Boron isotope evidence for shallow fluid transfer across subduction zones by serpentinized mantle

Abstract

Serpentinites formed by alteration of oceanic and forearc mantle are major volatile and fluid-mobile element reservoirs for arc magmatism, though direct proof of their dominance in the subduction-zone volatile cycles has been elusive. Boron isotopes are established markers of fluid-mediated mass transfer during subduction. Altered oceanic crust and sediments have been shown to release in the subarc mantle 11 B-depleted fluids, which cannot explain 11 B enrichment of many arcs. In contrast to these crustal reservoirs, we document high δ 11 B values retained in subduction-zone Alpine serpentinites. No 11 B fractionation occurs in these rocks with progressive burial: the released 11 B-rich fluids uniquely explain the elevated δ 11 B of arc magmas. B, O-H, and Sr isotope systems indicate that serpentinization was driven by slab fluids that infiltrated the slab-mantle interface early in the subduction history.