Primitive Olivine-phyric Shergottite Nwa 5789: Petrography, Mineral Chemistry and Cooling History Imply a Magma Similar To

Introduction: Knowledge of martian igneous and mantle composition is crucial for understanding mantle evolution including early differentiation, mantle convection and the chemical alteration of the surface. Primitive magmas provide critical information on their mantle source regions but most Martian meteorites crystallized from fractionated melts [1]. The new martian meteorite NWA 5789, discovered in North West Africa, is an olivine-pheric shergottite. NWA 5789 may have special significance among the martian meteorites because it appears to have the most primitive magnesian martian magma yet found besides Yamato 980459. With NWA 5789 potentially being a primitive melt it will help elucidate igneous geology and geochemistry of Mars. Sample and method: A chip of 5.6g of NWA 5789 was split and two thick sections (each 1.5-2.0mm thick) were cut, doubly polished and analyzed. Mineral analyses were obtained using the electron microprobe (Cameca SX100) at NASA JSC. Operating conditions were: 15kV accelerating voltage, 20nA beam current, focused beam and measurement times of 20-40s per element. Standards included well characterized natural and synthetic materials. Petrography and Mineralogy: NWA 5789 is an igneous rock with a porphyritic texture of olivine megacrysts and smaller grained phenocrysts (pyroxene, olivine, spinel) in a crystalline mesostasis. The mesostasis itself is composed of radiating sprays of plagioclase, clinopyroxene, spinel and silica. The volume proportions of minerals are: 19% olivine, (14% megacrysts), 38% pyroxene, 1-2% chromite, and 41% mesostasis.