H2O in basaltic glasses from the slow-spreading Carlsberg Ridge: Implications for mantle source and magmatic processes

Abstract

H2O in the mantle is of great interest in the fields of geochemistry and geophysics on account of its significant influence on the melting and physical properties of mantle rocks. The abundance of water in derivative mantle melts beneath mid-ocean ridges can provide important insights into the abundance and distribution of water in the mantle. For this study, we analyzed the H2O, CO2, and major- and trace-element contents in basaltic glass samples collected from the slow-spreading Carlsberg Ridge from 59°E to 66°E in the Indian Ocean. All of the studied samples are found to be normal mid-ocean ridge basalt (N-MORB) in composition. The H2O concentrations range from 0.13 to 0.29wt% with a mean value of 0.22±0.10wt%. Along the ridge from 59°E to 66°E, H2O/La, H2O/Ce, and trace-element ratios show systematic variation whereby H2O/La and H2O/Ce ratios increase with longitude, whereas K2O/Ti2O and (La/Sm)N decrease. The Nb/U (36.4–42.3) and Ce/Pb (17.6–20.8) ratios of the samples are lower in comparison with typical values reported for global MORBs and OIBs (ocean island basalts) (Nb/U=ca. 47, Ce/Pb=ca. 25). In addition, the ratios of incompatible trace elements display wide variation (e.g.,Ba/La and Nb/Zr values of 2.04–6.02 and 0.02–0.07, respectively). The limited variation in the degree of mantle melting cannot yield the observed level of variation in the basaltic glass trace-element ratios. Therefore, we suggest that the mantle source of the studied MORBs has been mixed with subducted oceanic/continental crust and that mantle heterogeneity is the main factor responsible for the chemical variation of MORB along the Carlsberg Ridge axis from 59°E to 66°E.