Mesoarchean melting and Neoarchean to Paleoproterozoic metasomatism during the formation of the cratonic mantle keel beneath West Greenland

Highly siderophile element (HSE) concentration and ¹⁸⁷Os/ ¹⁸⁸Os isotopic heterogeneity has been observed on various scales in the Earth's mantle. Interaction of residual mantle peridotite with infiltrating melts has been suggested to overprint primary bulk rock HSE signatures originating from partial melting, contributing to the heterogeneity seen in the global peridotite database. Here we present a detailed study of harzburgitic xenolith 474527 from the Kangerlussuaq suite, West Greenland, coupling the Re–Os isotope geochemistry with petrography of both base metal sulfides (BMS) and silicates to assess the impact of overprint induced by melt-rock reaction on the Re–Os isotope system. Garnet harzburgite sample 474527 shows considerable heterogeneity in the composition of its major phases, most notably olivine and Cr-rich garnet, suggesting formation through multiple stages of partial melting and subsequent metasomatic events. The major BMS phases show a fairly homogeneous pentlandite-rich composition typical for BMS formed via metasomatic reaction, whereas the ¹⁸⁷Os/ ¹⁸⁸Os compositions determined for 17 of these BMS are extremely heterogeneous ranging between 0.1037 and 0.1981. Analyses by LA-ICP-MS reveal at least two populations of BMS grains characterized by contrasting HSE patterns. One type of pattern is strongly enriched in the more compatible HSE Os, Ir, and Ru over the typically incompatible Pt, Pd, and Re, while the other type shows moderate enrichment of the more incompatible HSE and has overall lower compatible HSE/incompatible HSE composition. The small-scale heterogeneity observed in these BMS highlights the need for caution when utilizing the Re–Os system to date mantle events, as even depleted harzburgite samples such as 474527 are likely to have experienced a complex history of metasomatic overprinting, with uncertain effects on the HSE.