Post-shock Annealing and Post-annealing Shock: Implications for the Thermal and Shock Histories of Ordinary-chondrite Parent Bodies

A common view of the thermal and shock histories of ordinary-chondrite (OC) parent bodies is that, for the most part, thermal and shock metamorphism were distinguishable and successive events; e.g., the decay of 26 Al heated chon-dritic planetesimals and subsequent impacts produced various shock effects. Although collisions have long been acknowledged as capable of producing significant heating (including melting) of target rocks, shock events are generally not thought responsible for whole-rock annealing. Because olivine is readily affected by shock processes , OC with olivine grains exhibiting sharp optical extinction have been considered by many workers to have never been significantly shocked. However, shock-induced damage of the olivine crystal lattice can be annealed out at relatively low temperatures [1,2]; thus, annealing can make a chondrite appear less shocked than it was at an earlier point in its history. I call this hypothetical earlier point the rock's maximum prior shock level. I examined the shock features of 75 type-5 and-6 OC of shock-stages S1 and S2 to test this idea. No more than a few of these chondrites are likely to be paired. In addition to long-recognized shock effects in olivine, pyroxene and plagioclase (including the presence of high-pressure phases), petrographic shock indicators in OC include silicate darkening [3], chromite veinlets [3], narrow glassy pseudo-tachylite-like veins [4], metal and sulfide veins [5], polycrystalline troilite [6], martensite [7], rapidly solidified metal-sulfide intergrowths [8], irregularly shaped troilite grains inside metal [9], metallic Cu [9], chromite-plagioclase assemblages (cpa) [10], and glassy melt pockets [11]. The petrologic characteristics of the Spade H-chondrite impact-melt breccia [12] demonstrate that post-shock annealing can occur. The impact-melt origin of this rock is indicated by the low modal abundances of relict chondrules (1.8 vol.%) and of coarse (i.e., ≥200-µm-size) isolated mafic silicate grains (1.8 vol.%) relative to those of mean H6 chondrites (11.4 and 9.8 vol.%, respectively), consistent with ~80% melting of Spade. The shock event that melted Spade caused significant silicate darkening and produced chromite veinlets and cpa. Molten plagioclase was partly volatilized; K and Na were mobilized and produced a relatively large range in plagioclase composition. The flowing plagioclase melts partially surrounded nearby mafic silicate grains. Metallic Fe-Ni cooled rapidly below the Fe-Ni solvus and transformed into martensite. Spade probably reached a maximum prior shock level approximately equivalent to shock-stage S6; those olivine grains that did not melt as a result of the impact acquired mosaic extinction. In contrast, …