Program > Browse abstracts by speaker > Gorojovsky Lauren

Oxygen fugacity (fO₂) recorded by volcanic liquids and their source rocks significantly influences Earth's geochemical evolution by controlling the composition of C-O-H-S gases released into the atmosphere and governing the distribution of economically valuable metals (e.g., Cu, Sn, Te, Pt) between the mantle and crust. Central to understanding magmatic redox states is sulfur, which can exist in both oxidised (SO₄²⁻, SO₂) and reduced (S²⁻, H₂S) forms in melts and vapours. Accurate knowledge of sulfur speciation and solubility as functions of fO₂, temperature, and melt composition is essential for understanding these processes.

Previous works have characterised sulfate (CS⁶⁺) and sulfide (CS²⁻) solubilities in silicate melts at atmospheric pressure (1 atm) between 1500 and 1200 °C. Recent work by Boulliung and Wood (2023) suggested a potential non-linear increase in sulfate solubility at lower reciprocal temperatures. To further investigate this, we experimentally determined sulfate and sulfide solubilities in silicate melts across a temperature range of 1250 to 1050 °C. Experiments involved equilibrating natural and synthetic melts over durations of 1–5 days under controlled oxygen (fO₂) and sulfur (fS₂) fugacities using gas mixtures comprising air, SO₂, CO, and CO₂ at ambient pressure.

We modelled the resulting sulfate and sulfide capacities using reciprocal solution theory, yielding excellent consistency with prior experiments conducted at 1 atm within overlapping temperature ranges. Our results demonstrate a clear, non-linear decrease in sulfate solubility with decreasing reciprocal temperature, shifting the sulfide-sulfate transition (S⁶⁺/ΣS = 0.5) from QFM +1 at 1200 °C to approximately QFM +2 at 1050 °C. Additionally, our findings allow us to simulate C-O-H-S degassing pathways under conditions relevant to subduction zones, demonstrating that sulfur degassing under hydrous conditions can drive significant oxidation, potentially explaining the elevated oxidation states of subduction zone magmas compared to those at mid-ocean ridges.