The distribution of lithium in peridotitic and pyroxenitic mantle lithologies - an indicator of magmatic and metasomatic processes

Lithium concentrations in orthopyroxene, clinopyroxene, olivine, garnet or spinel from equilibrated spinel peridotite, garnet peridotite and garnet pyroxenite xenoliths and from metasomatised peridotites and pyroxenites were measured using the ion-microprobe (SIMS). With respect to changes in physical and chemical parameters, we present new data on lithium contents in mantle minerals and its partitioning behaviour in (1) equilibrated and (2) metasomatised samples. Lithium is preferentially incorporated into olivine, ranging between 1 and 2 ppm in equilibrated unmetasomatised peridotites. Pyroxenes from peridotitic xenoliths have Li concentrations on the order of several hundred ppb up to 1.3 ppm, while pyroxenes from pyroxenites have somewhat higher abundances (1-3 ppm with a maximum of 19 ppm). The following partitioning relationships have been established: ol > cpx greater than or equal to opx much greater than grt or sp for garnet and spinel peridotites, respectively, and cpx 2 opx > grt for garnet pyroxenites. The intercrystalline partitioning of Li is independent of T, P and bulk composition (for ultramafic to mafic compositions), making Li a suitable tracer element for chemical processes such as metasomatism. We estimate a bulk Li content of 1.0-1.5 ppm for fertile to moderately depleted lithospheric mantle. Low Li abundances in mantle peridotites and pyroxenites emphasise its incompatibility during partial melting and fractional crystallisation. However, elevated Li concentrations are present in some pyroxenites, presumably due to complete crystallisation of trapped partial melts. Metasomatised samples from peridotite massifs (Pyrenees and Ivrea Zone) and mantle nodules from two Victorian volcanic fields (Australia) clearly show enrichment of Li in both olivine and clinopyroxene, whereby the distribution of Li between olivine and clinopyroxene has generally not achieved equilibrium. Disequilibrium is manifested by preferential Li enrichment in either olivine or clinopyroxene depending on the type of metasomatic agent involved (carbonatite vs. mafic silicate melt). Differences in absolute Li abundances and in its partitioning behaviour allow the identification not only of metasomatic overprints, but also of magmatic processes such as partial melting, crystal fractionation and accumulation. The sensitivity of Li as such a chemical tracer gives an additional criterium for recognising cryptic metasomatism. (C) 2000 Elsevier Science B.V. All rights reserved.