The multicomponent exchange of metals between magmatic fluids and silicate melts

Magmatic fluids are an integral part of volcanic eruptions and the transport of metals through the crust. In order to understand this transport and the evolution of magmatic fluids, we performed experiments on rhyolitic melts saturated with an aqueous fluid at 800 °C and 200 MPa and measured the major and trace element composition, as well as the chlorine and fluorine content of coexisting fluids and melts. We find that most trace elements are largely fluid immobile, i.e. partition coefficients of < 1, with the exception of some transition metals, such as Cr, Ni, Cu, and Zn. Fluid mobility is primarily affected by the chlorine concentration of the fluid where increasing chlorine concentration in the fluid increases metal mobility. Such experimental observations have been previously parameterized using empirical relationships between partition coefficients and fluid salinity; however, such relationships do not consider metal speciation or fully capture the fluid-melt exchanges in which cations (major and trace elements) compete for available ligands (Cl, F, OH, etc.). In order to better characterize the behavior of metal in magmatic fluids we utilize existing thermodynamic databases and experimental fluid compositions to calculate the equilibrium concentration of aqueous hydroxide, chloride, and fluoride species in the fluid phase. The equilibrium concentrations of each species were then used to calculate apparent equilibrium constants and characterize the exchange of 42 cations between the fluid and silicate melts for 129 aqueous species. We find that these apparent equilibrium constants vary as a function of the HCl and HF content of the experimental fluid. We further present a model based on these experimentally determined apparent equilibrium constants that is capable of calculating fluid-melt equilibria. This model can subsequently be used to predict fluid-melt partition coefficients for metals, as well as chlorine and fluorine, over a wide range of P-T-X conditions.