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

摘要

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 mark-ers of fluid-mediated mass transfer during subduction. Altered oceanic crust and sediments have been shown to release in the subarc mantle B-depleted fluids,which cannot explain 11B enrichment of many arcs. In contrast to these crustal reservoirs,we document high values retained in subduction-zone Alpine serpentinites. No 11B fractionation occurs in these rocks with progressive burial:the released 11B-rich fluids uniquely explain the elevated 11B 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.