Water Content of the Dehydration Melting Layer in the Topmost Lower Mantle

Abstract

A dehydration melting layer is predicted near the 660-km depth boundary by the phase transition of hydrous ringwoodite in the mantle transition zone to dry bridgmanite and ferropericlase in the lower mantle. However, the water content in the melt, which controls the melt density and thus gravitational stability, remains poorly constrained. In this study, the water content in hydrous melt under 660-km depth conditions is estimated as a function of temperature. At 2000 K, corresponding to topmost lower mantle geotherm, hydrous silicate melt should contain ∼20 wt.% water, which will cause the melt to be gravitationally unstable. The melt fraction at 660-km depth is estimated to be >0.5 vol.%, which may significantly reduce rock viscosity and seismic velocity. FEI

Temperature Dependence of H2O Solubility in Al‐Free Stishovite

“…However, the water content in the hydrous silicate melt in a complex system is only 150,000–250,000 wt. ppm at 1600–1800°C (Fei, 2021), and therefore, the water activity in Litasov et al. (2007)'s study should be much lower than in this study.…”

Water Solubility in Fe‐Bearing Wadsleyite at Mantle Transition Zone Temperatures

“…If ringwoodite in the lower part of the mantle transition zone is nearly H 2 O‐saturated (Fei et al., 2017), dehydration melting should occur at the 520‐km discontinuity caused by the phase transformation from ringwoodite to wadsleyite under H 2 O‐saturated conditions driven by upwelling flow in mantle plumes. By assuming 1.0 wt.% H 2 O in ringwoodite (Fei & Katsura, 2020), 0.65 wt.% H 2 O in wadsleyite (this study), a H 2 O content of ∼16% in hydrous silicate melt at 2100 K (Fei, 2021), and a ringwoodite or wadsleyite volume fraction of ∼55% (Frost, 2008), a mass balance calculation indicates a melt fraction of 1.2 vol.% in the dehydration melting layer. Such a high fraction is sufficient to completely wet the grain boundaries of wadsleyite and thus reduce its viscosity and seismic velocity.…”

Viscosity of anhydrous and hydrous peridotite melts

“…These melts are expected to be ultramafic and iron-enriched . The FeOtot in the melts can reach 20 -30 mol.% because of the high partitioning coefficient of Fe/Mg between melt and solids, while the water content can reach about 20 -50 wt.% (Fei, 2021;Ghosh and Schmidt, 2014;Nakajima et al, 2019). Although the pressure conditions for the 660-km discontinuity (about 23 GPa) are much higher than the experimental conditions in this study (ambient pressure), the pressure effect on melt viscosity is relatively small (Xie et al, 2021(Xie et al, , 2020.…”

“…Such a reduction in melt viscosity may further enhance horizontal flow, leading to slab stagnation at the bottom of the mantle transition zone as imaged seismologically (Fukao and Obayashi, 2013). On the other hand, melt at the 660-km discontinuity is gravitationally unstable because of its low density compared to the mantle transition zone (Fei, 2021;Nakajima et al, 2019). Its low viscosity may enhance the upwelling of the melt, which returns water from the subducting slabs back to the mantle transition zone.…”

“…The resulting peridotite magma oceans facilitated core formation through silicate-metal segregation (e.g., Li and Agee, 1996;Wade and Wood, 2005) and originated the atmosphere and hydrosphere through degassing (e.g., Karato et al, 2020;Sossi et al, 2020). Moreover, high-pressure experiments demonstrated that highly ultramafic and Mg-rich melts (such as picrites and kimberlites) could be formed at present by partial melting of peridotite in the upper and lower mantle (e.g., Fei, 2021;Kawamoto and Holloway, 1997;Mysen and Boettcher, 1975;Sinmyo et al, 2019), with far-reaching implications for the transport of incompatible elements such as hydrogen (Karato et al, 2020).…”