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Published September 28, 2006 | Published
Journal Article Open

Mantle melting as a function of water content beneath back-arc basins


Subduction zone magmas are characterized by high concentrations of H_(2)O, presumably derived from the subducted plate and ultimately responsible for melting at this tectonic setting. Previous studies of the role of water during mantle melting beneath back-arc basins found positive correlations between the H_(2)O concentration of the mantle (H_(2)O_o ) and the extent of melting (F), in contrast to the negative correlations observed at mid-ocean ridges. Here we examine data compiled from six back-arc basins and three mid-ocean ridge regions. We use TiO_2 as a proxy for F, then use F to calculate H_(2)O_o from measured H_(2)O concentrations of submarine basalts. Back-arc basins record up to 0.5 wt % H_(2)O or more in their mantle sources and define positive, approximately linear correlations between H_(2)O_o and F that vary regionally in slope and intercept. Ridge-like mantle potential temperatures at back-arc basins, constrained from Na-Fe systematics (1350°–1500°C), correlate with variations in axial depth and wet melt productivity (∼30–80% F/wt % H_(2)O_o ). Water concentrations in back-arc mantle sources increase toward the trench, and back-arc spreading segments with the highest mean H_(2)O_o are at anomalously shallow water depths, consistent with increases in crustal thickness and total melt production resulting from high H_(2)O. These results contrast with those from ridges, which record low H_(2)O_o (<0.05 wt %) and broadly negative correlations between H_(2)O_o and F that result from purely passive melting and efficient melt focusing, where water and melt distribution are governed by the solid flow field. Back-arc basin spreading combines ridge-like adiabatic melting with nonadiabatic mantle melting paths that may be independent of the solid flow field and derive from the H_(2)O supply from the subducting plate. These factors combine significant quantitative and qualitative differences in the integrated influence of water on melting phenomena in back-arc basin and mid-ocean ridge settings.

Additional Information

© 2006 American Geophysical Union. Received 15 March 2005; accepted 29 March 2006; published 28 September 2006. Many thanks to M. Hirschmann, R. Katz, C. Langmuir, D. Wiens, B. Taylor, A. Kent, J. Sinton, P. Asimow, G. Hirth, and P. Kelemen for sharing results, unpublished data, and discussion, which were all critical to the successful completion of this study. We also thank E. Klein, J. Dixon, and an anonymous reviewer for careful, thoughtful, and thorough reviews. This work was funded by support from NSF OCE-0001897 and the NSF MARGINS Program as well as by an NSF graduate research fellowship. Development and writing of this manuscript was also supported by a Carnegie Postdoctoral Fellowship.

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