Plank, T. and J. Wade (2005) Constraints from Water on Mantle Melting and Slab Fluid Composition. Eos. Trans. AGU, 86(52) Fall Meet. Suppl., Abstract V52A-01.
Water drives mantle melting and fluid migration in subduction zones, but most models for these phenomena have been developed without constraints from water measurements in arc magmas. For example, the Central American volcanic arc (CAVA) records systematic variations in La/Yb, Ba/La and d18O, and these proxies have been used to predict the extent of mantle melting during decompression [1] and water-addition [2].
Here we use water concentrations in olivine-hosted melt inclusions from arc tephra, along with estimates derived from a clinopyroxene hygrometer [3], to test different models for mantle melting and slab fluid composition along the CAVA (from Nicaragua to Costa Rica). We use Ti as a proxy for mantle melt fraction (F) and invert H2O concentrations in CAVA magmas to obtain those in the mantle source (H2Oo), as in [4]. The relationship between F and H2Oo is nominally linear for Costa Rica mantle, with wet melting productivity dF/dH2O = 30 (wt%/wt%), higher than that used in [2], but consistent with experimentally-determined and MELTS-calculated productivity at 50 degrees above the dry solidus. This predicts mantle temperature beneath Costa Rica of at least 1350°C, and allows for a small (1-2% F) decompression-melting contribution, relative to the wet melting contribution (8-20% F).
The percent of wet melting correlates locally with Ba/La, but not regionally, and so the use of Ba/La as a wet melting proxy [1] should be limited to single volcanoes or clusters. The water content of the CAVA melting region varies from 2500-9000 ppm H2O but does not decrease monotonically from Nicaragua to Costa Rica as does Ba/La. The relationship between H2Oo and Ba/La is thus complex, and requires a large along-strike decrease in Ba/La and H2O/La in the slab fluids towards the southeast. Such variation appears to be driven largely by La concentration, reflecting more dilute fluids (higher H2O/La) beneath Nicaragua and more solute-rich fluids (e.g., sediment melts with high La/ H2O) beneath Costa Rica.
[1] Walker, J.A., M.J. Carr, L.C. Patino, C.M. Johnson, M.D. Feigenson, R.L. Ward Contrib. Mineral. Petrol.
[2] Eiler, J.M., M.J. Carr, M. Reagan, E. Stolper, 2005. Geochem. Geophys. Geosyst.
[3] Plank, T., Benjamin, E., Wade, J., Grove, T.L. 2004. Fall Mtg, AGU.
[4] Kelley, K., Plank, T., Newman, S., Stolper, E.M., Grove, TL, Hauri, E. 2005. JGR.