Petrology, Mineralogy and Origin of Olivine Megacrysts of the Martian Meteorite NWA 8716

Martian meteorites provide direct evidence and critical constraints on the evolution of the Martian mantle. Olivine-phyric shergottites are suggested to carry geochemical information of the primitive Martian mantle. Detailed petrological and mineralogical analyses were made for the NWA (Northwest Africa) 8716 olivine-phyric shergottite which is composed of large olivine megacrysts (0.5-1.8 mm) set in a matrix of relatively small olivine (50-400 μm), and pyroxene with interstitial maskelynite, and other accessory minerals. Both the melt inclusion within olivine phenocryst and the calculated bulk rock display an LREE (Light Rare Earth Element)-depleted pattern ([La/Yb]_{CI} 0.06 to 0.1) which is in concordant with some other olivine-phyric shergottites. This indicates NWA 8716 is derived from a depleted Martian magma. The origin of olivine megacrysts is a crucial factor on whether NWA 8716 can be counted to represent composition of a primitive, mantle-derived magma. The crystal size distribution analysis indicates excesses of large crystals which may be due to the entrainment of cumulates. Calculations of Fe-Mg and rare earth element partition reveal that NWA 8716 did not crystallize in a closed system. But estimated compositions of the parent melt are not in large discrepancy with what observed, indicating the megacrysts crystallized from a magma similar to its parent melt. NWA 8716 should have derived from a less evolved depleted reservoir.