petrographically identified shock loading, indicating a coalescing of Fe-Ni metal at or below this scale. Our results demonstrate that collective degreesmore » of metal grain preferred orientation increase with greater degrees of impact-related compaction and shock loading. Ductile metal grains in L chondrites begin to show foliation at peak shock pressures < 5 GPa, pressures great enough to compact and indurate loosely bound chondritic material, and our results constitute evidence for multiple generations of impact events acting on the L parent body or bodies.« less