Mantle amphibole trace-element and isotopic signatures trace multiple metasomatic episodes in lithospheric mantle, western Victoria, Australia

Abstract

Peridotite xenoliths from western Victoria, Australia, contain varying proportions of amphibole (±apatite). The xenoliths also exhibit varying degrees of metasomatic enrichment as indicated by the rare-earth element (REE) patterns of clinopyroxene and amphibole analysed in situ by laser-ablation microprobe ICP–MS and by whole-rock trace-element patterns. Trace-element patterns allow the samples to be divided into three groups. Group A is characterised by flat REE patterns for whole-rock, clinopyroxene and amphibole, and small negative or absent high-field-strength element (HFSE) anomalies; Group B features high levels of light-REE enrichment, pronounced negative HFSE anomalies and low Ti/Eu in whole-rock trace-element patterns; and Group C shows positive HFSE anomalies in whole-rock patterns. The different trace-element patterns are consistent with metasomatism by different agents or combinations of agents, which have acted upon the already modally metasomatised sub-continental lithospheric mantle (SCLM) beneath western Victoria. Geochemical signatures suggest that Group A samples have undergone silicate melt metasomatism, Group B samples have undergone metasomatism by a carbonate-rich or carbonatitic fluid and Group C samples may record metasomatism by a fractionated silicate fluid with hydrous component.