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A metastable Fo-III wedge in cold slabs subducted to the lower part of the mantle transition zone: A hypothesis based on first-principles simualtions
Yining Zhang;  Yanyao Zhang;  Yun Liu;  Xi Liu
2019
Source PublicationMinerals
Volume9Issue:3Pages:1-29
Abstract

The metastable olivine (Ol) wedge hypothesis assumes that Ol may exist as a metastable phase at the P conditions of the mantle transition zone (MTZ) and even deeper regions due to inhibition of the phase transitions from Ol to wadsleyite and ringwoodite caused by low T in the cold subducting slabs. It is commonly invoked to account for the stagnation of the descending slabs, deep focus earthquakes and other geophysical observations. In the last few years, several new structures with the forsterite (Fo) composition, namely Fo-II, Fo-III and Fo-IV, were either experimentally observed or theoretically predicted at very low T conditions. They may have important impacts on the metastable Ol wedge hypothesis. By performing first-principles calculations, we have systematically examined their crystallographic characteristics, elastic properties and dynamic stabilities from 0 to 100 GPa, and identified the Fo-III phase as the most likely metastable phase to occur in the cold slabs subducted to the depths equivalent to the lower part of the MTZ (below the ~600 km depth) and even the lower mantle. As disclosed by our theoretical simulations, the Fo-III phase is a post-spinel phase (space group Cmc21), has all cations in sixfold coordination at P < ~60 GPa, and shows dynamic stability for the entire P range from 0 to 100 GPa. Further, our static enthalpy calculations have suggested that the Fo-III phase may directly form from the Fo material at ~22 GPa (0 K), and our high-T phase relation calculations have located the Fo/Fo-III phase boundary at ~23.75 GPa (room T) with an averaged Clapeyron slope of ~−1.1 MPa/K for the T interval from 300 to 1800 K. All these calculated phase transition pressures are likely overestimated by ~3 GPa because of the GGA method used in this study. The discrepancy between our predicted phase transition P and the experimental observation (~58 GPa at 300 K) can be explained by slow reaction rate and short experimental durations. Taking into account the P-T conditions in the cold downgoing slabs, we therefore propose that the Fo-III phase, rather than the Ol, highly possibly occurs as the metastable phase in the cold slabs subducted to the P conditions of the lower part of the MTZ (below the ~600 km depth) and even the lower mantle. In addition, our calculation has showed that the Fo-III phase has higher bulk seismic velocity, and thus may make important contributions to the high seismic speeds observed in the cold slabs stagnated near the upper mantle-lower mantle boundary. Future seismic studies may discriminate the effects of the Fo-III phase and the low T. Surprisingly, the Fo-III phase will speed up, rather than slow down, the subducting process of the cold slabs, if it metastably forms from the Ol. In general, the Fo-III phase has a higher density than the warm MTZ, but has a lower density than the lower mantle, as suggested by our calculations.

KeywordCold Slabs Crystallographic Feature And Structural Evolution FIrst-principles Simulation Metastable Olivine Wedge Metastable Fo-iii Wedge Thermodynamic Properties mantle Transition Zone
Indexed BySCI
Language英语
Document Type期刊论文
Identifierhttp://ir.gyig.ac.cn/handle/42920512-1/10389
Collection矿床地球化学国家重点实验室
Affiliation1.The Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education of China, Beijing 100871,China
2.School of Earth and Space Sciences, Peking University, Beijing 100871, China
3.State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences,Guiyang 550002, China
Recommended Citation
GB/T 7714
Yining Zhang;Yanyao Zhang;Yun Liu;Xi Liu. A metastable Fo-III wedge in cold slabs subducted to the lower part of the mantle transition zone: A hypothesis based on first-principles simualtions[J]. Minerals,2019,9(3):1-29.
APA Yining Zhang;Yanyao Zhang;Yun Liu;Xi Liu.(2019).A metastable Fo-III wedge in cold slabs subducted to the lower part of the mantle transition zone: A hypothesis based on first-principles simualtions.Minerals,9(3),1-29.
MLA Yining Zhang;Yanyao Zhang;Yun Liu;Xi Liu."A metastable Fo-III wedge in cold slabs subducted to the lower part of the mantle transition zone: A hypothesis based on first-principles simualtions".Minerals 9.3(2019):1-29.
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