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

Serpentinites formed by alteration of oceanic and forearc mantle are major volatile and fl uid-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 fl uid-mediated mass transfer during subduction. Altered oceanic crust and sediments have been shown to release in the subarc mantle 11B-depleted fl uids, which cannot explain 11B enrichment of many arcs. In contrast to these crustal reservoirs, we document high 11B values retained in subduction-zone Alpine serpentinites. No 11B fractionation occurs in these rocks with progressive burial: the released 11B-rich fl uids uniquely explain the elevated 11B of arc magmas. B, O-H, and Sr isotope systems indicate that serpentinization was driven by slab fl uids that infi ltrated the slab-mantle interface early in the subduction history.