CSEDI: Alkaline-Earth Carbonate Melts at Deep Earth Conditions

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Carbonates are common rock-forming minerals that contain significant carbon in their crystal structures, and thus represent a dominant reservoir for carbon in the Earth System. Despite their primary role in the long-term carbon cycle, we currently have a very limited understanding of their melting behavior at the relevant conditions deep inside the Earth. The carbon hosted in carbonate-bearing rocks is carried into the Earth by subducting oceanic plates, where it is either returned to the surface through volcanism or dragged further down into the deep mantle, depending on the depth of carbonate melting. The melting properties of carbonates thus directly control the long-term component of Earth's carbon cycle. This project is focused on better understanding the details of carbonate melting at extreme pressures and temperatures by combining diverse research techniques using experiments, simulations, and statistical modeling. In addition to improving our fundamental understanding of the deep carbon cycle, this project supports developing both technical and leadership expertise; the project is being led by an early career research scientist who will join the co-investigators in the training of two graduate students in a variety of cutting edge research techniques that are also broadly applicable to future material properties research.The thermodynamic melting properties of carbonates directly control the long-term component of Earth's carbon cycle, and yet this key area has remained poorly understood as a result of practical challenges for carbonate melting experiments. These difficulties include decomposition prior to melting at pressures below ~3 GPa, as well as extreme sensitivities of the melting behavior to pressure, temperature, and composition (including water content), which severely complicate analysis of such experiments. The goal of this proposal is to combine multiple experimental and theoretical methods to characterize the thermodynamics of solid and molten alkaline earth carbonates, in the (Ca,Mg,Sr,Ba)CO3 system, which encompasses the dominant Mg-Ca bearing components most relevant to the deep Earth. The team's experimental objectives are to: (1) carry out conductivity-based in situ melt detection experiments up to 25 GPa for each endmember, (2) conduct liquid one-bar density and sound speed measurements to determine how each component affects melt density and compressibility, and (3) perform laser-heated diamond anvil cell measurements to constrain the high pressure-temperature (PT) equations of state of the solidus phases. These are combined with theoretical objectives to: (1) simulate the alkaline earth carbonate system using empirical molecular dynamics to predict high PT properties for both melt and mineral phases and (2) employ Bayesian statistical analysis to obtain a global self-consistent thermodynamic model of the (Ca,Mg,Sr,Ba)CO3 system. The final thermodynamic picture provided by this work represents a major stepping-stone on the path to incorporating carbonate melting into a variety of widely-used thermodynamic databases that underpin our understanding of geologic processes at extreme planetary conditions. Together the five techniques employed in this work, which rely on the cooperation and distinct expertise of the three principal investigators, will provide a comprehensive thermodynamic view of carbonate melting at deep mantle conditions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.