Boron Isotope Evidence for Shallow Fluid Transfer Across Subduction Zones by Serpentinized Mantle

Boron Isotope Evidence for Shallow Fluid Transfer Across Subduction Zones by Serpentinized Mantle M. Scambelluri (1), S. Tonarini (2), S. Agostini (2), E. Cannaò (1) (1) Dipartimento di Scienze della Terra, Ambiente e vita, University of Genova, Italy (2) Istituto di Geoscienze e Georisorse-CNR, Pisa, Italy In subduction zones, fluid-mediated chemical exchange between slabs and mantle dictates volatile and incompatible element cycles and influences arc magmatism. Outstanding issues concern the sources of water for arc magmas and its slab-to-mantle wedge transport. Does it occur by slab dehydration beneath arc fronts, or by hydration of fore-arc mantle and subsequent subduction of the hydrated mantle? So far, the deep slab dehydration hypothesis had strong support, but the hydrated mantle wedge idea is advancing supported by studies of fluid-mobile elements in serpentinized wedge peridotites and their subducted high-pressure (HP) equivalents. Serpentinites are volatile and fluid-mobile element reservoirs for subduction: their dehydration causes large fluid and element flux to the mantle.However, direct evidence for their key role in arc magmatism and identification of dehydration environments has been elusive and boron isotopes can trace the process. Until recently, the altered oceanic crust (AOC) was considered the 11B reservoir for arcs, which largely display positive δ11B. However, shallow slab dehydration transfers 11B to the fore-arc mantle and leaves the residual AOC very depleted in 11B below arcs. Here we present high positive δ11B of HP serpentinized peridotites from Erro Tobbio (Ligurian Alps), recording subduction metamorphism from hydration at low-grade to eclogite-facies dehydration. We show a connection among serpentinite dehydration, release of 11B-rich fluids and arc magmatism. The dataset is completed by B isotope data on other HP Alpine serpentinites from Liguria and Lanzo Massif. In general, the δ11B of these rocks is heavy (16 to + 30 permil). No significant B loss and 11B fractionation occurs with burial. Their B and 11B abundance shows that high budgets acquired during shallow hydration are transferred to HP fluids, providing the heavy-boron component requested for arcs. The B compositions of Erro-Tobbio are unexpected for slabs, deputed to loose B and 11B during dehydration: its isotopic composition can be achieved diluting in the mantle shallow subduction-fluids (30 km). The serpentinizing fluids and the fluid-transfer mechanism in Erro-Tobbio are clarified integrating B with O-H and Sr isotopes. Low δD (-102permil), high δ18O (8permil) of early serpentinites suggest low-temperature hydration by metamorphic fluids. 87Sr/86Sr (0.7044 to 0.7065) is lower than oceanic serpentinites formed from seawater. We conclude that alteration was distant from mid-ocean ridges and occurred at the slab-mantle interface or in forearc environments. We thus provide evidence for delivery of water and 11B at sub-arcs by serpentinized mantle altered by subduction-fluid infiltration atop of the slab since the early stages of burial, witnessing shallow fluid transfer across the subduction zone. Similarity of the B composition of Erro Tobbio with other Alpine serpentinized peridotites suggests that these materials might have spent much of their subduction lifetime at the plate interface, fed by B and 11Bich fluids uprising from the slab.