Volatile and mobile element recycling during subduction of the oceanic lithosphere. Insights from metasediments and serpentinites of the Alps.

We discuss some aspects of the release of light elements and volatile components during subduction of serpentinite and metasedimentary systems. Enrichment in B, CI, and Sr during serpentinization of the oceanic mantle is followed by their progressive release in the subduction fluids. Olivine-orthopyroxene rocks formed after significant serpentinite dehydration still contain bulk B, CI and Li considerably higher than in mantle reservoirs and may introduce light element anomalies into the upper mantle. The high to ultrahigh pressure metasediments underwent little or no devolatilization, nor N release, down to at least 90 km, acting as closed systems to the infiltration of externally derived fluids. They therefore reflux to the mantle appreciable amounts of exogenic N. This behaviour is supported by the oxygen isotope geochemistry of high and ultrahigh pressure rocks, showing closed system behaviour and limited fluid mobility at these depths. Many high pressure rocks thus do not represent the levels of fluid extraction from slabs; we suggest that the antigorite breakdown may represent a dehydration reaction creating a mobile, trace element-enriched, subduction fluid