Partitioning of Chlorine Between NaCl Brines and Ferro-Pargasite: Implications For the Formation of Chlorine-Rich Amphiboles In Mafic Rocks

Abstract

Determining the partitioning of chloride between fluid and rock has relevance to a wide range of geological processes, including high-grade metamorphism, economic deposit formation, and seawater-oceanic crust interactions. Calcium amphiboles, particularly those that are iron-rich, are a common host for chlorine and have the potential to reveal information about the ambient fluid composition, if the crystal-chemical controls on chlorine incorporation are known. In this study we investigated the incorporation of chlorine into synthetic ferro-pargasite [NaCa_2(Fe_4Al)(Al_2Si_6)O_(22)(Cl,OH)_2] formed mostly at 700 °C, 0.2 GPa, and oxygen fugacities (f_(O2)) between Co–CoO and wüstite–magnetite (−20.2 < log(f_(O2)) < −17.5) for durations of 3–28 days. Starting mixtures consisted of reagent oxides and metallic iron to which chlorine was added as either a stock solution of NaCl brine up to 5 molal (m) (equivalent to mole fraction of NaCl, X_(NaCl), up to 0.082), or by mixing NaCl ± water directly with the starting mixture to achieve higher concentration brines by homogenization at elevated pressures and temperatures. Good yields of amphibole were obtained, with the most common additional phases being plagioclase, hedenbergite, and halite. At low salinities the Cl content of the amphibole, determined by electron microprobe analysis (EMPA) of individual grains, increases linearly to about 0.045 anions per formula unit (apfu) as the brine concentration increases to about 1.7 m NaCl (X_(NaCl) = 0.03). Thermodynamic modeling of the partitioning of Cl– between brine and the amphibole yields a constant value for the equilibrium constant (K_a) of 0.78 ± 0.09 at 700 °C and 0.2 GPa over the range 0.1–2.0 m NaCl with the empirically derived relationship for the activity of Cl– in amphibole (ɑ^(ɑmph)_(Cl)_-) being ɑ^(ɑmph)_(Cl)_- ═ (X^(o(3)_(Cl))^(2/3). This activity-composition relationship suggests much less random mixing of Cl^–, OH^–, and O2^– at the O(3) site. At salinities above 1.7 m NaCl, the Cl content of the amphibole is fairly insensitive to the brine concentration. Only at halite saturation (83 m NaCl, or X_(NaCl) = 0.60, at 700 °C and 0.2 GPa) is there a noticeable increase in the Cl content of the amphibole, with a bimodal distribution centered at 0.2 and 0.7 Cl apfu observed in the highest salinity synthesis. The implication of this study is that (1) amphiboles with Cl contents above about 0.1 apfu (∼0.4 wt.% Cl) require ambient brines at halite saturation, and (2) oceanic crust reacting with homogeneous seawater could incorporate 50–200 ppm Cl via the presence of 8–31 wt.% ferro-pargasite in the oceanic crust, respectively.

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