Water Concentration in Single-Crystal (Al,Fe)-bearing Bridgmanite and its Possible Implications for the Dehydration Melting below 660 km Depth
Abstract
Global water cycle between Earth's surface and its deep interior plays a key role in understanding the physical, chemical and dynamic states of the planet. Early studies show as high as 1-3 wt% water (hydrogen) can be dissolved into the crystal structures of major transition zone minerals, wadsleyite and ringwoodite, in form of lattice-bonded hydrogen atoms (1,2). However, water solubility in lower-mantle bridgmanite, the most abundant mineral in the most volumetric layer of the planet, has remained poorly understood. The scientific challenge here was largely due to the difficulties in making large-size high-quality single-crystals of bridgmanite for reliable characterizations of its water concentration. In most early studies (3-5), the presence of hydrogen-rich inclusions is the most serious cause for the discrepancy on water concentration in their synthesized bridgmanite. Here we synthesized high-quality single-crystal Fe/Al-bearing bridgmanite of hundred s of micrometer size in a Kawai-type apparatus from the starting hydrous melt containing water . The crystals were examined to be inclusion- and precipitate-free with multiple analytical techniques, including scanning and transmission electron microscopy, and Mössbauer spectroscopy . Nano-secondary ion mass spectrometry (NanoSIMS) analyses were used for reliable water concentration measurements. Unpolarized and polarized Fourier-transform i nfrared ( FTIR ) analyses were used to identify characteristic OH-stretching bands, which could provide complimentary information on the hydrogen substitution mechanism in the (Al,Fe)-bearing bridgmanite. Our results show a significant amount of water is present in the bridgmanite. In this presentation, we will discuss the geophysical and geochemical consequences of the high water solubil ity in lower-mantle bridgmanite, with a focus on the role of water on dehydration melting at the topmost lower mantle near the transition zone.
1 Kohlstedt, D. et al. Contrib Mineral Petr 123 , 345 (1996). 2 Pearson, D. et al. Nature 507 , 221 (2014). 3 Bolfan-Casanova, N. et al. Geophys Res Lett 30 (2003). 4 Inoue, T . et al. Phys Earth Planet In 183 , 245 (2010). 5 Murakami, M . et al. Science 295 , 1885 (2002).- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMMR33A..04L
- Keywords:
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- 1042 Mineral and crystal chemistry;
- GEOCHEMISTRY;
- 3924 High-pressure behavior;
- MINERAL PHYSICS;
- 3924 High-pressure behavior;
- MINERAL PHYSICS;
- 3924 High-pressure behavior;
- MINERAL PHYSICS;
- 3999 General or miscellaneous;
- MINERAL PHYSICS;
- 3999 General or miscellaneous;
- MINERAL PHYSICS;
- 3620 Mineral and crystal chemistry;
- MINERALOGY AND PETROLOGY;
- 3620 Mineral and crystal chemistry;
- MINERALOGY AND PETROLOGY;
- 3672 Planetary mineralogy and petrology;
- MINERALOGY AND PETROLOGY;
- 5430 Interiors;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS;
- 7208 Mantle;
- SEISMOLOGY;
- 7208 Mantle;
- SEISMOLOGY