Petrogenesis of the NWA 7320 enriched martian gabbroic shergottite: Insight into the martian crust

1Arya Udry, 2Geoffrey H. Howarth, 3Thomas J. Lapen, 3Minako Righter
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.01.032]
1Department of Geoscience, University of Nevada Las Vegas, Las Vegas NV, USA
2Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa
3Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA
Copyright Elsevier

Northwest Africa (NWA) 7320 is classified as a gabbroic shergottite, the second to be recognized in the martian meteorite record. This interpretation is based on: (1) the calculated bulk-rock rare earth element (REE) concentrations, which show the highest Eu positive anomaly (Eu/Eu∗ = 2.2) of all the shergottites, reflecting accumulation of plagioclase; and (2) the highest modal abundance of maskelynitized plagioclase (50 mod.%) compared to the other shergottites. The three-phase symplectite (fayalite + hedenbergite + silica) is present in NWA 7320 and formed as a result of the breakdown of metastable pyroxene/pyroxenoid margins on coarse-grained pyroxenes. The latter is indicative of metastable overgrowths on pyroxene cores during the final stages of crystallization, followed by relatively slow cooling at subsolidus conditions. The NWA 7320 parental melt originated from an incompatible trace element enriched and oxidized (∼FMQ) source as indicated by Sm–Nd, Lu–Hf isotope systematics, ilmenite-titanomagnetite pairs, the partition coefficient of Cr in pyroxene, and merrillite REE compositions. The Ti/Al ratio of pyroxene in NWA 7320 indicates an initial crystallization depth of 30–70 km (P = 4–9 kbar). However, the largest impact craters on Mars are

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