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Ancient porosity preserved in ordinary chondrites: Examining shock and compaction on young asteroids
Author(s) -
Friedrich Jon M.,
Rubin Alan E.,
Beard Sky P.,
Swindle Timothy D.,
Isachsen Clark E.,
Rivers Mark L.,
Macke Robert J.
Publication year - 2014
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.12328
Subject(s) - chondrite , shock metamorphism , petrography , geology , parent body , geochemistry , asteroid , impact crater , porosity , olivine , mineralogy , meteorite , astrobiology , physics , geotechnical engineering
We use a combination of 2D and 3D petrographic examination and 40 Ar‐ 39 Ar analyses to examine the impact histories of a suite of seven ordinary chondrites (Baszkówka, Miller, NWA 2380, Mount Tazerzait, Sahara 98034, Tjerebon, and MIL 99301) that partially preserve their ancient, but postaccretionary, porosity ranging from 10 to 20%. We examine whether materials that seem to be only mildly processed (as their large intergranular pore spaces suggest) may have more complex shock histories. The ages determined for most of the seven OC s studied here indicate closure of the 40 Ar‐ 39 Ar system after primary accretion, but during (Baszkówka) or shortly after (others) thermal metamorphism, with little subsequent heating. Exceptions include Sahara 98034 and MIL 99301, which were heated to some degree at later stages, but retain some evidence for the timing of thermal metamorphism in the 40 Ar‐ 39 Ar system. Although each of these chondrites has olivine grains with sharp optical extinction (signaling an apparent shock stage of S1), normally indicative of an extremely mild impact history, all of the samples contain relict shock indicators. Given the high porosity and relatively low degree of compaction coupled with signs of shock and thermal annealing, it seems plausible that impacts into materials that were already hot may have produced the relict shock indicators. Initial heating could have resulted from prior collisions, the decay of 26 Al, or both processes.