Ca-Sr distribution among amphibole, clinopyroxene, and chloride-bearing solutions

Abstract

The distribution of Sr between a 1 M (Ca,Sr)Cl2 solution, (Ca,Sr)-tremolite and (Ca,Sr)-diopside was determined at 750 °C and 200 MPa. The synthesized crystals of (Ca,Sr)-tremolite (2000 x 30 μm) and (Ca,Sr)-diopside (1500 x 20 μm) were large enough for accurate electron microprobe analysis. The experimental results indicate that Ca2+ can be replaced completely by Sr2+ on the M4-site in tremolite and on the M2-site in diopside. The compositions of the product fluid were analyzed by atomic absorption spectroscopy. In both the (Ca,Sr)-tremolite-fluid and (Ca,Sr)-diopside-fluid systems, Sr strongly fractionated into the fluid. For bulk compositions having low Sr concentrations, mineral/fluid partition coefficients, D(Sr)(mineral/fluid), of 0.045 for (Ca,Sr)-tremolite/fluid and 0.082 for (Ca,Sr)-diopside/fluid were derived. The experimental results were evaluated thermodynamically assuming Henry's law and simple mixing properties for SrCl2 and CaCl2 in the fluid. The mixing energies of the solids were calculated using a regular solution model. In the (Ca,Sr)-tremolite-(Ca,Sr)Cl2 system, Δμ°is 59.0 kJ and W(CaSr)(amph) = 9.8 kJ. In the system (Ca,Sr)-diopside-(Ca,Sr)Cl2 Δμ°is 30.8 kJ and W(CaSr)(px) is 11.7 kJ. The high Δμ°values and, to a much lesser extent, the W(CaSr) values cause the strong fractionation of Sr into the fluid. The moderate values for W(CaSr)(amph) and W(CaSr)(px) strongly suggest that complete solid solution exists for (Ca,Sr)-tremolite and (Ca,Sr)-diopside at experimental run conditions. However, for the (Ca,Sr)-tremolite and (Ca,Sr)-diopside joins, limited miscibilities were calculated below 316 and 430 °C, respectively. The experimentally derived thermodynamic properties were used to determine Ca/Sr ratios of Sr-rich metasomatic fluids that penetrated a metaeclogite in Bjorkedalen, southwest Norway. The derived Ca/Sr ratios from amphibole-fluid equilibria are in good agreement with those calculated from plagioclase-fluid equilibria.