The influence of redox condition on crystal structures of bridgmanite at megabar pressure
Abstract
Iron can incorporate into bridgmanite in different oxidation state and by several substitution mechanisms, which may depend on the chemical composition, P-T conditions and oxygen fugacity. Numerous studies have shown that the iron content and occupied site in the bridgmanite structure may strongly affect both elastic and conductive properties of the lower mantle, but it is still a challenge to determine it unambiguously in situ under lower mantle P-T conditions.
In this study, both enstatite (Mg0.85Fe0.15)SiO3 and Fe3+-bearing silicate gel Mg0.85Fe0.15SiOx were prepared as the starting materials to represent different redox state. Coarse-grained polycrystalline bridgmanite phases with different starting materials were synthesized in the laser-heated DAC at megabar pressure and 2400 K, along with H phase and silica (CaCl2 structure) as the minor phases. High-quality diffraction spots belonging to individual bridgmanite grains were collected by rotation method at high pressure and at ambient conditions separately and then extracted using multigrain method 1. In the runs where silicate gel was used as the starting material, bridgmanite has a small unit cell, similar that MgSiO3 bridgmanite measured in previous work 2 at both high pressure and ambient conditions after decompression (Figure 1). However, the detailed structure of the bridgmanite from the gel in our study is different from that of Mg-bridgmanite, implying that the smaller unit cell volume in this study may not be simply caused by the loss of iron. On the other hand, the unit cell volume of bridgmanite synthesized from En85 in this study is comparable to that in previous work using the same kind of starting material 3. These features may result from different iron oxidation states and ferric iron substitution in bridgmanite structure. In situ single-crystal XRD combined with the multigrain method in this study provides constraints on the detailed structure of bridgmanite at high pressure. Reference - O. Sørensen et al., Z. Kristallogr. 227,63-78 (2012). - B. Ballaran et al., Earth Planet. Sci. Lett. 333, 181-190 (2012) - Lundin et al., Phys. Earth Planet. Inter. 168,97-102 (2008). Figure 1. Variation with pressure of the unit cell volume of bridgmanite. The solid points stand for the unit cell volume of bridgmanite from Mg0.85Fe0.15SiO3 gel in this study.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMMR33C0139M
- Keywords:
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- 3924 High-pressure behavior;
- MINERAL PHYSICSDE: 3954 X-ray;
- neutron;
- and electron spectroscopy and diffraction;
- MINERAL PHYSICSDE: 3612 Reactions and phase equilibria;
- MINERALOGY AND PETROLOGYDE: 3621 Mantle processes;
- MINERALOGY AND PETROLOGY