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-Tobbio peridotite (Western Alps), a mantle slice that was exposed to the seafloor of the Mesozoic Ligurian-Piedmontese Tethys and was later involved in Alpine subduction. Hydrothermal alteration by seawater-derived fluids led to replacement of the mantle assemblage by Cl-bearing serpentine (0.35 wt% Cl), brucite (0.2 wt%), Cl- and alkali-bearing phyllosilicates (0.2 wt% Cl; 0.2–0.55 wt% Na2O; 1–5 wt% K2O). Relics of these hydrous phases occur in olivine + titanian clinohumite + antigorite assemblages formed at 2.5 GPa and 550–600°C during partial devolatilization and veining of the hydrothermally altered peridotite. The high-pressure phases lack chlorine and alkalis and are coeval with 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 mantle minerals and deposition of hydrous phases in the veins. The data presented suggest that the seafloor hydrothermal signature was inherited by the eclogitic fluid due to partitioning of chlorine and alkalis into the fluid phase. The presence of salty brines in eclogitized hydrous peridotites can indicate deep recycling of seawater-derived fluids. Hydrous ultramafic systems can therefore act as large-scale carriers of seawater into the mantle.