VulcansFluids:The behaviour of halogens and sulphur in natural high temperature processes

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Chlorine, fluorine and sulphur are important components of the gases emitted by volcanoes and of the high temperature water-rich fluids responsible for transport and deposition of many economically important metals. The aims of this project are therefore to characterise fully the behaviour of chlorine, fluorine and sulphur in volcanic melts and thereby to determine the conditions under which these elements are released to the environment during degassing and ore-forming processes. The results have applications to processes occurring on Earth, Mars and other protoplanetary and asteroidal bodies The project uses two novel experimental approaches. In one we fix the partial pressure (fugacity) of Cl2 (or F2) at high temperatures and pressures using mixtures of silicate, silver metal, silver iodide and silver chloride (or silver fluoride). During the experiment the metal and silver halide compounds form 2 immiscible liquids controlling the Cl (or F) content of the silicate melt. A similar approach is used to fix S2 fugacity at high temperature and pressure using silver/silver sulphide or platinum/platinum sulphide mixtures (at fixed oxygen fugacity). After the experiment the silicate (which is now a glass) is analysed by microbeam methods to determine the exact contents of the element(s) of concern, Cl, F and S. The second novel experimental method uses a furnace in which the molten silicate sample is stirred at high temperature in order to determine relative volatilities of Cl, F, S and trace metals. Our results will be integrated with the numerous available data on fluid-melt partitioning of Cl, S and F to determine the fugacities of these elements in brines and lower density fluids and hence their metal-transport capacities. It is known that there are large differences between the Cl/F and Cl/S ratios of the Earth, Moon, Mars and Vesta, all bodies which have undergone extensive melting and volatile loss. Our results will enable us to estimate the different conditions under which volatile loss occurred during formation of planets and asteroids. Chlorine and sulphur are principal agents in the transport of metals as chlorides in volcanic fluids and in the precipitation of metal sulfides in porphyry copper deposits. Our results will enable us to identify potential volcanic sites of economic metal deposits based on the compositions of fluids and gases evolved during their evolution.