Fluid/melt partitioning of Re, Mo, W, Tl and Pb in the system haplobasalt-H2O-Cl and the volcanic degassing of trace metals

The partition coefficients (Kdxf/m) of Re, Mo, W, Tl and Pb between fluid (H2O + Cl) and a haplobasaltic melt in the CaO-MgO-Al2O3-SiO2 system were measured between 1300 and 1400 °C at 1 GPa and fluid chlorine molarities from 7.7 to 27 mol/L. Dissociation of hydroxides added to a trace metal-doped oxide powder, produced a fluid phase in equilibrium with melt at the run conditions. The Kdxf/m values were determined by mass balance of trace metals added with those determined in glass from the run products. At 1300 °C and fluid molarity of 7.7 mol/L, KdRef/m = 9.8 ± 1.8, KdMof/m = 11.8 ± 1.6, KdWf/m = 3.7 ± 1.6, KdTlf/m = 4.5 ± 1.4 and KdPbf/m = 2.4 ± 1.8. Both Mo and Re were shown to partition most strongly into the fluid at all temperatures and fluid chlorinities. At 1400 °C, KdPbf/m and KdTlf/m were the only elements showing any correlation with fluid chlorinities. At 1400 °C, Re and Mo completely partitioned into the fluid and thus our calculated KdRe, Mof/m are minima. Controls on fluid/melt partitioning are explored in detail. Calculated values of KdRe, Tl, Pbf/m were combined with the diffusivity of these elements in a 1-D bubble growth model to evaluate the change in metal concentrations and metal ratios expected during the degassing of magma. The model results were compared with emanation coefficients for trace metals from natural volcanoes. The magnitudes of the modeled Re/Tl and Re/Pb in fluids using the diffusivity and partitioning values at 1300 °C and the lowest fluid chlorinities were less than that observed from their emanation coefficients.

Research highlights
► Pb and Tl fluid/melt partition coefficients depend on Cl concentration in fluid.
► Re and Mo partition strongly into high Cl concentration fluids.
► Behavior of fluid/melt partition coefficients in H2O–Cl fluids is not linear.
► Modeling shows that small gas bubbles coalesce to preserve metal fractionation.
► New measurements of basalt-fluid partition coefficients for Mo, Re, W, Tl and Pb at 1 GPa.