Lithium behavior during cooling of a dry basalt: An ion-microprobe study of the lunar meteorite Northwest Africa 479 (NWA 479)

Northwest Africa 479 (NWA 479) is a lunar meteorite recovered in 2000 from Morocco. This unbrecciated low-Ti basalt is paired with NWA 032. The texture of NWA 032/479 indicates a simple crystallization history and a fast cooling, followed by an impact event. The occurrence of high-pressure polymorphs of olivine (ringwoodite and wadsleyite) in shock-melt veins indicates shock-pressures of at least 20 GPa.Lithium abundances and isotopic compositions were measured by ion microprobe in pyroxene, olivine crystals, and magmatic inclusions. The δ⁷Li values in the magmatic inclusions indicate that the NWA 479 parental melt was enriched in ⁷Li (δ⁷Li = +15‰). The behavior of Li depicted by the phenocrysts is complex and is not controlled by their major element compositions. Li abundances and δ⁷Li values range respectively from 3.2 to 11.8 μg/g and +2.4 to +15.1‰ in olivine and from 2.8 to 18.4 μg/g and −0.2 to + 16.1‰ in pyroxene phenocrysts. Neither hot desert weathering, closed-system fractional crystallization, involvement of a low-δ⁷Li reservoir, degassing of NWA 479 parental melt, nor shock metamorphism correctly explain the Li distribution in the phenocrysts. We propose that the wide range of δ⁷Li values displayed by the phenocrysts results from the large diffusivity differences between ⁶Li and ⁷Li. It is shown that this difference is able to produce large isotopic heterogeneities in a very short time.