In Situ Determination of Thermoelastic Properties of Magnesite at High Pressure and Temperature with Implications to Seismic Detectability of Moderately Carbonated Lithologies in the Earth's Mantle

Subduction of carbonate-bearing oceanic plates into Earth's interior recycles carbon from the surface to the deep mantle. The subducted carbonates can significantly affect mantle properties and dynamics. Magnesite is recognized as one of the major potential carbon hosts in the deep mantle because of its stability up to deep lower mantle conditions. However, despite many previous studies on the equation of state and elastic properties of magnesite, large discrepancies still exist for its elastic moduli and their pressure and temperature derivatives. Here we report in situ density and elastic wave velocity measurements on a natural magnesite at simultaneous high pressure-temperature conditions up to similar to 8 GPa-1073 K in a multi-anvil apparatus using ultrasonic and synchrotron X-ray techniques. Global fitting of the data set to finite strain equations yields KS0 = 114.0 +/- 1.2 GPa, KS ' ${K}_{S}<^>{\prime }$ = 4.9 +/- 0.3, partial derivative KS partial derivative TP ${\left(\frac{\partial {K}_{S}}{\partial T}\right)}_{P}$ = (-0.019 +/- 0.002) GPa/K, G0 = 68.6 +/- 0.4 GPa, G ' = 1.6 +/- 0.1, and partial derivative G partial derivative TP ${\left(\frac{\partial G}{\partial T}\right)}_{P}$ = (-0.018 +/- 0.001) GPa/K. Compared to other major upper mantle and transition zone minerals, magnesite has intermediate VP, the lowest VS, and the lowest density. Thus, magnesite possesses higher VP/VS ratio than other mantle minerals in normal mantle regions, whereas this feature is less pronounced in subduction zone environments. Modeling of the velocity profiles of carbonated lithologies along different geotherms suggests that moderately-enriched magnesite domains are unlikely to be detected seismically in the Earth's mantle. Carbon can be transported into Earth's interior by subducting slabs in the form of carbonates. Magnesite (MgCO3) is recognized as one of the most important oxidized carbon hosts inside the Earth. Accurate knowledge of the thermoelastic properties of magnesite at Earth's mantle conditions is thus needed in order to evaluate its seismic detectability and effect on mantle seismic structure and dynamics. Despite many previous efforts in determining the properties of magnesite, large discrepancies still exist for its elastic moduli and their pressure and temperature derivatives. In this study, we determined both the density and elastic wave velocity of magnesite at high pressure and temperature conditions up to similar to 8 GPa-1073 K using in situ ultrasonic measurements combined with synchrotron X-ray techniques in a multi-anvil press. Our high-quality data tightly constrained the thermoelastic parameters of magnesite, which were then used to model the velocity and density of magnesite and magnesite-bearing rocks in the Earth's mantle along different geotherms. Our results show that despite the fact that magnesite has the lowest VS compared to other major mantle minerals, moderately-enriched magnesite domain is difficult to detect in the Earth's mantle based on its seismic velocity. In situ density and elastic wave velocity measurements on magnesite at simultaneous high P-T The thermoelastic properties of magnesite are tightly constrained by high-quality P-rho-T-VP-VS data set Moderately-enriched magnesite domains are difficult to detect seismically in the Earth's mantle