Experimental investigation of chlorine partitioning between silicate melt and calcium amphibole
Chlorine (Cl) is a minor but ubiquitous volatile component in the earth atmosphere, hydrosphere and inner crust and mantle, and it can play important roles in a variety of ecological and geological processes. At high temperature and pressure, chlorine present in fluids or silicate melts forms complexes with a large number of trace elements and the formation of ore deposits (e.g., Rare Earth Elements, noble metals such as Pt, Pd, Au) is often closely linked to the presence of these Cl-bearing species. Thus, chlorine plays a crucial role in the transport processes of rare metals in fluids and silicate melts. One main problem for geologists is that the determination of the fugacity or activity of chlorine prevailing in natural systems is extremely difficult because (1) activity-composition relationships for Cl-bearing solid phases are not calibrated and (2) only few Cl-bearing solid phases are surviving surface alteration processes. Silicate minerals may be suited to trace Cl concentrations in high temperature natural systems but only a few of them contain Cl. Among these minerals, amphibole is a common mineral group formed in a large variety of volatile-bearing (H2O, F, Cl, etc) environments and this mineral is stable over a wide pressure and temperature range. Amphibole, as an important halogen host, is thus a potential probe for tracing the role of halogens, especially Cl and F, during metamorphic magmatic processes. This proposal plans to investigate experimentally the partitioning of Cl between silicate melts and amphiboles at high pressure and temperature (200-400 MPa, 700–900 °C). The ultimate goal is to provide a thermodynamic model predicting Cl fugacities prevailing in natural systems from the Cl content of amphiboles. The results should be of broad interests in many geological sub-disciplines, such as halogen cycle in the deep earth, volcanic degassing, ore-forming process, hydrothermal cycles, and etc.