Stability of Al-bearing superhydrous phase B at the mantle transition zone and the uppermost lower mantle

2018 ◽  
Vol 103 (8) ◽  
pp. 1221-1227 ◽  
Author(s):  
Sho Kakizawa ◽  
Toru Inoue ◽  
Hideto Nakano ◽  
Minami Kuroda ◽  
Naoya Sakamoto ◽  
...  
Keyword(s):  
Transition Zone ◽  
Lower Mantle ◽  
Mantle Transition Zone

scholarly journals Garnet-controlled very low velocities in the lower mantle transition zone at sites of mantle upwelling

Terra Nova ◽  
2018 ◽  
Vol 30 (5) ◽  
pp. 333-340 ◽  
Author(s):  
Thorsten J. Nagel ◽  
Erik Duesterhoeft ◽  
Christian Schiffer
Keyword(s):  
Transition Zone ◽  
Lower Mantle ◽  
Mantle Upwelling ◽  
Mantle Transition Zone

scholarly journals High-pressure phase of brucite stable at Earth’s mantle transition zone and lower mantle conditions

2016 ◽  
Vol 113 (49) ◽  
pp. 13971-13976 ◽  
Author(s):  
Andreas Hermann ◽  
Mainak Mookherjee
Keyword(s):  
High Pressure ◽  
Transition Zone ◽  
Lower Mantle ◽  
Direct Detection ◽  
Geophysical Methods ◽  
High Pressure Phase ◽  
Mantle Transition Zone ◽  
Earth’S Mantle ◽  
3D Network ◽  
Pressure Phase

We investigate the high-pressure phase diagram of the hydrous mineral brucite, Mg(OH)2, using structure search algorithms and ab initio simulations. We predict a high-pressure phase stable at pressure and temperature conditions found in cold subducting slabs in Earth’s mantle transition zone and lower mantle. This prediction implies that brucite can play a much more important role in water transport and storage in Earth’s interior than hitherto thought. The predicted high-pressure phase, stable in calculations between 20 and 35 GPa and up to 800 K, features MgO6 octahedral units arranged in the anatase–TiO2 structure. Our findings suggest that brucite will transform from a layered to a compact 3D network structure before eventual decomposition into periclase and ice. We show that the high-pressure phase has unique spectroscopic fingerprints that should allow for straightforward detection in experiments. The phase also has distinct elastic properties that might make its direct detection in the deep Earth possible with geophysical methods.

Subducting slab morphology and mantle transition zone upwelling in double-slab subduction models with inward-dipping directions

2019 ◽  
Vol 218 (3) ◽  
pp. 2089-2105 ◽  
Author(s):  
Tianyang Lyu ◽  
Zhiyuan Zhu ◽  
Benjun Wu
Keyword(s):  
Transition Zone ◽  
Lower Mantle ◽  
Numerical Models ◽  
Local Maximum ◽  
Substantial Effect ◽  
Mantle Flow ◽  
Slab Geometry ◽  
Mantle Transition Zone ◽  
Mantle Viscosity ◽  
Deep Mantle

SUMMARY Lithospheric plates on the Earth's surface interact with each other, producing distinctive structures comprising two descending slabs. Double-slab subduction with inward-dipping directions represents an important multiplate system that is not yet well understood. This paper presents 2-D numerical models that investigate the dynamic process of double-slab subduction with inward dipping, focussing on slab geometry and mantle transition zone upwelling flow. This unique double-slab configuration limits trench motion and causes steep downward slab movement, thus forming fold piles in the lower mantle and driving upward mantle flow between the slabs. The model results show the effects of lithospheric plate properties and lower-mantle viscosity on subducting plate kinematics, overriding plate stress and upward mantle flow beneath the overriding plate. Appropriate lower-mantle strength (such as an upper–lower mantle viscosity increase with a factor of 200) allows slabs to penetrate into the lower mantle with periodical buckling. While varying the length and thickness of a long overriding plate (≥2500) does not have a substantial effect on slab geometry, its viscosity has a marked impact on slab evolution and mantle flow pattern. When the overriding plate is strong, slabs exhibit an overturned geometry and hesitate to fold. Mantle transition zone upwelling velocity depends on the speed of descending slabs. The downward velocity of slabs with a large negative buoyancy (caused by thickness or density) is very fast, inducing a significant transition zone upwelling flow. A stiff slab slowly descends into the deep mantle, causing a small upward flow in the transition zone. In addition, the temporal variation of mantle upwelling velocity shows strong correlation with the evolution of slab folding geometry. In the double subduction system with inward-dipping directions, the mantle transition zone upwelling exhibits oscillatory rise with time. During the backward-folding stage, upwelling velocity reaches its local maximum. Our results provide new insights into the deep mantle source of intraplate volcanism in a three-plate interaction system such as the Southeast Asia region.

