Unique igneous textures and shock metamorphism of the Northwest Africa 7203 angrite: Implications for crystallization processes and the evolutionary history of the angrite parent body

Abstract

Northwest Africa (NWA) 7203 is a quenched angrite, showing mineralogical features typically not present in other quenched angrites. NWA 7203 exhibits textures whose grain size varies from fine grains (<10 mu m) to coarse grains (similar to 3 mm), while other quenched angrites show only single-sized textures. Fine-grained and coarse-grained lithologies have nearly the same bulk compositions. Cooling rates were estimated to be similar to 80 degrees C h(-1) for fine-grained lithologies and similar to 1 degrees C h(-1) for coarse-grained lithologies. Mg-rich olivines (similar to Fo(64)) were found only in fine-grained lithologies. Crystallization of NWA 7203 started in the fine-grained lithologies with Mg-rich olivine grains acting as seeds for crystallization. Coarse-grained lithologies were subsequently formed under conditions of slower cooling. NWA 7203 shows clear shock metamorphic textures unlike other quenched angrites except for NWA 1670. We confirm that the oxygen isotopic ratios of NWA 7203 plot on the angrite fractionation line within uncertainty. However, the obtained Pb-Pb age of NWA 7203 is 4543 +/- 19 Ma, younger than the ages of other quenched angrites, which might be a result of disturbance by shock metamorphism. The finding of shock metamorphism of NWA 7203 suggests that some angrites might be derived from asteroids that remained large (>10 km in diameter) during the late heavy bombardment.