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Subduction of oceanic plates releases large amounts of chlorine and fluorine into the mantle. These halogens are transported into the crust through hydrous melting, where they may be incorporated into minerals such as biotite, apatite, and amphibole. Halogen concentrations are measured in volcanic or plutonic material, while the concentration of Cl and F released from the subducting slab must be calculated based upon the amount of crystallized material and the partition coefficients of each mineral. As amphibole is the most common halogen bearing igneous mineral, it is commonly studied as a bearer of Cl and F. However, the partition coefficient of F between amphibole and a hydrous melt has not been agreed upon by previous studies. Here we show that F is moderately to highly compatible in amphibole, in agreement with other experiments performed at crustal conditions. As amphibole may be able to incorporate a large amount of F, cryptic amphibole crystallization may raise the Cl/F ratio of residual magma, which will then be transported to the surface bearing this geochemical signature, even with little crystallized amphibole present in erupted material. This provides further evidence for the occurrence of cryptic amphibole crystallization, previously predicted based on REE studies and phase equilibria. A better understanding of the halogen reservoirs present in the crust will allow for more accurate estimates of the amount of Cl and F released by subducting slabs. 

Oxygen fugacity is an important but difficult parameter to constrain for primitive arc magmas. In this study, the partitioning behavior of Fe3+/Fe2+ between amphibole and glass synthesized in piston cylinder and cold-seal apparatus experiments is developed as an oxybarometer using x-ray absorption spectroscopy. The amphibole oxybarometer is applicable to hydrous magmas at subduction zone settings, and is here applied to amphibole in mafic enclaves, cumulates, and a basaltic tephra erupted from Shiveluch volcano in Kamchatka with measured Fe3+/FeTotal. The fO2 of primitive melts at the volcano is approximately NNO+2 and is faithfully recorded in amphibole from an amphibole-rich cumulate and the basaltic tephra. Apparently higher fO2 recorded by amphibole in mafic enclaves likely results from partial dehydrogenation of amphibole during residence in a shallow andesite storage region. Using a combination of the new oxybarometer and diffusion modeling, we identify three pulses of mafic magma recharge within two weeks of, a month before, and two to three months before eruption, and find that, at each of these times, the host andesite was recharged by at least two magmas at varying stages of differentiation. Application of the amphibole oxybarometer not only gives insight to magmatic fO2 but also potentially details of shallow magmatic processes.