Carbonate metasomatism at mantle wedge conditions, evidence from trace element and stable isotope (Fe, Zn) signatures of orogenic peridotites

The composition and redox state of the mantle wedge over geological time can be impacted by fluid transfer from the slab during subduction. Although arc magmas are oxidised and enriched in fluid-mobile elements relative to mid-ocean ridge basalts (MORB), the nature of the fluid phase (aqueous or melts) produced by the slab in mantle wedge remains debated. Here we compare the elemental and isotopic (Fe and Zn isotopes) composition of both unmetasomatised and metasomatized ultramafic rocks from the Western Alps, respectively the Balmuccia and Finero massifs, to identify and characterise the relative effects of subduction-related processes on mantle peridotite composition. The metasomatism of Finero massif is evidenced by Light Rare Earth Element (LREE), U and Th enrichment coupled with isotopically light Zn and Fe signatures and an increase in oxygen fugacity relative to the MORB mantle-like peridotites of the Balmuccia massif. Negative correlations between LREE/HREE and U/Th ratios in metasomatized samples suggest preferential transport of Th relative to U in the infiltrating phase. Based on experimental constraints on fluid/melt partitioning, these observations are most consistent with Th dissolution in slab-derived melts. On the other hand, the light Zn isotope signatures in the Finero peridotites relative to those of Balmuccia peridotites are inconsistent with metasomatism by silicate melts and melt extraction processes. Trace elements and Zn isotopes results can be reconciled through the metasomatism of the mantle wedge by carbonate bearing fluids or melts in an open system. This process favours the formation of isotopically light metasomatic minerals in the Finero peridotite and the subsequent release of isotopically heavy CO2-rich fluids or melts, probably during massif exhumation.