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A first-principles study of water in wadsleyite and ringwoodite: Implication for the 520 km discontinuity

Wang, Wenzhong; Wu, Zhongqing; (2022) A first-principles study of water in wadsleyite and ringwoodite: Implication for the 520 km discontinuity. American Mineralogist , 107 (7) pp. 1361-1368. 10.2138/am-2021-7929.

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Abstract

The seismic discontinuity around 520 km is believed to be caused by the phase transition from wadsleyite to ringwoodite, the dominant minerals in the mantle transition zone (MTZ). Both wadsleyite and ringwoodite can contain more than 1.0 wt% water at MTZ’s conditions, but it is not well known how water affects the wadsleyite-ringwoodite transition. Here we investigated water partitioning between wadsleyite and ringwoodite and the water effect on this phase boundary using first-principles calculations. Our results show that the presence of water will shift the phase boundary to higher pressures, and the width of the two-phase coexistence domain in the Mg2SiO4-H2O system is insignificant at mid-MTZ conditions. For the (Mg0.9Fe0.1)2SiO4 system, the incorporation of 1.0 wt% water can narrow the effective width of two-phase coexistence by two-thirds. Together with elastic data, we find that velocity and impedance contrasts are only mildly changed by the water partitioning. We suggest that compared to the anhydrous condition, the presence of 1.0 wt% water will increase velocity gradients across the wadsleyite-ringwoodite transition by threefold, enhancing the detectability of the 520 km discontinuity.

Type: Article
Title: A first-principles study of water in wadsleyite and ringwoodite: Implication for the 520 km discontinuity
DOI: 10.2138/am-2021-7929
Publisher version: https://doi.org/10.2138/am-2021-7929
Language: English
Additional information: This version is the author accepted manuscript. For information on re-use, please refer to the publisher’s terms and conditions.
Keywords: Water partitioning, wadsleyite, ringwoodite, 520-km discontinuity, two-phase coexistence, mantle transition zone
UCL classification: UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL
URI: https://discovery.ucl.ac.uk/id/eprint/10153288
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