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

Abstract

The influence of water on the dynamics of the oceanic upper mantle is re-evaluated based on recent experimental constraints on the solubility of water in mantle minerals and earlier experimental studies of olivine rheology. Experimental results indicate that the viscosity of olivine aggregates is reduced by a factor of # 140 in the presence of water at a confining pressure of 300 MPa and that the influence of water on viscosity depends on the concentration of water in olivine. The water content of olivine in the MORB source is estimated to be 810 +/- 490 H/106 Si, a value greater than the solubility of water in olivine at a confining pressure of 300 MPa (# 250 H/106 Si). We therefore conclude that the viscosity of the mantle in the MORB source 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 constraints to estimate the depth at which melting initiates beneath mid-ocean ridges. These calculations indicate that melting begins at a depth of # 115 km, consistent with other geochemical observations. Owing to the relatively small amount of water present in the MORB source, only # 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 # 60-70 km as a result of the extraction of water from olivine during the MORB melting process. In the mid-ocean ridge environment, the mantle viscosity at depths above this discontinuity may be large enough to produce lateral pressure gradients capable of focusing melt migration to the ridge axis. These observations indicate that the base of an oceanic plate is defined by a compositional rather than thermal boundary layer, or at least that the location of the thermal boundary layer is strongly influenced by a compositional boundary, and that the evolution of the oceanic upper mantle is strongly influenced by a viscosity structure that is controlled by the extraction of water from olivine at mid-ocean ridges.