Shock Heating and Subsequent Cooling of Basaltic Shergottites: the Cases

Introduction: Shock metamorphism is one of the most fundamental processes in the history of Martian meteorites. Especially, shergottites experienced strong shock effects (>30 GPa) most likely when they were ejected from Mars [e.g., 1]. " Maskelynitization " of plagioclase and formation of shock melts are major effects due to this severe shock. QUE94201 (QUE) and Dhofar 378 (DHO) are basaltic shergottites that are mainly composed of pyroxene and plagioclase glass [e.g., 2-3]. These two shergottites suffered severe shock, which generated abundant impact melt and vesiculated flow textures. In spite of similar degrees of shock between two meteorites, they are distinct in several mineralogical aspects. In this abstract, we discuss their differences to understand differences in their shock heating and subsequent cooling histories. QUE94201: QUE is composed of subequal amounts of pyroxene (pigeonite and augite) and pla-gioclase glass. Plagioclase glass is isotropic and is similar to " maskelynite " in other shergottites. There is no area showing birefringence even where plagioclase glass is in contact with either shock melt or fusion crust. Plagioclase glass shows " normal " chemical zon-ing from An-rich core (An 65 Ab 35) to relatively Ab-rich rim (An 59 Ab 40 Or 1). A wide shock melt vein cut across the center of the thin section studied (Fig. 1a). Several grains of plagioclase glass are entrained in this shock melt vein, and some show flow textures, suggesting melting of plagioclase (Fig. 1b). These plagioclase glasses are enriched in Fe (FeO = up to 6 wt%). In the shock melt vein, dendritic pyroxene grains (up to 100 µm long) recrystallized (some nucleated on pyrrhotite spherules of ~200 µm in diameter) (Fig. 1c). This suggests that pyrrhotite precipitated from the melt before the crystallization of pyroxene. Plagioclase recrystalli-zation is not observed in the melt. The melt is enriched in P (P 2 O 5 = ~7 wt%) although it is heterogeneous. Dhofar 378: DHO is also composed of subequal amounts of clinopyroxenes and plagioclase like QUE, and contains abundant vesicles (~1 mm in diameter) (Fig. 2a). The sample shows the presence of many blackened areas similar to shock melt, but FEG-SEM observation revealed that they were composed of re-crystallizing fine-grained pyroxene and plagioclase (Fig. 2c). The most notable difference of DHO from QUE94201 is seen in plagioclase mineralogy. Although some plagioclase grains display flow textures like QUE, plagioclase in DHO shows a fibrous crystalline texture (Figs. 2b and d). …