Tracing Deep Melt Infiltration in the Mantle Using Sulphide, HSE, and Re–Os Isotope Systematics from Ligurian Pyroxenite–Peridotite Associations

Abstract Deep melt infiltration and melt–peridotite interaction are fundamental processes in generating chemical and isotopic heterogeneity in the upper mantle. These processes promote the formation of pyroxenite-bearing mantle domains and thus play a key role in the development of veined and compositionally enriched mantle sources commonly invoked to explain oceanic and continental magmatism. However, natural analogues of such pyroxenite-bearing fertile mantle domains are rare, and the processes controlling the chemical modification of mantle sources through pyroxenite emplacement remain poorly constrained. We present an integrated petrological and geochemical study of pyroxenite–peridotite associations from the External Liguride ophiolites (Northern Apennines, Italy), combining whole-rock HSE abundances and Re–Os isotopes with spatially constrained analyses of individual base-metal sulphides (BMS). These mantle sequences represent an exceptionally well-preserved record of (spinel-facies) melt infiltration and pyroxenite emplacement at ca. 430 Ma within ancient subcontinental lithospheric mantle. Peridotites show near–primitive upper mantle HSE patterns and unradiogenic 187Os/188Os ratios, indicating that bulk-rock HSE systematics were largely unaffected by pyroxenite emplacement. In contrast, pyroxenites exhibit large variations in HSE abundances and Os isotopic compositions, defining three compositional groups that correlate with major-element chemistry and layer thickness. Clinopyroxenites display strongly fractionated, melt-like HSE signatures, whereas websterites show progressively less fractionated patterns approaching those of the host peridotite, reflecting progressively greater chemical equilibration and buffering by the host peridotite. Forward modelling indicates that the observed Pd/Ir systematics of pyroxenites can be reproduced by variable extents of sulphide mixing and reactive melt percolation. Single-sulphide analyses reveal significant HSE and Os isotopic heterogeneity at the grain scale, along a pyroxenite-peridotite profile, documenting localized sulphide hybridization during melt infiltration. These results demonstrate that base-metal sulphides preserve a sensitive record of melt-peridotite chemical equilibration and exert a primary control on HSE redistribution during mantle melt infiltration. Our results provide new constraints on the development of mantle heterogeneity and on the long-term chemical evolution of subcontinental lithospheric mantle domains.