Plume-ridge interaction along the Galapagos spreading center, 90 30'W to 98 W: a hydrous melting model to explain variations in observed glass compositions

vii, 74 leavesThe Galapagos Spreading Center (GSC) between 90°30'W and 98°W shows manifestations of its interaction with the nearby Galapagos plume by way of variations in lava geochemistry, crustal thickness, and morphology along the ridge axis. Rock samples with an average spacing of ~9 km were analyzed for major elements and dissolved H2O. Samples were classified as E-MORBs, T-MORBs, or N-MORBs based on K/Ti ratios. E-MORBs dominate the GSC east of 92.6°W. T-MORBs are mainly found between 92.6°W and 95.5°W. West of the propagating rift tip at 95.5°W, N-MORBs dominate. High K/Ti EMORBs are also characterized by higher H2O, Al2O3, and Na2O, and lower FeO*, SiO2, and CaO/Al2O3 relative to N-MORB at similar values of MgO. These compositional characteristics are consistent with lower mean extents of partial melting relative to NMORB. We developed a melting equation to assess the change in lava composition and mean fraction of partial melting (F) produced by contributions from the zone of hydrous melting whose presence is caused by the depression of the mantle solidus by H2O. We use our hydrous melting equation to model the source composition, the depth of the additional hydrous melting zone, the productivity in the hydrous region, and the upwelling rate that may combine to match our measured crustal thickness values and concentrations of K, Na2O, H2O, and Ti in lavas from the GSC. Model results indicate that GSC N-MORBs were created by F ~0.06 from a source with ~34±1 ppm K, 133±3 ppm H2O, 2250±50 ppm Na2O, and 1050±25 ppm Ti. E-MORB values can be predicted in a number of ways. Higher upwelling rates in the E-MORB region require less source enrichment than low upwelling rates. Upwelling rate has the strongest effect on F . The crustal thickness and glass compositional variations in the "enriched" region of the GSC can best be explained by only a slight increase in the temperature of the mantle (11±11°C), coupled with a moderately enriched mantle source and upwelling of 1.5-3.5 times passive upwelling rates. The transitional region requires only slight upwelling (UwUo =1.5) and a source enriched only in K