Stability and Thermochemistry of some Chlorine-bearing Calcium Amphiboles

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Chlorine-rich fluids are important agents in basic geological processes such as (i) transporting metal ions in aqueous brines, (ii) controlling the temperatures and depths in the Earth where hydrated crustal rocks react to denser anhydrous rocks (e.g., granulites and eclogites), and (iii) controlling the nature of the aqueous phase coexisting with silicate melts. These geological processes control, respectively, the formation of economic mineral deposits, the dynamics of mountain building and plate tectonics, and the style of volcanic eruptions. The details of how chlorine-rich fluids interact with crustal rocks are poorly understood, ranging from the near-surface incorporation of chlorine at the seawater-to-rock interface in oceanic crust to the migration of brines in mid- to lower-crustal levels. One of the most common rock-forming minerals into which chlorine is known to substitute at weight-percent levels is calcium-rich amphibole. The amount of chlorine that substitutes into amphiboles depends on the concentration of chlorine in the ambient brine or melt as well as the chemical composition of the host amphibole. A starting point for examining this complex interplay is to determine what compositional variables in the host amphibole exert the greatest control on its chlorine content. The first goal in this proposal is to examine the role that potassium plays, which, based on prior results obtained in this lab, now appears to be extremely important in controlling the chlorine content of amphiboles. Second, the effect of chlorine on the upper-thermal stability of this normally hydrous mineral will be investigated. Third, basic thermochemical data for one or more chlorine-rich amphiboles will be determined. Any rigorous analysis of the substitution of chlorine into an amphibole ultimately requires the fundamental thermochemical data for a given chlorine end-member amphibole. Obtaining these data, which simply do not exist at this time, would be a major contribution to the sciences. This proposal provides educational opportunities for undergraduate and graduate students, and welcomes the participation of under-represented minorities. Opportunities will also be made available for the training of pre-service Earth Science, Chemistry, Physics, and elementary teachers.Research in this proposal will focus on the following three main goals. First, the effect of several key compositional variables (ferrous iron content, potassium content) on the incorporation of chlorine into several calcium amphiboles will be examined. This will build on research already done at Binghamton University on potassium-free calcium amphiboles which were generally found to have very limited chlorine contents. Second, the upper-thermal stability of chlorine-rich amphiboles will be determined by phase-equilibrium methods and compared to the stabilities of their hydroxyl-rich equivalents. At present there are no experimental data demonstrating the effect of chlorine on the thermal stability of an amphibole, so that this information will be of considerable importance. Third, thermochemical data (volume, heat capacity, enthalpy of formation, and third-law entropy) will be obtained by a combination of direct measurements and derivation from the phase-equilibria data. Volume and heat capacity measurements can be made at Binghamton University, while the enthalpy of formation will entail measurements at the University of California at Davis. The third-law entropy will be derived from the thermal stability boundary. Thermochemical data on chlorine-rich amphiboles are absent in the scientific literature, yet such information is fundamental to any rigorous treatment of the partitioning of chlorine between amphibole-bearing rocks and the brine with which it equilibrates. Results from this study should be of interest to those working in a wide range of geological settings, including ocean-rock interactions, mantle chloride metasomatism, deep crustal and upper-mantle metamorphism, and ore-forming bodies and skarns.