WATER IN THE OCEANIC UPPER-MANTLE - IMPLICATIONS FOR RHEOLOGY, MELT EXTRACTION AND THE EVOLUTION OF THE LITHOSPHERE

The influence of water on the dynamics of the oceanic upper mantle is re-evaluated based on recent experimental constraints on the solubilit y of water in mantle minerals and earlier experimental studies of oliv ine theology. Experimental results indicate that the viscosity of oliv ine aggregates is reduced by a factor of similar to 140 in the presenc e of water at a confining pressure of 300 MPa and that the influence o f water on viscosity depends on the concentration of water in olivine. The water content of olivine in the MORE source is estimated to be 81 0+/-490 H/10(6) Si, a value greater than the solubility of water in ol ivine at a confining pressure of 300 MPa (similar to 250 H/l0(6) Si). We therefore conclude that the viscosity of the mantle in the MORE sou rce region is 500+/-300 times less than that of dry olivine aggregates . The dependence of the solubility of water in olivine on pressure and water fugacity is used in conjunction with other petrological constra ints to estimate the depth at which melting initiates beneath mid-ocea n ridges. These calculations indicate that melting begins at a depth o f similar to 115 km, consistent with ether geochemical observations. O wing to the relatively small amount of water present in the MORE sourc e, only similar to 1-2% melt is produced in the depth interval between the water-influenced solidus and the dry solidus. A discontinuity in mantle viscosity can develop at a depth of similar to 60-70 km as a re sult of the extraction of water from olivine during the MORB melting p rocess. In the mid-ocean ridge environment, the mantle viscosity at de pths above this discontinuity may be large enough to produce lateral p ressure gradients capable of focusing melt migration to the ridge axis . These observations indicate that the base of an oceanic plate is def ined by a compositional rather than thermal boundary layer, or at leas t that the location of the thermal boundary layer is strongly influenc ed by a compositional boundary, and that the evolution of the oceanic upper mantle is strongly influenced by a viscosity structure that is c ontrolled by the extraction of water from olivine at mid-ocean ridges.