Mobility of volatile (H2O & CO2)-bearing melts in the upper mantle: from carbonatite magmatism to the dynamics of kimberlitic eruptions

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Volatiles (H2O and CO2) play a critical role in the internal dynamics and thermal evolution of planetary interiors, and define the habitability conditions at their surface. Understanding the exchanges and fluxes of volatiles between deep and surficial reservoirs thus remain a primary goal of the Earth Sciences community. Yet, the quantitative modelling of these processes is largely limited by the lack of fundamental constraints on the mobility and migration rates of volatile-bearing melts, that are the main conveyors for the distribution of volatiles. This project addresses this critical gap of knowledge by combining high pressure density measurements in volatile-bearing melts and thermodynamic modelling to gain quantitative insights into the deep volatile cycles and related geophysical processes inside the planet. During the first funding period, we focused on carbonate-rich melts and constrained the effect of hydration on the density/mobility of carbonate melts, the compressibility of dissolved volatiles in the melt and the timescale of carbonate melt extraction from subducted lithologies. Moreover, we calibrated the first continuous density model for carbonate melts in the system MgO-CaO-Na2O-K2O-Li2O-H2O-CO2 at conditions that span magmatic processes beyond the mantle transition zone, i.e. up to 30 GPa and 2300 K. After a successful first funding period, the extension project will focus on intermediate silica under-saturated carbon-rich melts for which density data is virtually not available at relevant mantle conditions in contrast to volatile-bearing silica-rich melts that have been extensively investigated. Density data in carbonatite and kimberlite melts at upper mantle conditions will be obtained using a combination of synchrotron X-ray absorption/radiography and sink-float methods in large volume presses. The new density data will enable the extension of the carbonate melt density model into a global density model for the range of volatile-bearing melts stabilized in the upper mantle, i.e. from carbonates (CO2-rich) to carbonated basalts (SiO2-rich melts). The new model will allow predictions of the mobility of volatile-bearing melts and modelling the migration/ascent/emplacement of melts through the mantle, including the dynamics of diamond-bearing kimberlite eruptions, and their geophysical signature in the solid mantle. Ultimately, the results of this project will lead to a new understanding of volatile mobility and recycling in the deep Earth.