Journal
EARTH AND PLANETARY SCIENCE LETTERS
Volume 494, Issue -, Pages 92-98Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.epsl.2018.05.003
Keywords
delta-AlOOH; thermal equation of state; water; dehydration; ULVZ
Categories
Funding
- National Science Foundation of China [41522403]
- National Basic Research Program of China [2014CB845904]
- Chinese Academy of Sciences [XDB18000000]
- National Science Foundation - Earth Sciences [EAR-1128799]
- U.S. Department of Energy - GeoSciences [DE-FG02-94ER14466]
- DOE-BES [DE-AC02-06CH11357]
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In this study, we present new experimental constraints on the phase stability and thermal equation of state of an important hydrous phase, delta-AlOOH, using synchrotron X-ray diffraction up to 142 GPa and 2500 K. Our experimental results have shown that delta-AlOOH remains stable at the whole mantle pressure-temperature conditions above the D '' layer yet will decompose at the core-mantle boundary because of a dramatic increase in temperature from the silicate mantle to the metallic outer core. At the bottom transition zone and top lower mantle, the formation of delta-AlOOH by the decomposition of phase Egg is associated with a similar to 2.1-2.5% increase in density (rho) and a similar to 19.7-20.4% increase in bulk sound velocity (V-Phi). The increase in rho across the phase Egg to delta-AlOOH phase transition can facilitate the subduction of delta-AlOOH to the lower mantle. Compared to major lower-mantle phases, delta-AlOOH has the lowest rho but greatest V-Phi, leading to an anomalous low rho/V-Phi ratio which can help to identify the potential presence of delta-AlOOH in the region. More importantly, water released from the breakdown of delta-AlOOH at the core-mantle boundary could lower the solidus of the pyrolitic mantle to cause partial melting and/or react with Fe in the region to form the low-velocity FeO2Hx phase. The presence of partial melting and/or the accumulation of FeO2Hx phase at the CMB could be the cause for the ultra-low velocity zone. delta-AlOOH is thus an important phase to transport water to the lowermost mantle and helps to understand the origin of the ultra-low velocity zone. (C) 2018 Elsevier B.V. All rights reserved.
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