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

Abstract

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 ∼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, (Formula presented.) = 4.9 ± 0.3, (Formula presented.) = (−0.019 ± 0.002) GPa/K, G0 = 68.6 ± 0.4 GPa, G′ = 1.6 ± 0.1, and (Formula presented.) = (−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.