Low degree melting under the Southwest Indian Ridge: the roles of mantle temperature, conductive cooling and wet melting

Both low mantle temperatures and conductive cooling have been suggested as the cause of the atypically thin oceanic crust and the incompatible element enrichment characteristic of very slow-spreading ridges. Here we present a model of melting under the Southwest Indian Ridge (SWIR), which takes into account mantle temperature, conductive cooling, source composition and wet melting. The model parameters are constrained by oceanic crustal thickness, lava chemistry and isotopic composition and water content. The results suggest that conductive cooling to a depth of around 20 km, expected in areas with a full spreading rate of 15 mm/yr, is necessary to generate the SWIR lava chemistry, but not that from faster spreading rate ridges at 23°N on the Mid-Atlantic Ridge or 45°N on the Juan de Fuca Ridge. The mantle potential temperatures of ∼1280°C, estimated for the SWIR lavas, are close to the global average of the upper mantle. Mantle water contents of 150–300 ppm can explain the observed melt water contents and allow sufficient melting at depth to explain the observed heavy rare earth element depletions in the melts.