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 delta(7)Li values in the magmatic inclusions indicate that the NWA 479 parental melt was enriched in Li-7 (delta(7)Li = +15 parts per thousand). The behavior of Li depicted by the phenocrysts is complex and is not controlled by their major element compositions. Li abundances and delta(7)Li values range respectively from 3.2 to 11.8 mu g/g and +2.4 to +15.1 parts per thousand in olivine and from 2.8 to 18.4 mu g/g and -0.2 to + 16.1 parts per thousand in pyroxene phenocrysts. Neither hot desert weathering, closed-system fractional crystallization, involvement of a low-delta(7)Li 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 delta(7)Li values displayed by the phenocrysts results from the large diffusivity differences between Li-6 and Li-7. It is shown that this difference is able to produce large isotopic heterogeneities in a very short time. Copyright (c) 2005 Elsevier Ltd.