Magnesium isotope record of fluid metasomatism along the slab-mantle interface in subduction zones

Fluids released from subducting slabs play an important role in mantle metasomatism and arc magmatism. Magnesium could be mobile in subduction-zone fluids and Mg isotopic signatures of mélange rocks may provide information on crust-mantle interactions at subduction zones. To investigate the behaviour of Mg isotopes during fluid metasomatism along the slab-mantle interface, we report Mg isotopic compositions of a set of well-characterized samples from the ultramafic blocks in the Franciscan Complex of California. The partially serpentinized and completely serpentinized peridotites have δ26Mg values (from −0.26 to −0.14) clustered around the mantle value, indicating small Mg isotope fractionation during peridotite serpentinization. Samples with serpentine completely replaced by talc have lower MgO contents and heavier Mg isotopic compositions (δ26Mg of −0.13 to −0.01) than serpentinized peridotites, implying that Mg was non-conservative during this metasomatic replacement. The shift toward higher δ26Mg values of talc-dominated metaperidotites reflects the preferential loss of light Mg isotopes into fluids via Rayleigh distillation during the dehydration reactions that produced talc from serpentine. On the other hand, samples with talc completely replaced by tremolite have lighter Mg isotopic compositions (δ26Mg of −0.50 to −0.41) and higher CaO contents than the talc-dominated samples. These features suggest a metasomatic process involving isotopically light carbonate-rich fluids derived from the subducting slab. Collectively, large Mg isotope fractionation occurs during multistage fluid-rock interactions within the Franciscan subduction channel, which provides direct evidence for significant Mg isotopic variation during fluid metasomatism along the slab-mantle interface. This may account for the observed heterogeneous Mg isotopic compositions of arc lavas. Magnesium isotopes thus could be a potentially useful tracer of crust-mantle interactions at subduction zones.