A shock-metamorphic model for silicate darkening and compositionally variable plagioclase in CK and ordinary chondrites

Abstract

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 (characterized by olivine grains with undulose to mosaic extinction) 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 (988°C) renders metal and troilite susceptible to melting and mobilization during shock heating. A few OC also contain thin melt veins of chromite; this implies that localized shock temperatures reached ≥ 1635°C in these meteorites. Silicate darkening is also evident in CK carbonaceous chondrites, where magnetite and pentlandite grains form analogous curvilinear trails. It is possible that magnetite and pentlandite in CK chondrites were mobilized during shock metamorphism and dispersed through silicate interiors; alternatively, metal and troilite may have been dispersed and then transformed into magnetite and pentlandite during subsequent oxidation of these chondrites. Unshocked OC tend to have plagioclase with uniform compositions; shocked OC tend to have plagioclase with more variable (albeit 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.