Experimental investigation of the degassing behavior of Hg via proxy heavy metals in natural and synthetic silicate melts

The degassing behavior of Bi, Tl, Au, Cd and Pb from silicate liquids have been investigated at 0.1 MPa. These metals were chosen on the basis that they may be used as a proxy for Hg. Natural basalt, dacite, and synthetic rhyolite (Ab-Or-Qz minimum eutectic) were doped with a heavy metal cocktail (Bi, Pb, Tl, Au, Re, Sb, Sn, In, Cd, Mo, As, Cu) and diffusion experiments were conducted in air at 0.1 MPa between 1200 - 1450 °C in open Pt capsules. Trace element concentration gradients for Bi, Tl, Au, Cd and Pb were measured normal to the polished melt(glass)/gas interface by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) after the experiment. Diffusion coefficients (Dx) were extracted for elements using a semi-infinite one-dimensional diffusion model. The effects of ligands on diffusivity were measured by adding Cl or S to experimental runs. Results show Au and Tl (which bracket Hg in the periodic table) are the most volatile in dacite at 1260°C (logDAu = -10.66 ± 0.20 m2/s and logDTl = -11.03 ± 0.15 m2/s ) followed by Cd, Pb and Bi (logDCd = -11.58 ± 0.07 m2/s, logDPb = -11.77 ± 0.2 m2/s, logDBi = -12.11 ± 0.2 m2/s;). The effect of starting composition on metal diffusion shows the diffusivity of Tl at 1260°C decreases from logDTl = -10.84 ± 0.22 m2/s in basalt to logDTl = -11.03 ± 0.15 m2/s and logDTl = -11.60 ± 0.20 m2/s in dacite and synthetic Ab-Or-Qz respectively. Decreasing diffusivity would be expected with increasing silica content that inhibits diffusion. We apply our diffusivities in a 1-D bubble growth model to observe differences in metal ratios in gas bubbles that would form in volcanic plumes. In this way, we see the changing heavy metal ratios in the gas phase may be useful in eruption prediction. This work may also help better constrain the behavior/mobility of Hg (using these metals as a proxy) in nature and its loading effect from volcanoes on the atmosphere and biosphere.