Experimental degassing of moderately volatile chalcophile elements from silicate melt with and without sulfur present

Document Type

Article

Publication Date

8-1-2025

Abstract

We quantify the evaporation of moderately volatile chalcophile elements (MVCEs, here Cu, Zn, Ge, As, Ag, Cd, In, and Pb) from silicate melts. Our approach is to subject MVCE-bearing silicate melt to variable temperature (1480–1773 K), fO2 (10−9 to atmospheric) and fS2 (0 or 10−3 to 10−2), and duration (5 min to 24 h) using a gas-mixing furnace where evaporation can occur. Chemical analyses are completed using core to rim transects for major elements and MVCEs. We process our MVCE data through layered inverse models to empirically determine evaporation reaction stoichiometry, apparent thermodynamic data, and MVCE diffusivity for experiments that undergo diffusion-limited evaporative loss. The diffusivity values recovered for MVCEs between 1481 and 1673 K range from 6.5 × 10−12 to 1.62 × 10−9 m2/s. Apparent evaporation enthalpies range from 213 to 1099 kJ/mol and entropies range from 85 to 621 J/mol K. We observe sulfur to have element-specific effects on MVCE volatility (Cu, Zn, Ge, and In). The volatility of Cu and Zn decreases with the presence of S, while Ge and In volatility increase with S present. We report previously unquantified evaporation thermodynamic data for Ag and As from silicate melt. Our thermodynamic data applied to magma oceans suggest that degassing under relatively cool (∼1573 K), highly reduced conditions (IW-3 to −5) can produce MVCE depletions broadly similar to those associated with incomplete nebular condensation, but key fractionations remain, particularly in In/Cd ratios. We also predict that magmatic evaporation under more oxidized conditions yields MVCE fractionations highly dissimilar to those associated with nebular condensation but broadly consistent with observations of volcanic outgassing.

Publication Source (Journal or Book title)

Geochimica Et Cosmochimica Acta

First Page

153

Last Page

172

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