Shock metamorphic effects in phosphates from ordinary chondrites

Abstract

Studies of shock metamorphic effects in apatite and merrillite in nine ordinary chondrites (OCs) of petrologic types 3.5–6 and shock metamorphic stages S1–S5 using transmission electron microscopy (TEM) reveal a correlation between the extent of brittle deformation in phosphates and the shock metamorphic stage of six host meteorites. No correlation is observed in thermally annealed and partially melted phosphates in Kyushu (L6), Paragould (L5), and Hamlet (LL3.5 − 3.9). Apatites in several shocked equilibrated (petrologic type 6) OCs show micro- and nano-scale heterogeneities in volatile elements, suggesting they were locally mobilized during shock metamorphism rather than during thermal metamorphism. In Alfianello (L6, S5) and Kyushu (L6, S5), maskelynite associated with apatite shows clear evidence for melting. We suggest that maskelynite formed during melting processes rather than solid-state deformation, which has significant implications for geochronology and reflects the time of impact rather than the crystallization age of phosphates. Our study demonstrates the inadequacy of optical microscopy methods currently applied to determine shock metamorphic stages of chondrites; incorporation of micro and nanostructural observations will improve the accuracy of these determinations. We suggest that integration of detailed observations of shock and thermal metamorphism and fluid alteration is required for a comprehensive understanding of the secondary processes that modified most small Solar System bodies.