Experimental insights into the mineralogy and melt-rock reactions produced by lunar cumulate mantle overturn

Abstract

Hybridization of the lunar mantle during the overturn (sinking) of Fe- and Ti–rich ilmenite-bearing clinopyroxenite cumulates (IBC) in the lunar interior is called upon to explain the high TiO₂ abundances of lunar basalts. Chemical reactions that occur after juxtaposition of IBC and mantle peridotite are poorly constrained. We experimentally investigated these reactions in experiments that adjoin an IBC glass against presynthesized dunite in a reaction couple at temperatures of 1100–1300 °C and pressures of 0.5–2.02 GPa for 0.33–31.66 h. These conditions produced experiments near to well above the solidus temperature of the IBC. Near solidus experiments produce garnet in the IBC at 2 GPa. Supersolidus experiments exhibit dissolution of olivine material into the IBC melt and the formation of clinopyroxene at the IBC melt-dunite interface. Dunite dissolution is attributed to the olivine undersaturated composition of the IBC melt. In both near- and supersolidus experiments, compositional variations produced by solid-state diffusion across the IBC melt-dunite interface are observed. When pressure increases, temperature decreases, or IBC melts become closer to olivine saturation, dissolution slows, and the effects of solid-state diffusion in the dunite become more evident. Similar chemical exchange reactions would occur in the lunar mantle as downwelling IBC and lunar peridotites are juxtaposed by cumulate overturn. Hybridized lunar mantle sources are expected to contain 47–84% normative peridotite and 16–53% IBC. Simple numerical simulations suggest that in addition to dissolution–precipitation reactions, mechanical mixing may be required to produce volumetrically significant hybridized mantle sources over geologically-relevant timescales.