Volatilities and diffusivities of Tl, Ag, Cu, Pb, Cd, Zn, Ga, and As from a Cl-bearing shoshonitic basalt and their application to volcanic degassing

Abstract

The emission of trace metals during volcanic eruptions is modulated by their diffusion rates through the silicate melt and their affinity for the gas phase. However, due to the multicomponent nature of natural magmas, the prevailing controls on emission rates remain poorly understood. To constrain how the presence of Cl affects the diffusivities and volatilities of trace metals (Cl, Tl, Ag, Cu, Cd, Zn, Pb, Ga and As) in a nominally anhydrous shoshonitic basalt, a series of degassing experiments was conducted at 1 atm and 1200, 1300 and 1400 °C, with initial Cl contents of ~0.6 and ~ 1.2 wt% Cl for durations of 1 and 4 h. The resulting concentration gradients perpendicular to the gas-melt interface attest to the diffusive transport of trace metals within the silicate framework in response to their evaporative loss. Diffusivities scale inversely with the ionic field strength, with monovalent cations diffusing at rates (~10−10 m2 s−1) two orders of magnitude faster than trivalent cations (~10−12 m2 s−1). The presence of Cl causes a near-uniform increase in diffusivity of roughly 0.5 logarithmic units across all trace metals. Evaporation rates, defined as the rates at which volatile elements are lost from the melt surface to the coexisting vapor phase, are found to be fastest for Tl, Ag and Cd (~10−9–10−8 m s−1), whereas Ga and As (~10−10–10−9 m s−1) are the least volatile trace metals. Thermodynamic calculations indicate that all evaporating metal-bearing species are present as chlorides in the gas phase, except for As. A positive correlation is observed between evaporation mass transfer coefficients from this study and the gas-melt partition coefficients determined for volcanic gases, with Ga becoming relative more volatile, while As, Cd and Tl becoming less volatile in the experiments than observed in natural volcanic settings. Furthermore, modeling of bubble growth in magmas reveal that diffusive fractionation of slow- and fast-diffusing trace metals may substantially change the concentration ratio of the two species at the bubble-melt interface, with profound implications for the interpretation of volcanic gas compositions.