Partial re-equilibration of highly siderophile elements and the chalcogens in the mantle: A case study on the Baldissero and Balmuccia peridotite massifs (Ivrea Zone, Italian Alps)

Abstract

The conditions at which melt percolation and reactive infiltration of depleted mantle peridotites fractionate highly siderophile elements (HSE) and cause re-equilibration of 187Os/188Os in mantle rocks are still poorly constrained. In a comparative study of the Paleozoic Balmuccia (BM) and Baldissero (BD) peridotite tectonites (Ivrea-Verbano Zone, Northern Italy), the influence of partial melting and melt infiltration on abundances of HSE, chalcogens (S, Se and Te) and 187Os/188Os have been studied.At BM, Re depletion ages (TRD) of lherzolites and replacive dunites display a uniform distribution with a maximum near 400–500Ma. BD peridotites also display a Paleozoic distribution peak but a significant number of samples yielded Proterozoic TRD. The predominance of Paleozoic Re depletion ages in both bodies is consistent with Sm–Nd ages and the late Paleozoic magmatic and geodynamic evolution of the Ivrea-Verbano Zone. The different extents of preservation of ancient 187Os/188Os in BM and BD peridotites are interpreted to reflect different degrees of isotopic homogenization and chemical re-equilibration with incompatible element-depleted infiltrating melt during the Paleozoic. The differences between the two bodies are also reflected by differences in HSE and chalcogen abundances, with BD displaying large scatter among HSE patterns, slight re-enrichment of Re relative to Au, and linear trends of Pd, Se and Te with Al2O3. The differences in distributions of model ages and heterogeneity in HSE abundances support the view that the lithophile element, HSE and chalcogen variations of different suites of lherzolites likely reflect different extents of reactive melt infiltration in mantle peridotites, with partial re-equilibration and melt extraction in open system environments. However, the variable re-equilibration of BM and BD lherzolites apparently did not produce significant differences in HSE ratios such as Os/Ir, Ru/Ir, Rh/Ir, and Pd/Pt, which are in the range of primitive mantle values, whereas Au, Re and S are somewhat depleted.The good linear correlation of S with Al2O3 in both suites reflects sulfide removal controlled by sulfur solubility in silicate melt, or co-precipitation with pyroxenes and spinel, and indicates very similar bulk partition coefficients for S and Al. S/Se and Se/Te in the lherzolites change little with decreasing Al2O3. Results for BM lherzolites are consistent with sulfide–silicate melt partitioning as the dominant control on abundances of the HSE, S, Se and Te during low to moderate degrees of melt extraction (DPt>DPd>DTe⩾DSe⩾DS≈DRe).Replacive dunites at Balmuccia have low abundances of Re, Au, Pd and chalcogens, but variable and higher abundances of Os, Ir and Ru, high S/Se and Se/Te, yet 187Os/188Os similar to BM lherzolites. The residual HSE and chalcogen compositions differ from those in dunites of subduction-related ophiolites. The composition and contact relations of the BM dunites with the host rocks likely reflect focused flow of sulfur-undersaturated melt after open system melting and re-equilibration of the lherzolites. The compositional record of the Balmuccia massif thus reflects the composition of different types of melts and their interaction with the peridotites at different P–T conditions.