Water in Earth's Lower Mantle

Abstract

Secondary ion mass spectrometry measurements show that Earth's representative lower mantle minerals synthesized in a natural peridotitic composition can dissolve considerable amounts of hydrogen. Both MgSiO3-rich perovskite and magnesiowüstite contain about 0.2 weight percent (wt%) H2O, and CaSiO3-rich perovskite contains about 0.4 wt% H2O. The OH absorption bands in Mg-perovskite and magnesiowüstite were also confirmed with the use of infrared microspectroscopic measurements. Earth's lower mantle may store about five times more H2O than the oceans.

Lower mantle hydrogen partitioning between periclase and perovskite: A quantum chemical modelling

“…Lower mantle minimum/maximum Bulk Water Content (hereafter BWC; Table 2) figures were inferred to be smaller than 1000 and 2000 ppm, respectively, using a conservative H 2 O estimate in line with BSE (1100-3000 ppm; Palme and O'Neill, 2003;Ohtani, 2005;Marty, 2012), and combining it with the mantle mass fractions (Table 2). The resulting BWC estimates are in agreement with earlier studies (Murakami et al, 2002;Ohtani, 2005;Hernàndez et al, 2013). For this reason, 800 and 1500 ppm reference average values were chosen as lower and upper BWC estimates, respectively; precise values depend on the geochemical model and will be considered below, in the Section 8.…”

“…Moreover, experimental Kd pe=pvk H 2 O values span a disquieting range of 0. 2-75 (Bolfan-Casanova et al, 2000;Murakami et al, 2002Murakami et al, , 2002Litasov et al, 2003;Ohtani, 2005;Inoue et al, 2010). This could be explained by relevant experimental difficulties both in performing hydrous experiments at extreme pressures and temperatures and in analysing small H 2 O concentrations in the resulting tiny crystals (Keppler and Smyth, 2006 and reference therein).…”

“…On the basis of some experimental studies, Fe-free periclase seems to accommodate almost no water in its structure (Bolfan-Casanova et al, 2000; conversely, other authors (Murakami et al, 2002;Litasov and Ohtani, 2007) assign high water incorporation capability to this phase, with H 2 O-uptake ranging between 0.1 and 0.2 wt% (1000-2000 ppm). Such discrepancies might mirror the main H incorporation mechanisms in the structure of the samples, their being out of P-T thermodynamic equilibrium and/or an analytical strategy for water measurement.…”

Rapid decrease of MgAlO2.5 component in bridgmanite with pressure

“…Hence, H 2 O is expected to be incorporated in bridgmanite as MgHAlO 3 in the lower mantle because of high pressures. Murakami et al (2002) and Litasov et al (2003) reported that synthetic Al-Fe-bearing bridgmanite in mantle peridotite can contain 1000-2000 ppm water at 26 GPa and 1500-1923 o K, which is significantly higher than that in synthetic MgSiO 3 bridgmanite at 24-25 GPa and 1573-1873 o K (Bolfan-Casanova et al, 2000; Geochemical Perspectives Letters Letter Litasov et al, 2003). Following Equation 2, the amount of 2.2 mol.…”

Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating

“…The masses of exoplanet oceans are unknown. Earth's mantle contains a few ocean masses of water (Bell and Rossman, 1992;Murakami et al, 2002;Marty and Yokochim, 2006), so it is possible that the surface of a planet could acquire a new ocean via outgassing after desiccation and become habitable again. Thus, we cannot conclude with certainty that a planet with a desiccated atmosphere will never be habitable in the future.…”