Shock conditions recorded in NWA 8159 martian augite basalt with implications for the impact cratering history on Mars
Tom G. Sharp, Erin L. Walton, Jinping Hu, Carl Agee
Geochimica et Cosmochimica Acta
In Press, Accepted Manuscript, Available online 24 November 2018
“NWA 8159 is an augite-rich martian basalt, formed by cooling of a relatively evolved, Ca-rich, Ti-poor and LREE-depleted lava, under relatively oxidizing conditions, during the early Amazonian. In addition to its distinct igneous petrogenesis and high fO2, NWA 8159 is also set apart from most martian shergottites with respect to the low degree of shock metamorphism required to preserve crystalline igneous plagioclase (An50-65). In this study, mineral transformations within and adjacent to shock veins in NWA 8159 were investigated using scanning electron microscopy, Raman spectroscopy and transmission electron microscopy to better constrain the unusal shock history of this meteorite. The transformation of olivine to ahrensite (Fe-ringwoodite) along shock vein margins, and tissintite and coesite formed from igneous mineral (labradorite and silica) grains entrained as clasts within shock veins has been documented in this study. We report on a previously unidentified mineral assemblage of Ca-Na-majoritic garnet, sodic-clinopyroxene and stishovite crystallized from shock melt. This mineral assemblage indicates a crystallization pressure of approximately 16 GPa, which is within the range of previous shock pressure estimates for this meteorite (15–23 GPa). The presence of a majoritic garnet-bearing assemblage throughout veins up to 0.6 mm wide indicates that the sample remained at high-pressure throughout the melt vein quench. Based on thermal models, the sample must have remained at high pressure for ∼100 ms. This shock duration is an order of magnitude longer than those experienced by more highly shocked shergottites such as Tissint or Zagami (>30 GPa; 10–20 ms) and would seem to imply a relatively large impact event. Recent numerical models demonstrate that a range of shock pressures and durations are realized by rocks within the ejected spall zone of a hypervelocity impact. The shock conditions experienced by NWA 8159 therefore do not require an impact event distinct from other shergottites. Rather, our findings suggest that this meteorite originated from near the martian surface at the edge of the impact site. The shock history of NWA 8159 provides a picture of Mars consistent with that derived from remote observation; that of a random cratering process that samples a geologically long-lived and complex planet.”
