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Shock and annealing in the amphibole- and mica-bearing R chondrites
Rubin, Alan E.
AA(Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567 USA)
Meteoritics & Planetary Science, Volume 49, Issue 6, pp. 1057-1075.
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MIL 11207 (R6) and LAP 04840 (R6) contain hornblende and phlogopite; MIL 07440 (R6) contains accessory titan-phlogopite and no hornblende. All three meteorites have been shocked: MIL 11207 contains extensive sulfide veins, pyroxene that formed from dehydrated hornblende, and an extensive network of plagioclase glass; MIL 07440 contains chromite-plagioclase assemblages, chromite veinlets and blebs, pincer-shaped plagioclase patches, but no sulfide veins; LAP 04840 contains olivine grains with chromite-bleb-laden cores and opaque-free rims, rare grains of pyroxene that formed from dehydrated hornblende, and no sulfide veins. These meteorites appear to have been heated to maximum temperatures of approximately 700-900 °C under conditions of moderately high PH2O (perhaps 250-500 bars). All three samples underwent postshock annealing. During this process, olivine crystal lattices healed (giving the rocks the appearance of shock-stage S1), and diffusion of Fe and S from thin sulfide veins to coarse sulfide grains caused the veins to disappear in MIL 07440 and LAP 04840. This latter process apparently also occurred in most S1-S2 ordinary chondrites of high petrologic type. The pressure-temperature conditions responsible for forming the amphibole and mica in these rocks may have been present at depths of a few tens of kilometers (as suggested in the literature). A giant impact or a series of smaller impacts would then have been required to excavate the hornblende- and biotite-bearing rocks and bring them closer to the surface. It was in that latter location where the samples were shocked, deposited in a hot ejecta blanket of low thermal diffusivity, and annealed.
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