Abstract
The determination of the speciation of ions and molecules in supercritical aqueous fluids under pressure is critical to understanding their mass transport in the Earth’s interior. Unfortunately, there is no experimental technique yet available to directly characterize species dissolved in water at extreme conditions. Here we present a strategy, based on first-principles simulations, to determine ratios of Raman scattering cross-sections of aqueous species under extreme conditions, thus providing a key quantity that can be used, in conjunction with Raman measurements, to predict chemical speciation in aqueous fluids. Due to the importance of the Earth’s carbon cycle, we focus on carbonate and bicarbonate ions. Our calculations up to 11 GPa and 1000 K indicate a higher concentration of bicarbonates in water than previously considered at conditions relevant to the Earth’s upper mantle, with important implications for the transport of carbon in aqueous fluids in the Earth’s interior.
Highlights
The determination of the speciation of ions and molecules in supercritical aqueous fluids under pressure is critical to understanding their mass transport in the Earth’s interior
In particular we present the results of first-principles molecular dynamics (FPMD) simulations of Na2CO3 and NaDCO3 aqueous solutions at high P (HP)-HT conditions
The Raman spectra were obtained using FPMD simulations, which account for an-harmonic effects, critical to describe fluids
Summary
The determination of the speciation of ions and molecules in supercritical aqueous fluids under pressure is critical to understanding their mass transport in the Earth’s interior. An important component of geofluids in the Earth’s crust and mantle, may carry a significant amount of carbon, but its dissolved forms in aqueous geofluids are still poorly known, limiting our overall understanding of mass transport of carbon in the Earth. This lack of knowledge is due to experimental difficulties in measuring which species are present in C–H–O fluids at extreme conditions, as well as to the use, on the theoretical side, of simplified models[6,7]. Several estimates of the dependence of the Raman cross-sections of aqueous solutions at high P (HP) and high
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