Enhanced waveguide effect for deep-focus earthquakes in the subducting Pacific slab produced by a metastable olivine wedge

2016 ◽  
Vol 121 (9) ◽  
pp. 6779-6796 ◽  
Author(s):  
Takashi Furumura ◽  
Brian L. N. Kennett ◽  
Simanchal Padhy
Keyword(s):  
Waveguide Effect ◽  
Pacific Slab ◽  
Deep Focus ◽  
Metastable Olivine

scholarly journals A Metastable Fo-III Wedge in Cold Slabs Subducted to the Lower Part of the Mantle Transition Zone: A Hypothesis Based on First-Principles Simulations

Minerals ◽  
2019 ◽  
Vol 9 (3) ◽  
pp. 186 ◽  
Author(s):  
Yining Zhang ◽  
Yanyao Zhang ◽  
Yun Liu ◽  
Xi Liu
Keyword(s):  
Phase Transition ◽  
Transition Zone ◽  
First Principles ◽  
Lower Mantle ◽  
Seismic Velocity ◽  
Spinel Phase ◽  
Metastable Phase ◽  
Mantle Transition Zone ◽  
Deep Focus ◽  
Metastable Olivine

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.

scholarly journals The waveguide effect of metastable olivine in slabs

2000 ◽  
Vol 27 (4) ◽  
pp. 581-584 ◽  
Author(s):  
Keith D. Koper ◽  
Douglas A. Wiens
Keyword(s):  
Waveguide Effect ◽  
Metastable Olivine

Seismic evidence for a metastable olivine wedge in the subducting Pacific slab under Japan Sea

2008 ◽  
Vol 270 (3-4) ◽  
pp. 300-307 ◽  
Author(s):  
Guoming Jiang ◽  
Dapeng Zhao ◽  
Guibin Zhang
Keyword(s):  
Japan Sea ◽  
Pacific Slab ◽  
Metastable Olivine

Recycling of carbon from the stagnant paleo-Pacific slab beneath Eastern China revealed by olivine geochemistry

Lithos ◽  
2021 ◽  
pp. 106249
Author(s):  
Junhua Yao ◽  
Guoliang Zhang ◽  
Shuai Wang ◽  
Jianxin Zhao
Keyword(s):  
Eastern China ◽  
Pacific Slab

scholarly journals Internal melt figures in ice by rapid adiabatic compression

1994 ◽  
Vol 40 (134) ◽  
pp. 132-134
Author(s):  
R.E. Gagnon ◽  
C. Tulk ◽  
H. Kiefte
Keyword(s):  
Phase Transformations ◽  
Grain Boundaries ◽  
Single Crystals ◽  
Adiabatic Compression ◽  
Air Bubbles ◽  
Water Ice ◽  
Deep Focus ◽  
And Behavior ◽  
First Time

AbstractSingle crystals and bicrystals of water ice have been adiabatically pressurized to produce, and clearly illustrate, two types of internal melt figures: (1) dendritic figures that grow from nucleation imperfections on the specimen’s surface, or from air bubbles at grain boundaries, into the ice as pressure is elevated; and (2) compression melt fractures, flat liquid-filled disks, that nucleate at imperfections in the crystal and grow with the application of pressure eventually to sprout dendritic fingers at the periphery. The transparency of the ice permitted visualization of the growth and behavior of the figures, and this could be an important tool in understanding the role of phase transformations in deep-focus earthquakes. Correlation between figure size and pressure is noted for the first time.

Sharpening Deep Focus

1980 ◽  
Vol 34 (2) ◽  
pp. 62-64
Author(s):  
Joe Heumann ◽  
Charles H. Harpole
Keyword(s):  
Deep Focus
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