Abstract
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, View MathML 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.%: View MathML . decreases from 0.123 ± 0.004 to 0.021 ± 0.014 while View MathML increases from 0.0021 ± 0.0031 to 0.07 ± 0.01. Similar results are observed for clinopyroxene: View MathML decreases from 0.153 ± 0.004 to 0.083 ± 0.004 while View MathML 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.
Highlights
Volatile species (H2O, CO2, S, Cl, and F) recycling in subduction zones is the main driver of mantle wedge melting and production of arc magmas
We address the effect of H2O on F and Cl partition coefficients between anhydrous minerals (olivine, orthopyroxene, clinopyroxene, and garnet), amphibole, and hydrous basaltic melts (0.2–5.9 wt.% dissolved H2O), at mantle wedge conditions (1.2 and 2.5 GPa and 1,180°C and 1,430°C)
No bubbles were observed in basaltic glasses, suggesting that the maximum of H2O content added to the starting materials was less than that needed for water saturation in these melts at pressure and temperature conditions of this study’s experiments
Summary
Volatile species (H2O, CO2, S, Cl, and F) recycling in subduction zones is the main driver of mantle wedge melting and production of arc magmas. Previous studies have shown that primitive arc magmas recorded in olivine-hosted melt inclusions are enriched in volatiles, especially H2O, compared to mid-ocean ridge basalts and/or their melt inclusions In contrast to H2O and other volatile species, F and Cl are not significantly degassed from mafic magmas at the depth of melt inclusion formation, if at all (Carroll and Webster 1994; Métrich and Wallace 2008), because of their comparatively high solubility and low concentration in mafic magmatic liquid (Webster 2004; Dalou et al 2014). The F and Cl contents recorded by arc melt inclusions represent the product of fractional crystallization from primary melts, and they retain valuable information on the source composition of arc magmas
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