Prominent thermal anomalies in the mantle transition zone beneath the Transantarctic Mountains

Geology ◽  
2020 ◽  
Vol 48 (7) ◽  
pp. 748-752
Author(s):  
Erica L. Emry ◽  
Andrew A. Nyblade ◽  
Alan Horton ◽  
Samantha E. Hansen ◽  
Jordi Julià ◽  
...  
Keyword(s):  
Transition Zone ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Low Velocity ◽  
Mantle Transition Zone ◽  
Thermal Anomalies ◽  
Victoria Land ◽  
Convective Thermal ◽  
Mantle Processes ◽  
Transantarctic Mountains

Abstract The Transantarctic Mountains (TAMs), Antarctica, exhibit anomalous uplift and volcanism and have been associated with regions of thermally perturbed upper mantle that may or may not be connected to lower mantle processes. To determine if the anomalous upper mantle beneath the TAMs connects to the lower mantle, we interrogate the mantle transition zone (MTZ) structure under the TAMs and adjacent parts of East Antarctica using 12,500+ detections of P-to-S conversions from the 410 and 660 km discontinuities. Our results show distinct zones of thinner-than-global-average MTZ (∼205–225 km, ∼10%–18% thinner) beneath the central TAMs and southern Victoria Land, revealing throughgoing convective thermal anomalies (i.e., mantle plumes) that connect prominent upper and lower mantle low-velocity regions. This suggests that the thermally perturbed upper mantle beneath the TAMs and Ross Island may have a lower mantle origin, which could influence patterns of volcanism and TAMs uplift.

scholarly journals Evolution of diamond-forming systems of the mantle transition zone: ringwoodite peritectic reaction (Mg,Fe)2SiO4 (experiment AT 20 GPa)

2019 ◽  
Vol 64 (9) ◽  
pp. 986-994
Author(s):  
А. V. Spivak ◽  
Yu. А. Litvin ◽  
Е. S. Zakharchenko ◽  
D. А. Simonova ◽  
L. S. Dubrovinsky
Keyword(s):  
Experimental Study ◽  
Transition Zone ◽  
Lower Mantle ◽  
Peritectic Reaction ◽  
Mantle Transition Zone ◽  
Earth’S Mantle ◽  
Silicate Carbonate ◽  
Melting Relations ◽  
Carbonate Melt

The peritectic reaction of ringwoodite (Mg,Fe)2SiO4 and silicate-carbonate melt with formation of magnesiowustite (Fe,Mg)O, stishovite SiO2 and Mg, Na, Ca, K-carbonates is revealed by experimental study at 20 GPa of melting relations of the multicomponent MgO-FeO-SiO2-Na2CO3-CaCO3-K2CO3 system of the Earth’s mantle transition zone. A reaction of CaCO3 and SiO2 with the formation of Ca-perovskite CaSiO3 is also detected. It is shown that the peritectic reaction of ringwoodite and melt with the formation of stishovite physic-chemically controls the fractional ultrabasic-basic evolution of both magmatic and diamond-forming systems of the deep horizons of the transition zone up to its boundary with the Earth’s lower mantle.

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 Receiver function mapping of mantle transition zone discontinuities beneath Alaska using scaled 3-D velocity corrections

2019 ◽  
Vol 219 (2) ◽  
pp. 1432-1446 ◽  
Author(s):  
A M van Stiphout ◽  
S Cottaar ◽  
A Deuss
Keyword(s):  
Transition Zone ◽  
Lower Mantle ◽  
Seismic Velocity ◽  
Receiver Function ◽  
Receiver Functions ◽  
North American Plate ◽  
S Wave ◽  
Mantle Transition Zone ◽  
Velocity Models ◽  
Subducting Slab

SUMMARY The mantle transition zone is the region between the globally observed major seismic velocity discontinuities around depths of 410 and 660 km and is important for determining the style of convection and mixing between the upper and the lower mantle. In this study, P-to-S converted waves, or receiver functions, are used to study these discontinuities beneath the Alaskan subduction zone, where the Pacific Plate subducts underneath the North American Plate. Previous tomographic models do not agree on the depth extent of the subducting slab, therefore improved imaging of the Earth structure underneath Alaska is required. We use 27 800 high quality radial receiver functions to make common conversion point stacks. Upper mantle velocity anomalies are accounted for by two recently published regional tomographic S-wave velocity models. Using these two tomographic models, we show that the discontinuity depths within our CCP stacks are highly dependent on the choice of velocity model, between which velocity anomaly magnitudes vary greatly. We design a quantitative test to show whether the anomalies in the velocity models are too strong or too weak, leading to over- or undercorrected discontinuity depths. We also show how this test can be used to rescale the 3-D velocity corrections in order to improve the discontinuity topography maps. After applying the appropriate corrections, we find a localized thicker mantle transition zone and an uplifted 410 discontinuity, which show that the slab has clearly penetrated into the mantle transition zone. Little topography is seen on the 660 discontinuity, indicating that the slab has probably not reached the lower mantle. In the southwest, P410s arrivals have very small amplitudes or no significant arrival at all. This could be caused by water or basalt in the subducting slab, reducing the strength at the 410, or by topography on the 410 discontinuity, preventing coherent stacking. In the southeast of Alaska, a thinner mantle transition zone is observed. This area corresponds to the location of a slab window, and thinning of the mantle transition zone may be caused by hot mantle upwellings.

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