Effects of H2O on the crystal chemistry of bridgmanite in the deep lower mantle

Abstract

The lower mantle is dominated by bridgmanite. Bridgmanite is nearly dry in coexistence with a hydrous phase [1]. Under the deep lower mantle conditions, the alumina-rich niccolite-type silica phase (Nt-phase) with a formula of Si0.7Al 0.3O1.85Hx can contain up to 4.6 wt.% water via the Si4+ = Al3+ + H+ charge-coupled substitution. [2]. Further, the presence of H2O changes the crystal chemistry of bridgmanite in the deep lower mantle. In contrast to iron-depletion in dry bridgmanite under similar pressure-temperature conditions [3], we found that ferrous iron is stable in bridgmanite under hydrous conditions. With multigrain diffraction methods allowing separation of individual grains in a multiphase assemblage, in combination with chemical analysis on recovered samples, we have obtained effects of H2O on chemical composition and crystal chemistry of bridgmanite under deep lower mantle conditions [4]. Our results imply that the existence of a water reservoir might be a major origin of lateral heterogeneity in the deep lower mantle. [1]        T. Ishii, E. Ohtani, and A. Shatskiy, Earth and Planetary Science Letters 583, 117441 (2022). [2]        L. Liu, H. Yuan, Y. Yao, Z. Yang, F. A. Gorelli, N. Giordano, L. He, E. Ohtani, and L. Zhang, Geophysical Research Letters 49, e2021GL097178 (2022). [3]        L. Zhang et al., Science 344, 877 (2014). [4]        L. Zhang, H. Yuan, Y. Meng, and H.-K. Mao, Engineering 5, 441 (2019).