Phase stabilities of carbonates and reaction between carbonates and mantle minerals at high pressures and temperatures studied in the LH-DAC by Raman and time-resolved laser fluorescence spectroscopy

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The scientific goal of the present project is the same as the overall goal of the Research Unit, namely to better understand the structures, properties and reactions of carbonates at extreme conditions. In order to achieve this goal, we propose to determine the stabilities of carbonates and reactions between carbonates and mantle minerals at geophysically relevant conditions using laser heated DAC-based studies. We will complement the experimental studies with DFT-based atomistic models in order to be able to quantitatively interpret spectra and to derive thermophysical parameters.Phase stability of carbonates at high pressure and temperature: The stabilities of end members in the system (Ca,Mg,Fe)CO3 have been extensively investigated. CaCO3 and MgCO3 are chemically stable as pure phases at pressure and temperature conditions corresponding to the deep lower mantle, but undergo a sequence of structural phase transitions. In contrast, pure FeCO3 breaks down into new phases of iron carbonates with unexpected stoichiometry and iron oxides. Most of the new phases of iron carbonates are stabile only at high pressures and temperatures. Previous studies of the behaviour of solid solutions in the system (Ca,Mg,Fe)CO3 are generally limited to ambient temperature studies at higher pressures or to higher temperature at moderate pressures. There is therefore a significant gap in our knowledge concerning the stabilities of binary and ternary solid solutions in the system (Ca,Fe,Mg)CO3 at mantle conditions. Therefore, we propose to continue our studies to determine the phase diagrams for binary or ternary solid solutions of (Ca,Fe,Mg)CO3 using in situ x-ray diffraction and in situ Raman and fluorescence spectroscopy at high pressure and temperature.Reaction between carbonates and mantle minerals: The existence of carbonatite melt in the mantle was deduced from the analyses of inclusions in diamonds. Such carbonatitic melts can be responsible for the redistribution of chemical elements in the mantle. However, most previous studies aimed at understanding the distribution of trace elements in the mantle have relied on quenched samples. As there are no data on the partitioning of trace elements during carbonate-silicate interactions at lower mantle conditions, the present proposal aims to close this gap. While some studies have shown that x-ray fluorescence spectroscopy, XRF, can be employed for the in situ investigation of partitioning of trace elements, the applicability of XRF for in situ studies at high p,T is limited. Here, we therefore propose to use doped samples, which will allow us to determine the distribution of trace elements during the reaction using in situ time-resolved laser fluorescence spectroscopy, Raman spectroscopy and x-ray diffraction in laser heated diamond anvil cell experiments.