Northwest Africa 6693: A new type of FeO-rich, low-Δ17O, poikilitic cumulate achondrite

Abstract

Northwest Africa 6693 is a new type of achondrite, with a unique combination of oxygen-isotopic composition (low Δ17O: −1.08‰; also δ17O=1.19‰) and FeO-rich, low mg bulk composition. A mode (in vol%) shows 70% pyroxene, 16% olivine and 13% feldspar, along with 0.6% Cr-spinel, and 0.4% NiFe metal (awaruite). Its coarse-poikilitic texture, with pigeonite oikocrysts up to 14mm, as well as the subchondritic MgO/SiO2 of the rock’s bulk composition, indicate origin as an igneous cumulate. The cumulus phases included pigeonite and olivine, and the parent magma was probably also saturated with feldspar, which occurs mainly as anhedral, yet optically continuous, grains intergrown with the pyroxene. The mafic silicates are uniformly ferroan: pigeonite near En57Wo3.2 and olivine near Fo49. The feldspar is uniformly albitic, near Ab92, except for a single tiny grain of Ab57Or43. However, the albite features diverse K/Ca (Or/An) ratios: ranging from consistently ∼0.46 in one end of the oblong NWA 6693 stone, to 5.2 in an olivine-rich enclave that consists mostly of micrographic olivine–feldspar intergrowth. Also, siderophile and incompatible element data show heterogeneity among samples from different regions of this large cumulate. The rock was probably neither an orthocumulate nor an adcumulate, and the proportion of “trapped liquid” probably varied from place to place. After initial crystallization, a shock event caused very minor brecciation, and pervasively mobilized linear-arcuate trails of microinclusions (minute oxides, mostly) and bubbles. A minor proportion of additional melt was formed within, and/or infiltrated into, the rock and formed discrete overgrowth mantles, recognizable based on unusual scarcity of microinclusions, on some pyroxenes. Final cooling, based on mineral-equilibration temperatures, occurred at a moderate rate by intrusive-igneous standards.Olivine, metal, and sulfide phases are all very Ni-rich (e.g., olivine NiO averages 0.77wt%). Evidently the partitioning of NiO into the parent melt was extraordinarily high, which suggests a commensurately high oxygen fugacity. The V/(Al+Cr) ratio within spinel suggests that fO2 was IW+2. The bulk-rock composition features strong depletions in sulfur and chalcophile elements, but nonvolatile lithophile elements are only subtly fractionated from chondritic. Even siderophile element concentrations are near-chondritic; Co, Ni, Ir and Os are all at 0.7–1.0×CI chondrites, and Au at 0.55×CI. The limited fractionation of the siderophile elements may reflect igneous processing in a parent body so pervasively oxidizing that FeNi metal did not play its usual planetary role as agent for efficient sequestration of siderophile elements. More generally, the limited fractionation among nonchalcophile, nonvolatile elements suggests that the parent melt was not produced by extensive fractional crystallization; i.e., the high FeO and low mg of NWA 6693 were probably in large measure already properties of the original primary magma, produced from an extremely oxidized (FeO-rich) variety of primitive material. NWA 6693 indicates that high oxygen fugacity inherited from oxidized-chondritic building blocks may persist within small bodies, despite melting extensive enough to engender igneous cumulates.