THERMOCHEMICAL VALUES OF CHLORO-HASTINGSITE AND QUANTIFYING CHLORINE CONCENTRATIONS OF HYDROTHERMAL SYSTEMS

The chloride content of hydrothermal fluids is of interest because it aids the transport of ore-forming metals, affects the temperature of dehydration reactions at a given depth, and controls the formation of Cl-bearing minerals in hydrothermally-altered rocks. At present there are no thermochemical data available for Cl-bearing amphiboles. Initial values of ΔH_f and S° for chloro-hastingsite (NaCa₂(Fe₄Fe³⁺)(Al₂Si₆)O₂₂Cl₂ = ClHast) have been derived from the experimentally determined upper-thermal stability curve of a Cl-bearing calcium amphibole.

The breakdown of amphibole (Na_0.4(Ca_1.8Na_0.2)(Fe_3.7Fe³⁺_0.8Al_0.5)(Al1.4Si_6.6)O₂₂(OH_1.5Cl_0.5) was investigated in the range of 1-3 kb, 580-700°C using synthetic phases made in the system Na₂O-CaO-FeO-Al₂O₃-SiO₂-H₂O-FeCl₂. Reversal mixtures consisted of equal proportions of the amphibole-rich reactant and its breakdown products (plag-An_80, fayalite, hedenbergite, halite, and FeCl₂). These were treated dry in Ag₅₀Pd₅₀ capsules in internally-heated gas vessels using H₂-argon mixed gas as the pressure medium, with the H₂ fugacity equivalent to log(fO₂) = +0.5ΔCoCoO. The reaction boundary was located at 614 ± 25°C/0.98 kb, 650 ± 20°C/2.1 kb, and 690 ± 20°C/3.0 kb. Based on the (narrow divariant) breakdown reaction: ClHast = 0.95An + 0.1Alb + 1.7Fay + 1.05Hed + 0.9NaCl + 0.55FeCl₂ + 0.25O₂, microprobe analyses of the phases, using the Holland and Powell (2011, JMG) database, and modeling the activity of ClHast after pargasite, values of -10,671.2 kJ/mol and 655.8 J/K·mol were obtained for ΔH_f and S°, respectively, at 298K and 1bar for ClHast, based on a volume of 28.48 kJ/kb and Cp (kJ/K·mol) = 1.3671 + 2.93E-05(T) + 93.725/(T²) – 9.583/(T^0.5).

With these data, expressions can be made to quantify Cl concentrations of brines equilibrated with Cl-bearing amphiboles. For example, Cl-bearing amphiboles in the amphibolitized part of the shear zone at Langoy, Norway (Kusebauch et al. 2015, JPetrol) indicate NaCl molalities of 0.6 – 4 m (4-19 wt% NaCl), which are probably low as fluid inclusions indicate higher salinities (> 30 wt% NaCl). Greater precision is expected for calculations of activity ratios (i.e., a_Cl-/a_OH-); however, these initial results illustrate the potential to extract absolute NaCl concentrations of fluids in hydrothermal systems.