Mantle metasomatism beneath western Victoria, Australia: I. Metasomatic processes in Cr-diopside lherzolites

Most Cr-diopside spinel lherzolite xenoliths from Bullenmerri and Gnotuk Maars, western Victoria, show modal metasomatism involving the growth of amphibole ± mica ± apatite at the expense of primary pyroxenes + spinel. The metasomatism is attributed to CO₂-rich fluids, observed in fluid inclusions. Median values of Ni, Cr, V, Sc, Y, Ti, K and Na are similar to those in depleted mantle xenoliths and the source regions of N-type MORB. Median concentrations of Ba, Th, U, Ta, Nb, Sr and REE range from 1-10 times "primordial" values. REE patterns of anhydrous lherzolites range from LREE-depleted ((La/Yb)_n ≈ 0.3) to LREE-enriched ((La/Yb)_n = 30-60), and show an inverse correlation of Nd/Sm with CaO. Amphibole-rich peridotites are enriched in LREE ((La/Yb)_n = 10-30), Zr and Ta, with high K/Rb. Mica-rich rocks are enriched in K, Rb, Ba, Ta and Ti, with low K/ Rb. Introduction of apatite leads to high σREE (with (La/Yb)_n = 40-100), Sr, U and Th contents. The distribution of trace and minor elements in the Iherzolites is thus controlled by the crystal chemistry of the primary and metasomatic phases. Micaceous xenoliths may be derived from thin selvedges on pyroxenite veins. Abundant amphibole lherzolites may form a matrix enclosing relict volumes of anhydrous lherzolites showing varying degrees of cryptic metasomatism. The overall pattern of trace-element enrichment in the mantle beneath Victoria will depend on the volumetric proportions of these rock types.