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We report the results of the first geochemical and isotopic survey of basaltic glasses dredged along the spreading ridge of the southern Mariana Trough (SMT; 15–17°N). This ridge is divided into two segments that have different axial depths, major and trace element compositions, water contents, and isotopic compositions of Sr, Nd, and Pb. Glasses from the shallower, northern segment (N-SMT; 16–17°N) are OL- and QZ-tholeiites that have compositions consistent with a higher degree of mantle melting relative to that of the OL tholeiites from the southern ridge segment (S-SMT; 15–16°N). The N-SMT glasses are similar to basalts erupted near 18°N in the Mariana Trough that have been the focus of previous studies. The more extensive melting inferred for the N-SMT correlates well with higher abundances of water and relative abundances of large ion lithophile and light Rare Earth elements that indicate involvement of a subduction component. The southern ridge segment is deeper and erupts compositions characteristic of lower degrees of melting; this correlates well with a lower proportion of the subduction component, including a suite that is indistinguishable from MORB. The strong correlation between degree of melting, water contents, and LIL elements indicates that hydrous fluxing as well as adiabatic decompression control melting of MORB-like mantle beneath back-arc basins. Details regarding the nature of this hydrous fluxing agent are not known, but it could be water-rich melts related to behind-the-arc volcanoes. These melts may be diverted by the back-arc convective regime, to become entrained in the zone of adiabatic upwelling, where they further stimulate melting. |
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