From Magma Oceans to Atmospheres: Copper Volatility as a Tracer of Magma Ocean Outgassing

Magma oceans represent a fundamental stage in the early evolution of rocky planets and are thought to be widespread during planetary accretion and differentiation. During this phase, extensive melting of the silicate mantle facilitates chemical exchange between the interior and the atmosphere, thereby exerting a primary control on the formation and evolution of primordial atmospheres. The composition of these primordial atmospheres is controlled by the release of volatile and moderately volatile elements from the magma ocean. Moderately volatile elements such as Cu are sensitive to evaporation under magma ocean conditions while remaining partially retained, making them valuable tracers of magma ocean outgassing. In addition to temperature, outgassing processes are governed by melt composition, oxygen fugacity, and ligand availability. As ancient magma oceans remain inaccessible to direct observation, their composition and outgassing behavior cannot be directly constrained. Experimental studies are therefore essential to understand magma ocean-atmosphere interactions.

In this study, outgassing experiments on Cu-bearing chemical systems under vacuum conditions are conducted using a high-temperature setup that combines a simultaneous thermal analyzer with a quadrupole mass spectrometer (NETZSCH STA 449 F3 Jupiter). This approach enables the simultaneous quantification of mass loss and identification of gas-phase species, allowing the determination of kinetic parameters, such as activation energies. The study initially focuses on simple chemical systems, including pure compounds such as Cu metal, Cu oxides, Cu sulfides, and Cu chlorides, and is subsequently extended to more complex silicate melts once the behavior of the simpler systems is well constrained. The outgassing behavior of Cu in silicate melts will be investigated as a function of melt composition, oxygen fugacity, and ligand availability, which are varied stepwise to assess their individual effects.

Initial experiments on simple chemical systems reveal pronounced differences in Cu volatility and gas-phase speciation depending on the starting material. Cu sulfides release CuS into the gas phase at temperatures as low as 600 °C, whereas Cu oxides decompose at around 900 °C, producing monoatomic Cu and O₂. In contrast, Cu metal exhibits significant evaporation only at temperatures above 1250 °C. These results indicate that Cu volatility is strongly influenced by its chemical environment.

Building on these results, the influence of ligand availability on Cu volatility is further investigated in reduced silicate melts. While the initial experiments primarily constrain evaporation kinetics, a comprehensive understanding of magma ocean outgassing also requires thermodynamic data. To address this, Knudsen cell experiments will be conducted in a later stage of the project. This method allows for determination of equilibrium vapor pressures and evaporation coefficients under controlled conditions, thereby providing direct constraints on the thermodynamics of Cu evaporation. By combining kinetic data from high-temperature outgassing experiments with thermodynamic constraints from Knudsen cell measurements, this study aims to better quantify the role of chemical environment in magma ocean outgassing and its implications for chemical evolution of primordial atmospheres.