The parental magma of the Nakhla achondrite: Ultrabasic volcanism on the shergottite parent body

Abstract

The Nakhla meteorite is an augite-rich igneous cumulate rock; magma trapped among the cumulus grains is now represented by fine-grained mesostasis and post-cumulus overgrowths on cumulus grains. The composition of the intercumulus magma, calculated by mass balance, is ultrabasic (SiO2 < 45%) and enriched in incompatible elements (LaLu = 6 × CI). The cumulus augite and intercumulus magma were in chemical equilibrium, implying that the intercumulus magma is a sample of Nakhla's parental magma. Complex processes like infiltration metasomatism were therefore unimportant. Nakhla contains olivine crystals which are as large or larger than the cumulus augite crystals. The largest olivine crystals, probably xenocrysts, preserve core compositions too Fe-rich to have been in equilibrium with the intercumulus magma. Other smaller olivine grains may have been phenocrysts. The closest terrestrial analogs to Nakhla (and the other nakhlites) are pyroxene-rich cumulate layers in differentiated picrite and komatiite flows and shallow sills. Spinel or garnet was fractionated from the parental magmas of the nakhlites or their parental mantle, based on the superchondritic CaOAl2O3 ratio of Nakhla's intercumulus magma and its lack of an Eu anomaly. The three nakhlites and Chassigny are half of the known samples of the Shergottite Parent Body (SPB). One may tentatively infer that ultrabasic volcanism was much more important on the SPB than on the moon, Eucrite Parent Body, or present-day Earth.