Geocheminar fall-2012

Meteorite Northwest Africa 6693 is a new type of cumulate achondrite, with a unique combination of oxygen isotopic composition (low ?17O) and FeO-rich, low MgO/FeO bulk composition. Cumulus phases include pigeonite (oikocrysts up to 14 mm) and olivine. The parent magma was probably also saturated with feldspar. Mafic silicates are uniformly ferroan (e.g. olivine near Fo49). The feldspar is albitic (near Ab92) with diverse K/Ca ratio: from consistently ~0.46 in one end of the stone, to 5.2 in an enclave of micrographic olivine-feldspar intergrowth. After initial crystallization, minor additional melt formed within, or infiltrated into, the rock, to form discrete overgrowths on some pyroxenes. Final cooling occurred at a moderate intrusive-igneous rate. Olivine, metal, and sulfide phases are all very Ni-rich (e.g., olivine NiO averages 0.77 wt%). Spinel V/(Al+Cr) suggests high oxygen fugacity: fO2 of IW+2. The bulk-rock composition features chalcophile depletions, but nonvolatile lithophile elements are only subtly fractionated from chondritic, and even siderophile concentrations are near-chondritic: Co, Ni, Ir and Os are all at 0.7-1.0×CI chondrites, and Au at 0.55×CI. Apparently the parent melt was not produced by extensive igneous fractionation, and the parent planetesimal was so pervasively oxidizing that FeNi metal did not efficiently sequester the siderophile elements. The high FeO and low MgO/FeO of NWA 6693 were probably in large measure already properties of the original primitive material.