From crustal protoliths to mantle garnet pyroxenites: insights from Os isotopes and highly siderophile elements

Abstract

Recycled pyroxenites are a key component of mantle heterogeneity, influencing magma generation and contributing to the radiogenic Os signature of oceanic basalts. However, natural examples of pyroxenites as proxies for crust-derived heterogeneities in the convective mantle are rare and recycling mechanisms remain debated. Here we report abundances of highly siderophile (HSE: Os, Ir, Pt, Pd, Re) and chalcogen (S, Se, Te) elements, along with Os isotope compositions, for garnet clinopyroxenites and websterites enclosed in fertile peridotites from the Jurassic Northern Apennine ophiolites. The pyroxenites provide fractionated HSE patterns—depleted in Os and Ir and enriched in Pt, Pd, and Re compared to host lherzolites. Present-day 187Os/188Os ratios range from moderately to highly radiogenic (0.154–2.475). Garnet clinopyroxenites have lower Os, higher Pd/Ir, and more radiogenic Os than websterites. Model calculations based on bulk HSE data indicate the garnet clinopyroxenites reflect sulfur saturation and base metal sulfide (BMS) crystallization from partial melts of eclogites derived from MORB-type gabbroic protoliths. Their Os isotope signature indicates long-term Re/Os enrichment and derivation from ancient mafic protoliths, likely older than 1.0 Ga. The HSE distribution and Os isotopic signature of the websterites, along with their sulfide mineralogy, support a hybrid origin involving contributions from both peridotite- and eclogite-derived components. The websterites may represent natural analogues of the second-stage pyroxenites involved in OIB magma genesis. Host lherzolites exhibit flat CI chondrite-normalized HSE patterns and near-chondritic Se/Te and Pd/Ir ratios, implying initial depletion of incompatible HSEs like Pd and Re, followed by BMS addition during melt percolation that also affected some pyroxenites. Se (78–539 ppb) and Te (13–44 ppb) are significantly enriched in the pyroxenites compared to the lherzolites. The low Se/Te ratios (6–15) in garnet clinopyroxenites could explain the Se–Te signature of plume-influenced E-MORB. We show that mantle pyroxenites with a crustal fingerprint display wide HSE and Os isotope variability, reflecting heterogeneity of crustal protoliths, melt–peridotite interactions, and late melt percolation during their decompression history.