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    Solid earth sciences

    201412201412

    Contrasting partition behavior of F and Cl during hydrous mantle melting: implications for Cl/F signature in arc magmas

    Dalou C, Koga K T, Le Voyer M, Shimizu N

    Fluorine, Chlorine, Partition coefficients, Lherzolite minerals, Hydrous silicate melt, Fluid induced melting, Subduction

    Figure 5. Cl versus F in, (A) arc melt inclusions, slab and mantle wedge, (B, C, and D) predicted melts. (A) Melt inclusions data are compiled from recent studies (see text). Each thin line represents a constant Cl/F ratio. (B) Melt compositions derived from anhydrous mantle (DMM, blue diamond) are predicted by the dark gray curve. Predicted melts' compositions calculated for fluid-induced melting of a spinel lherzolite are indicated by the dotted curve for low degree (α = 0.026), dashed for intermediate (α = 0.049), and solid curves for high (α = 0.059) and very high degree of melt production rate for a unit flux (α = 0.1). Fluid composition is from Straub and Layne ([2003]) (black star). Fluid-induced melting of an amphibole-bearing wehrlite is indicated by gray curves: dashed for α = 0.043 and dotted for α = 0.059. Each curve is graduated as a function of degree of melting (from 1% to 20%). See text for details on the model parameters. The gray line represents the mixing line between the fluid composition (Straub and Layne, [2003]) and the DMM. Compositions of aqueous fluid from Le Voyer et al. ([2010]) and slab melt from Le Voyer et al. ([2010]) are shown for comparison (gray star and white star, respectively). (C) F and Cl compositions of melts produced by fluid-induced melting of a lherzolite metasomatized by an aqueous fluid from Le Voyer et al. ([2010]). (D) F and Cl compositions of melts produced by fluid-induced melting of a lherzolite metasomatized by slab-derived melt from Le Voyer et al. ([2010]).

    We present the results of five experiments on F and Cl partitioning during hydrous mantle melting under conditions relevant to subduction zone magmatism (1.2–2.5 GPa, 1,180°C–1,430°C). For each experiment, we determined the F and Cl partition coefficients between lherzolitic mineral phases (olivine, orthopyroxene (opx), clinopyroxene (cpx), and garnet), amphibole, and hydrous basaltic melts (0.2–5.9 wt.% dissolved H2O). At constant pressure, show contrasting response to the combined effects of decreasing temperature from 1,310°C to 1,180°C and increasing H2O content in the melt from 0.2 to 5.9 wt.%: . decreases from 0.123 ± 0.004 to 0.021 ± 0.014 while increases from 0.0021 ± 0.0031 to 0.07 ± 0.01. Similar results are observed for clinopyroxene: decreases from 0.153 ± 0.004 to 0.083 ± 0.004 while increases from 0.009 ± 0.0005 to 0.015 ± 0.0008. Experimentally determined F and Cl partition coefficients were used in a hydrous melting model of a lherzolitic mantle metasomatized by slab fluid. In this model, we vary the amount of metasomatic slab fluid added into the mantle while its composition is kept constant. Increasing the amount of fluid results in an increase of both the degree of melting (due to the effect of H2O addition) and the F and Cl input in the mantle wedge. Because of the change of F and Cl partition coefficients with the increase of H2O, the observed variation in the F and Cl contents of the modeled melts is produced not only by F and Cl input from the fluid, but also by the changes in F and Cl fractionation during hydrous melting. Overall, the model predicts that the Cl/F ratio of modeled melts increases with increasing fluid fraction. Therefore, a variation in the amount of fluid added to the mantle wedge can contribute to the variability in Cl/F ratios observed in arc melt inclusions.