Sulfide chemistry of pyroxenites from the External Liguride Peridotite Massif, Italy - Implications for melt-rock interactions in the mantle.

The ophiolites of the Northern Apennine thrust and fold belt represent samples of deep subcontinental lithosphere from an oceancontinent transition in Mesozoic times. The External Liguride (EL) units consist of fertile lherzolites with MOR-type isotopic signatures and diffuse pyroxenite layers. Pyroxenites are a main contributor to mantle heterogeneity, yet their origin and petrogenesis is still disputed. Highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) and the Re-Os isotopic system, which are predominantly controlled by base metal sulfides (BMS), are considered powerful tools to study mantle processes offering a different perspective than conventional lithophile trace elements. To better understand the melt-rock interactions in the lithospheric mantle on an individual grain scale, 22 BMS from two EL pyroxenite samples from Monte Gavi and Rio Strega were analyzed for HSE and 187Os/188Os signatures. The Monte Gavi pyroxenite consists of a coarse-grained matrix of Cpx, Opx, Ol and secondary mica. Base metal sulfides occur as abundant (n=30-40 grains per thin section) large grains (up to 400 μm), systematically associated to mica and adjacent chlorite-filled fractures crosscutting the entire sample. They are Py, Pn and Cpy in close to equal proportions (40%:35%:25%, respectively) frequently intergrown with ilmenite and magnetite. The Rio Strega garnet clinopyroxenite has a fine-grained matrix of Opx, Cpx, Grt, Spl and Plag. Base metal sulfides are as abundant as in the Monte Gavi pyroxenite, smaller in size (50-100 μm) and show a different sulfide assemblage (Po:Pn:Cpy = 55%:25%:20%). They are located at Cpx-Plag grain boundaries showing no association with oxides. In the BMS, HSE concentrations span over two orders of magnitude, ranging from 1 ppb to 5 ppm, showing an overall broad, positive CI-chondrite normalized HSE pattern (Pd/Ir = 29-59; Re/Os = 2-17). Their 187Os/188Os signatures vary from unradiogenic to highly radiogenic (0.1081-0.3745) with one outlier at 1.0062. The pyroxenites, which formed by melt-rock reaction between a silicate melt and a peridotitic protolith, have thus inherited their 187Os/188Os signatures at the grain scale from both the mantle residue and metasomatic agent. Additionally, the BMS are comparatively less radiogenic than the respective bulk pyroxenite (187Os/188Os: 0.6906 for Monte Gavi, 1.0544 for Rio Strega), suggesting that radiogenic Os, and potentially other HSE, are also significantly hosted by platinum group minerals or silicates. Ultimately, this study highlights the necessity of considering not only bulk rock isotopic signatures, but also the mineral scale ones, when studying the heterogeneity of the terrestrial mantl