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Partitioning and outgassing of chlorine during ascent of a rhyolite magma from physical modelling and natural glass data

Samuel J. Mitchell 1, Alison Rust1

Abstract

The behaviour and availability of magmatic volatile components (e.g. Cl, a metal complexing agent) are key controls on the transport and concentration of valuable metals such as Cu. The evolution of volatile phase composition, and the ultimate destination of the volatile components (and metals), will differ for closed and open-system degassing. The evolution from closed to open-system degassing may permit fluids to leave the conduit into the surrounding wall rock by lateral transport if there is sufficient conduit overpressure. Silicic hydrous magmas rich in Cl are of interest for their potential to concentrate metals in fluids. We apply a 1D conduit magma-ascent code to model H2O-Cl degassing of a rhyolitic magma ascending up a conduit from pressures of ~200 MPa to 10 MPa. We use a suite of volatile data from the 2012 Havre eruption (melt inclusions up to 4500 ppm Cl and 6 wt.% H2O, down to 1000 ppm Cl and 0.7 wt.% H2O in the matrix glasses) to set the degassing endmembers. The model further determines conduit overpressure and the flux of lateral outgassing into the crust. We then apply a geochemical model, tracking melt-fluid partitioning of Cl, to estimate the concentration of particular metals (e.g. Cu, Au) in the outgassing fluids. We show that, while lateral outgassing by conduit overpressure is most prevalent in crustal depths less than 1000m, the highest metal concentrations and likelihood of brine accumulation occurs at depths of several km, and closer to initial magma storage regions where melt-fluid partitioning is greatest.