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Extreme Shock Effects in Relatively Enriched Shergottite Northwest Africa 4797

Introduction: All Martian meteorites have been ejected from Martian near-surface regions by impact events capable of accelerating material 5 km/s. The extreme physical conditions of impact, or 'shock', cause significant changes in the texture, mineralogy and isotopic composition of the rocks [1, 2]. Every Martian meteorite records some features associated with shock, with variable intensity, ranging from the least shocked nakhlites to the most strongly shocked shergottites [2]. The general convention has been to ascribe all shock features to impact ejection from Mars, with the exception of ALH 84001 and NWA 2737 [3, 4]; however, a recent study on shergottite Dhofar 378 (55–75 GPa; [5]), has shown that an Ar-Ar age of ~141 Ma most likely dates a major impact on Mars, with later ejection at 3 Ma in a smaller impact event. This suggestion raises the question whether the shock features observed in other shergottites were formed at the time of Mars ejection or earlier in Martian history. This is particularly true for highly shocked samples where the complex textures can lead to debate over the details of these processes, in particular on the number of shock events required to explain observed textures. This study reports new data on strongly shocked shergottite Northwest Africa (NWA) 4797, found in 2001 near Missour, Morocco as a single stone weighing 15.0 grams [6]. A total of 3.3 g was made available for study. Here, we focus on detailed analysis of shock metamorphic effects in NWA 4797 in order to constrain the equilibration shock pressure. Petrography: Microtextures were characterized using BSE and SE images using a JEOL 6301F Field Emission SEM with an 8 mm working distance and an accelerating voltage of 20 kV. Mineral modes for the thin section were determined by manual point counting on BSE images (n = 12,000). Igneous Lithologies and Oxygen Fugacity. NWA 4797 exhibits two igneous lithologies – poikilitic and non-poikilitic. The stone is composed principally of mm-size grains of olivine (40.3%), pyroxene (33.9%), and smaller (80 – 450 μm) regions of interstitial glass or recrystallized glass (after plagioclase; 9.1%). Minor phases include chromite (3.5%), merrillite (0.8%), and melt inclusions (0.4%). The sample is highly vesicu-lated (1.6%). Pyroxene abundances have been further refined using elemental X-ray maps; the modal abundances of pigeonite and augite are 22.1% and 11.8%, respectively.