The role of volatiles (H2O, CO2) in the mantle incipient melting captured by a multi-component model
Résumé
The link between volatiles and mantle melting has so far been illuminated by
experiments revealing punctually, at a given P-T condition and under a specific
chemical system, properties such as solubility laws, redox equilibria, and phase
equilibria. Our aim is to establish a multi-component model describing the Gibbs free
energy of melt produced by mantle melting in presence of CO2-H2O: that are
carbonatite-carbonated melt-nephilinite-basanite and basalt with increasing degree of
partial melting.
Near solidus melts are dominated by carbonate-rich compositions, evolving towards
basaltic compositions at higher temperatures. However, this carbonate-silicate
transition is complex, abrupt, and dependent on temperature, pressure and the chemical
composition of the system. In order to simulate partial melting in a variety of mantle
conditions, we established a parameterization of the mixing properties allowing the
complex activity-composition relationships for multi-component hydrated carbonated
melts to be accounted for. Using the Margules formalism, this parameterization is
calibrated on crystal-liquid, graphite-liquid, fluid-liquid and liquid-liquid equilibria
obtained by experimental studies in the P-T range 1-10 GPa and 900-1800°C. We so far
adjusted the activity of the SiO2 and CO2 melt components, which constitutes the main
part of the silicated and carbonated frameworks. The SiO2-CO2 interaction reveals a
strong non-ideality requiring a strongly asymetric Margules formulation. We also
determined the standard thermodynamic properties for the CO2 melt component and we
have refined the standard volume properties for liquid SiO2.