Mantle metasomatism beneath western Victoria, Australia: II. Isotopic geochemistry of Cr-diopside lherzolites and Al-augite pyroxenites

Cr-diopside spinel lherzolite xenoliths from Victoria show metasomatic introduction of amphibole ± mica ± apatite, accompanied by enrichments in Sr, LREE and other incompatible elements. The metasomatism can be related to fluids emanating from crystallizing basaltic magmas now represented by dykes of pyroxenite. Abundant garnet and spinel metapyroxenites are products of an older magmatic episode; igneous-textured wehrlite series xenoliths represent a younger (Recent ?) episode. Sr-Nd isotopic compositions of the lherzolites spread into the "enriched mantle" field (ε{lunate}_Nd < 0, ε{lunate}_Sr > 0). Nd isotopic systematics suggest a maximum age for LREE enrichment of <700 Ma. At 300-500 Ma, the metapyroxenites define a mixing hyperbola with the lherzolites, most of which have ε{lunate}_Sr > 0 at this time. More detailed mixing models require a third (apatite-rich component with present-day ε{lunate}_Sr and ε{lunate}_Nd ≈0). The least-modified metapyroxenites have very unradiogenic Nd and unsupported radiogenic Sr, suggesting a large component of older recycled crustal material. The origin of the "apatite component" is uncertain.