The Viscosity of Carbonate‐Silicate Transitional Melts at Earth's Upper Mantle Pressures and Temperatures, Determined by the In Situ Falling‐Sphere Technique

The circulation of carbon in Earth’s interior occurs through the formation, migration, and ascent of CO 2 ‐ bearing magmas throughout the convective mantle. Their chemical composition spans from carbonatitic to kimberlitic as a result of either temperature and pressure variations or local redox conditions at which partial melting of carbonated mantle mineral assemblages occurs. Previous experiments that focused on melting relations of synthetic CO 2 ‐bearing mantle assemblages revealed the stability of carbonate‐silicate melts, or transitional melts, that have been generally described to mark the chemical evolution from kimberlitic to carbonatitic melts at mantle conditions. The migration of these melts upward will depend on their rheology as a function of pressure and temperature. In this study, we determined the viscosity of carbonate‐silicate liquids (~18 wt% SiO 2 and 22.54 wt% CO 2 ) using the falling‐sphere technique combined with in situ synchrotron X‐ray radiography. We performed six successful experiments at pressures between 2.4 and 5.3 GPa and temperature between 1565 °C and 2155 °C. At these conditions, the viscosity of transitional melts is between 0.02 and 0.08 Pa˙s; that is, about one order of magnitude higher than what was determined for synthetic carbonatitic melts at similar P‐T conditions, likely due to the polymerizing effect of the SiO 2 component in the melt.