Chlorine isotope evidence for multicomponent mantle metasomatism in the Ivrea Zone

Abstract

Metasomatism of the Finero peridotite (Ivrea Zone, Italy) has previously been ascribed to mantle melts generated in a rift or plume setting, and/or to subduction fluids. We combine Cl, H, and O stable isotope geochemistry with petrologic and major and trace element data to further constrain sources and setting of the metasomatic agents and to assess fluid infiltration mechanisms. Relatively unmetasomatized samples from the Balmuccia peridotite serve as our control population (δ 37Cl = − 0.7 to + 0.4‰; n = 13). We analyzed 47 samples from four Finero rock types: (1) spinel harzburgite with minor amphibole ± phlogopite, (2) harzburgite with several percent phlogopite ± amphibole, (3) amphibole-rich segregations, and (4) phlogopite segregations. δ 37Cl values range from − 2.0 to + 2.1‰ and cannot be reconciled with metasomatism by a single homogeneous fluid. Three components describe most of the sample variability: (A) endmember spinel peridotite, as represented by the Balmuccia samples; (B) a “hydrated clinopyroxenite” component with low δ 37Cl and high Si, Al, Ca and δD responsible for formation of Type 3 amphibole segregations; and (C) a component with high δ 37Cl, δD, Cl, Cr, LILE, and HFSE that formed Type 4 phlogopite segregations. Type 1 and 2 peridotite compositions are consistent with different degrees of mixing between these three components. Centimeter-scale isotopic gradients indicate that much of the fluid infiltration was channelized rather than pervasive. The data support metasomatism in a forearc wedge, with fluids sourced from at least two subducted components with distinct chemical and isotopic compositions (e.g., δ 37Cl < − 2‰ and >+2‰). Overall, our results show that relatively shallow and heterogeneous fluid release events from the downgoing slab can profoundly modify the mantle wedge.