SERPENTINITE AS A CARRIER OF CHLORINE INTO THE MANTLE

Elemental recycling governs the composition of the oceans and atmosphere, while the composition of subducting fluids controls melt processes and magma generation over subduction zones, as well as the mineralogy of subducting metamorphic rocks. In recent years, evidence for high salinity fluids in deep metamorphic environments has been recognized by several authors, although the source of the salinity is not clear. In this study, high concentrations of water-soluble chlorides and sulfates have been measured in ODP serpentinite cores. Water-soluble Cl- concentrations as high as 0.3 wt% are found; overall Cl- contents can be in excess of 1%. The room-temperature water-soluble Cl- may reside as finely-disseminated NaCl along grain boundaries, having formed during high temperature hydrothermal interaction of hot peridotite with seawater. The NaCl will be carried down into the mantle in the serpentinites; estimated fluxes are 6x10^12g Cl/yr, greater than all other known non-exospheric Cl- sinks. The Cl- concentration in serpentinites must be controlled by ocean salinity. As a result, a negative feedback situation exists, with the serpentinite buffering the salinity of the oceans. During subduction and heating, serpentinite will undergo dehydration reactions, releasing H2O that will dissolve and incorporate the NaCl. These high salinity fluids may explain the following observations: high salinity fluid inclusions in eclogites, low calculated a(H2O) fluids in high-P metamorphic rocks, and intense metasomatism that has been observed in deep metamorphic rocks (e.g., whiteschists and Si-depleted rocks).