Major Element and Halogen (F, Cl) Mineral–Melt–Fluid Partitioning in Hydrous Rhyodacitic Melts at Shallow Crustal Conditions

Abstract

Twenty-four internally heated pressure vessel experiments were conducted at 810–860°C, 1·5–4·05 kbar, and oxygen fugacities (fO2) = NNO –0·5 to NNO +2 log units (where NNO is the nickel–nickel oxide buffer), using hydrous rhyodacitic starting glasses from Mt Usu, Japan. Aqueous solutions were added to experimental charges such that the volatile phase(s) coexisting with the crystalline phases (amphibole, plagioclase, and clinopyroxene) at run conditions buffered the F, Cl, S, and CO2 concentrations in the melt. The resultant phenocryst phases and glass chemistries were analysed by electron microprobe and Fourier transform infrared spectroscopy, and final fluid Cl– contents by chloridometer. All experiments produced homogeneous glasses and large euhedral phenocrysts with minimal compositional zonation. The results of the crystal–melt partitioning data are applicable to understanding the geochemical evolution of rhyodacitic melts at fluid-saturated shallow crustal levels, the composition of fluid phases later exsolved owing to second boiling, and the effects of halogens on amphibole crystal chemistry. The residual glasses from these experiments span the dacite–rhyolite compositional join, and are hydrous (>5 wt % H2O) with F concentrations from ≤100 ppm to 0·63 wt % and Cl from ∼130 ppm to 0·72 wt %. Measured final fluid Cl contents show that Cl strongly prefers the fluid phase over the melt phase in all experiments, with DClfluid/melt ranging from 3·5 to 22·7. Amphibole compositions are calcic, Mg-rich, and typical of those found in natural calc-alkaline arc magmas. They are particularly sensitive to changes in melt halogen chemistry, with maximum amphibole F contents of 2·59 wt % (maximum DFamph/melt values of ≥15) and maximum Cl contents of 0·12 wt % (maximum DClamph/melt values of 0·40). Integration of the amphibole data with other experimental data shows that Cl incorporation is a strong function of the Mg–Cl crystallographic avoidance principle, and that addition of F to the melt strongly decreases Cl partitioning at equivalent Mg# (DClamph/melt = 0·10 ± 0·02 in experiments that were not F-doped, compared with DClamph/melt = 0·05 ± 0·01 in F-doped experiments). Plagioclase compositions are relatively restricted, with anorthite contents An59–An39, and clinopyroxene is similarly calcic, containing a significant enstatite component (Wo49–Wo30). The behaviour of Fe in the glass and crystalline phases is most significantly affected by the fO2 of the experiment. Higher total Fe contents are found in amphibole and clinopyroxene from experiments with fO2 < NNO +1, but with little effect on the absolute DFeOmineral/melt. Plagioclase, however, shows relatively decreased DFeOplag/melt reflecting its preferential incorporation of Fe3+, and less Fe3+ expected in melts crystallized at lower fO2 conditions. Comparison of the data with recently formulated amphibole geothermometers, barometers, and plagioclase hygrometers shows calculated results that are consistent with the actual experimental conditions but discrepancies arise owing to halogen-induced major element variations in amphibole, indicating the importance of routinely measuring F and Cl in both natural and experimental amphibole.