Sulphide metasomatism of the Cordilleran lithospheric mantle: Insights from LA-ICP-MS mapping and basalt geochemistry

Sulphide minerals within xenoliths provide critical records for melting and metasomatism in the lithospheric mantle and crust. During the transport of these mantle and crustal xenoliths to the surface, high-temperature sulphide minerals cool and transform to a low-temperature assemblage that includes pyrrhotite, pentlandite, and chalcopyrite. However, the small size and complex intergrowths between these low-temperature minerals present analytical challenges for determining their trace element concentrations, and for estimating the bulk sulphide composition prior to cooling. Herein we combine geochemical mapping by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) with machine learning for a suite of basalt-hosted peridotite, pyroxenite, gabbro, and anorthosite xenoliths from British Columbia, Canada, to reconstruct the bulk sulphide composition and constrain the petrology of the Cordilleran lithospheric mantle. Sulphide mineral compositions extracted from the segmented geochemical maps for the peridotite and pyroxenite xenoliths yield chondritic to sub-chondritic Os/Ir and Pd/Ir ratios, indicating that these samples represent mantle residues that underwent low to moderate degrees of partial melting. However, reconstructed bulk sulphide compositions also yield supra-chondritic Re/Os ratios and high concentrations of relatively incompatible and highly chalcophile trace elements (e.g., Te, Ag, and Se). These enriched geochemical signatures indicate residual sulphide phases must have mixed with a metasomatic and compositionally fractionated sulphide melt. Small volumes of alkaline silicate melts partially preserved with sulphide, feldspar, and other metasomatic minerals in the same micro-textural settings further indicate that sulphide metasomatism occurred in the mantle and prior to volcanism. New two-pyroxene and olivine-spinel geothermometry results demonstrate that these sulphide-bearing lherzolite, websterite, and wehrlite xenoliths equilibrated at a range of temperatures (931–1129 °C). Sulphide minerals with similar compositions from peridotite xenolith localities across British Columbia of varying equilibration temperatures and volcanic ages (i.e., Oligocene to Pleistocene) suggests that sulphide metasomatism is a characteristic feature of the Cordilleran lithospheric mantle. • Machine learning used to segment and combine results from LA-ICP-MS maps. • Sulphide minerals yield PGE ratios that are consistent with mantle residues. • Metasomatic sulphide minerals yield higher concentrations of incompatible elements. • Metasomatic sulphide minerals occur with alkaline silicate melt. • Sulphide metasomatism is characteristic of the Cordilleran lithospheric mantle.