Single-crystal elasticity of the deep-mantle magnesite at high pressure and temperature

摘要

Magnesite (MgCO_3) is considered to be a major candidate carbon host in the Earth's mantle, and has been found to exist as an accessory mineral in carbonated peridotite and eclogite. Studying the thermal elastic properties of magnesite under relevant pressure-temperature conditions of the upper mantle is thus important for our understanding of the deep-carbon storage in the Earth's interior. Here we have measured the single-crystal elasticity of a natural magnesite using in situ Brillouin spectroscopy and X-ray diffraction in a diamond anvil cell up to 14 GPa at room temperature and up to 750 K at ambient pressure, respectively. Using the third-order Eulerian finite-strain equations to model the elasticity data, we have derived the aggregate adiabatic bulk, K_(S0), and shear moduli, Go, at ambient conditions: K_(S0) = 114.7 (±1.3) GPa and G_0 = 69.9 (±0.6) GPa. The pressure derivatives of the bulk and shear moduli at 300 K are (?K_S/?P)_T = 4.82 (±0.10) and (?G/?P)_T = 1.75 (±0.10), respectively, while their temperature derivatives at ambient pressure are (?Ks/?T)_P = -24.0 (±0.2) MPa/K and (?G/?T)_P = -14.8 (±0.7) MPa/K. Based on the thermal elastic modeling of the measured elastic constants along an expected normal upper-mantle geotherm and a cold subducting slab, magnesite exhibits compressional wave (V_P) anisotropy of approximately 46-49% and shear wave (V_S) splitting of 37-41% that are much larger than those of major constituent minerals in the Earth's upper mantle including olivine, pyroxene, and garnet. The modeled aggregate V_P and V_S velocity in moderately carbonated peridotite and eclogite containing approximately 10 wt.% magnesite (approximately 5 wt.% CO_2) show minimal effects of magnesite on the seismic profiles of these rock assemblages at upper mantle conditions, suggesting that the presence of magnesite is likely difficult to be detected seismically. However, due to its unusually high V_P and V_S anisotropies, magnesite with strong preferred orientations may exhibit sufficient V_P and V_S anisotropies that can have significant influences on seismic anisotropies of the regionally carbonated upper mantle.