Petrogenesis of the nakhlite meteorites: Evidence from cumulate mineral zoning

Abstract

A simple igneous petrogenesis for the meteorite Nakhla has previously been called into question because Mg Fe ratios in olivine indicate substantial disequilibrium between the predominant cumulus minerals (olivine and augite). Comparative analyses of simulated diffusive zoning and the observed cumulus mineral zoning for all three nakhlites (Nakhla, Governador Valadares, and Lafayette) show that their current compositions do not necessarily reflect parental magma compositions. Instead, diffusion has altered primary cumulus compositions to varying degrees, Nakhla being the least affected, and Lafayette being almost completely re-equilibrated. Only the cores of augite grains in Nakhla and Governador Valadares appear to have preserved their original compositions. Mineral zoning in each meteorite is strongly concentric around mesostasis areas, suggesting that reaction with intercumulus liquid has controlled the observed zoning. The presence of pigeonite and orthopyroxene overgrowths in Nakhla and Governador Valadares, and of poikilitic orthopyroxene enclosing olivine relicts in Lafayette, indicate substantial late-magmatic reactions. Two-pyroxene geothermometry for Lafayette indicates temperatures around 950°C, suggesting subsolidus equilibration as well. The nakhlites appear to be a series of relatively simple cumulate rocks which have undergone various amounts of late-magmatic and subsolidus diffusion, possibly reflecting their relative positions in a cooling cumulate pile.