Chlorine-rich fluids play an important role in many geological processes, including the formation of economic deposits, crustal and mantle metasomatism, and high-grade metamorphism. Furthermore, the chlorine content of a fluid is often one of the main variables, dictating the fluid’s properties (i.e., the propensity for mass transport). Calcium amphiboles have the potential to be used to determine the chlorinity of paleo fluids, given sufficient knowledge of how Cl is partitioned between amphibole and fluid. Amphiboles with Fe# [= Fe2+/(Fe2+ + Mg)] = 1.0 were synthesized along the hastingsite–potassic-hastingsite join in the presence of variably concentrated FeCl2 brines, ranging from 1 molal to 100 molal. Syntheses were done at 700 °C and 3 kbar at fO2 values near Ni-NiO for durations of 96-132 hours. All amphiboles were characterized by powder X-ray diffraction and electron microprobe and several samples were analyzed by Mössbauer spectroscopy to determine ferric iron content. Results showed that amphibole Cl content increased linearly with the mole fraction of Cl in the brine and has no obvious relationship with the K# (= K/(K+Na)). Amphibole Cl contents varied from ~0.05 atoms per formula unit (apfu), synthesized in the most dilute brines, to ~1.05 apfu, synthesized in the most concentrated brines. Amphibole yield was related to the K#, with higher amphibole yields for the more K-rich bulk compositions. The amphibole ferric iron fraction was dependent on the brine FeCl2 concentration, increasing from 0.176 at 1 molal FeCl2, to 0.310 at 24 molal FeCl2. For brines more concentrated than 24 molal FeCl2, the ferric iron fraction significantly decreased to 0.116 at 50 molal FeCl2. The significant decrease in ferric iron fraction also coincided with a transition from magnetite to fayalite as a coexisting phase. The ferric iron fraction seemed to influence the total occupancy of the A site through the following reaction: A+ + Fe2+ ⇋ ☐ + Fe3+, where ☐ is a vacancy. Trends between Fe3+ and Cl display both positive and negative correlations, raising further questions on the role of Fe3+ on Cl incorporation. The findings of this study indicate that for Fe# = 1.0 amphiboles, the Cl concentration of the fluid plays the dominant, or perhaps only, role in amphibole Cl incorporation, with the occupant of the A site being inconsequential.

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