HIGH SALINITY FLUID INCLUSIONS FORMED FROM RECYCLED SEAWATER IN DEEPLY SUBDUCTED ALPINE SERPENTINITE

The origin of high-pressure brines has been investigated in the Erro-T obbio peridotite (Western Alps), a mantle slice that was exposed to th e seafloor of the Mesozoic Ligurian-Piedmontese Tethys and was later i nvolved in Alpine subduction. Hydrothermal alteration by seawater-deri ved fluids led to replacement of the mantle assemblage by Cl-bearing s erpentine (0.35 wt% Cl), brucite (0.2 wt%), Cl- and alkali-bearing phy llosilicates (0.2 wt% Cl; 0.2-0.55 wt% Na2O; 1-5 wt% K2O). Relies of t hese hydrous phases occur in olivine + titanian clinohumite + antigori te assemblages formed at 2.5 GPa and 550-600 degrees C during partial devolatilization and veining of the hydrothermally altered peridotite. The high-pressure phases lack chlorine and alkalis and are coeval wit h fluid inclusions trapped in the syn-eclogitic veins. The inclusions are salt-saturated and contain up to 50 wt% Cl, Na, K, Mg and Fe. High fluid chlorinity was probably achieved during rehydration of relict m antle minerals and deposition of hydrous phases in the veins. The data presented suggest that the seafloor hydrothermal signature was inheri ted by the eclogitic fluid due to partitioning of chlorine and alkalis into the fluid phase. The presence of salty brines in eclogitized hyd rous peridotites can indicate deep recycling of seawater-derived fluid s. Hydrous ultramafic systems can therefore act as large-scale carrier s of seawater into the mantle.