ALBERT

Description: 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 Fe 2+ /(Fe 2+ + Mg) (= Fe#). To quantify this dependence, amphiboles with intended compositions along the pargasite–ferro-pargasite [NaCa 2 (Mg 4 Al)(Al 2 Si 6 )O 22 (Cl,OH) 2 –NaCa 2 (Fe 4 Al)(Al 2 Si 6 )O 22 (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 ( f O 2 ) 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 Fe 3+ /(Fe 2+ + Fe 3+ ) 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 ( X NaCl , 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 H 2 O/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.

Description: 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 4 Al)(Al 2 Si 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 ( ) being . This activity-composition relationship suggests much less random mixing of Cl – , OH – , and O 2– 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.