Experimental Investigation of Chlorine Substitution into Calcic Amphiboles

Suny At Binghamton, Binghamton NY

Investigators

Abstract

The results from this study should provide insights into such basic processes as the interaction of seawater with oceanic crustal rocks, the role of chlorine-rich fluids in producing mineralogical changes in deep-crustal rocks, and the compositions of fluids associated with ore-mineral formation. Chlorine-rich amphiboles have been observed in a wide range of geological settings yet little is known about how they form. The focus of this proposal is to investigate experimentally the partitioning of chlorine between the common rock-forming mineral amphibole and chlorine-bearing brines. Calibrating how the concentration of chlorine in the amphibole varies with both the composition of the host amphibole (such as its iron content) and the chlorine content of the ambient brine permits geologists to determine the chlorine content of fluids that were once in contact with the amphibole but are no longer present. Important outcomes from this project will be to identify what compositional variables in the amphibole are most important in controlling its ability to incorporate chlorine, and quantifying the distribution of chlorine between mineral and fluid. This research project will provide some of the first experimental data on the compositional controls on the substitution of chlorine into amphibole by examining some of the crystal-chemical correlations that have been proposed in the literature. Four specific correlations will be investigated: (i) substitution of Fe2+ for Mg at octahedral sites (i.e., the Fe#) and determining whether or not the Cl content varies linearly with the Fe#, (ii) occupancy of K at the crystallographic A site, (iii) substitution of tetrahedral Al for Si, and (iv) substitution of octahedral Fe3+ for octahedral Al. These compositional variables will be studied primarily using the amphiboles ferro-pargasite and hastingsite, both of which have been observed with high Cl contents in nature and which can be synthesized with sufficient size for routine electron microprobe analysis. A complementary set of experiments will be done where amphiboles are synthesized in brines of varying types and Cl concentration in order to determine the maximum Cl content, or saturation level, obtained in the amphibole at a given set of pressure?temperature?oxygen-fugacity conditions. The results of this study should improve our understanding of what compositional variables are most important in controlling the substitution of Cl into amphibole and quantify the partitioning of Cl between calcic amphiboles and an ambient brine. This information has applications to diverse areas in the geological sciences, including seawater-rock interaction, chorine concentration in fluids associated with Cl-metasomatism, the role of saline solutions in forming high-grade metamorphic rocks, and characterizing fluids that yield economically important deposits.