Abstract

Understanding how chlorine partitions between calcium amphiboles and an ambient fluid can be used to deduce the chlorine content of fluids involved in a wide range of geological processes, such as in high-grade metamorphism, deposition of economic minerals, chlorine metasomatism, and seawater-oceanic crust interactions. One of the most common correlations that is observed in calcium amphiboles is the increase in chlorine concentration with increasing Fe2+/(Fe2+ + Mg) (= Fe#). To quantify this dependence, amphiboles with intended compositions along the pargasite–ferro-pargasite [NaCa2(Mg4Al)(Al2Si6)O22(Cl,OH)2–NaCa2(Fe4Al)(Al2Si6)O22(Cl,OH)2] join were synthesized in the presence of NaCl brines ranging in concentration from 1 molal (m) NaCl to halite saturation. Syntheses were made over the range 700–950 °C and 0.2–0.5 GPa at oxygen fugacities (fO2) near the Co-CoO buffer and for durations of 46–576 h. The observed amphibole compositions varied between pargasite and hastingsite with increasing Fe# for an assumed constant Fe3+/(Fe2+ + Fe3+) ratio of 0.18. The most common additional phases were plagioclase and Ca-rich clinopyroxene with minor davyne in some syntheses. At any given bulk composition, amphiboles increase in Cl content with increasing brine concentrations, reaching a plateau in brines of about 0.1–0.2 mole fraction of NaCl (XNaCl, or 5–15 m NaCl). Amphiboles synthesized in brines below halite saturation showed a fairly linear increase in Cl content from essentially 0.0 to 0.11 atoms per formula unit (apfu) with increasing Fe content. Syntheses in brines above halite saturation showed a distinct jump in Cl contents up to 0.14–0.30 apfu for the two most Fe-rich bulk compositions. Isopleths of constant NaCl concentration have been fitted to all of the data, and a thermochemical analysis of the dependence of the Cl/OH ratio of amphibole to the activity ratio of H2O/HCl in the brine has been offered for amphiboles synthesized in the 1 m NaCl brines. Comparison of the Cl contents of synthetic amphiboles made here with natural pargasitic amphiboles shows that even the highest Cl contents observed here are only about 20% of the maximum Cl contents found in amphiboles from some localities. The main implication of this study is that there is very limited Cl uptake in amphiboles made in this chemical system in the presence of aqueous brines of highly variable concentrations, even for the most Fe-rich amphiboles. The high Cl contents observed in natural assemblages are likely controlled by brine, or perhaps halide or silicate melt chemistry, or crystal-chemical factors (e.g., K content) that exert a stronger effect than just the Fe# of the amphibole.

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