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Martian Dunite NWA 2737: Petrographic constraints on geological history, shock events, and olivine color

Received 16 June 2006; revised 26 July 2006; accepted 1 September 2006; published 17 April 2007. [1] Meteorite Northwest Africa (NWA) 2737 is the second known chassignite, an olivine-rich igneous rock with mineral compositions and isotopic ratios that suggest it formed on Mars. NWA 2737 consists of � 85% vol. olivine (Mg#, molar Mg/(Mg + Fe), of 78.3 ± 0.4%), which is notable because it is black in hand sample and brown in thin section. Other minerals include chromite, pyroxenes (augite, pigeonite, orthopyroxene), and diaplectic glass of alkali-feldspar composition. Aqueous alteration is minimal and appears only as slight dissolution of glass. NWA 2737 formed by accumulation of olivine and chromite from a basaltic magma; the other minerals represent magma trapped among the cumulus grains. Minerals are compositionally homogeneous, consistent with chemical equilibration in late and postigneous cooling. Two-pyroxene thermometry gives equilibration temperatures � 1150C, implying a significant time spent at the basalt solidus. Olivine-spinel-pyroxene equilibria give � 825C (possibly the T of mesostasis