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ACS earth and space chemistry 7(2), 471 - 478 (2023). doi:10.1021/acsearthspacechem.2c00326, The chemistry of bridgmanite is a crucial parameter for understanding the structure and dynamics of the Earth’s lower mantle. Incorporating Al, Fe$^{2+}$, and Fe$^{3+}$ into MgSiO$_3$ bridgmanite can significantly affect its physical properties. We investigated the substitution mechanisms of Fe$^{3+}$ in bridgmanite under SiO$_2$- and Fe$_2$O$_3$-saturated conditions at 27 GPa and 1700–2000 K using a multianvil apparatus. The fraction of Fe$^{3+}$ in bridgmanite increases from 0.085 to 0.202 atoms pfu with increasing temperature. We observed that only the charge-coupled mechanism takes place in bridgmanite under the studied conditions by forming the FeFeO$_3$ component, which increases from 4.0 ± 0.6 mol % at 1700 K to 9.9 ± 0.7 mol % at 2000 K. The absence of vacancies in bridgmanite under SiO$_2$-saturated conditions implies that bridgmanite in mid-ocean ridge basalt layers of subducted slabs in the lower mantle should have higher viscosity and lower electrical conductivity than that in the surrounding mantle. These differences in bridgmanite properties should affect lower-mantle dynamics, for example, enhance slab penetration more deeply into the lower mantle due to viscosity difference between mid-ocean ridge basalt and surrounding bridgmanite., Published by ACS Publications, Washington, DC

The 660-km seismic discontinuity (D660) is the boundary between the Earth’s lower mantle and transition zone and is commonly interpreted as the dissociation of (Mg,Fe)2SiO4 ringwoodite to (Mg,Fe)SiO3 bridgmanite plus (Mg,Fe)O ferropericlase (post-spinel transition). Prominent features of D660 are significant depressions to 750 km and multiplicity beneath cold subduction zones. Previous high-pressure experiments provided negative but gentle Clapeyron slopes (−1.3 to −0.5 MPa/K) of the post-spinel transition. Thus, the post-spinel transition cannot interpret the D660 depression. Therefore, another phase transition with a steep negative slope is required, and the akimotoite−bridgmanite transition in (Mg,Fe)SiO3 is one candidate.In the current study, we determined the boundaries of the post-spinel (RBP) and akimotoite−bridgmanite (AB) phase transitions in the MgO-SiO2 system over a temperature range of 1250–2085 K using advanced multi-anvil techniques with in situ X-ray diffraction. We judged a stable phase assemblage by observing relative increase/decrease in the ratio of coexisting high- and low-pressure assemblages at spontaneously and gradually decreasing pressure and a constant temperature from diffraction intensities. Since this strategy is strictly based on the principle of phase equilibrium, it excludes problems in determining phase stability caused by sluggish kinetics and surface energy.We found that the RBP boundary has a slightly concave curve, whereas the AB boundary has a steep convex curve. The RBP boundary is located at pressures of 23.2–23.7 GPa in the temperature range of 1250–2040 K. Its slope varies from −0.1 MPa/K at temperatures less than 1700 K to −0.9 MPa/K at 2000 K with an averaged value of −0.5 MPa/K. The slope of the AB boundary gradually changes from −8.1 MPa/K at low temperatures up to 1300 K to −3.2 MPa/K above 1600 K. Based on these findings, we predict that, beneath cold subduction zones, ringwoodite should first dissociate into akimotoite plus periclase, and then akimotoite transforms to bridgmanite with increasing depth; these successive transitions cause the multiple D660. Moreover, the steep negative boundary of the AB transition should result in cold-slab stagnation due to significant upward buoyancy. Our predictions are supported by the seismological observations beneath cold (e.g., Tonga, Izu-Bonin) subduction zones.

Nature 601, 69 - 73 (2022). doi:10.1038/s41586-021-04157-z, The 660-km seismic discontinuity is the boundary between the Earth���s lower mantle and transition zone and is commonly interpreted as the dissociation of ringwoodite to bridgmanite plus ferropericlase (post-spinel transition)1-3. A distinct feature of the 660-km discontinuity is its depression to 750 km beneath subduction zones4-10. However, in situ X-ray diffraction studies using multianvil techniques have demonstrated negative but gentle Clapeyron slopes of the post-spinel transition that do not allow a significant depression11-13. On the other hand, conventional high-pressure experiments face difficulties in accurate phase identification due to inevitable pressure changes during heating and the persistent presence of metastable phases1,3. Here, we determined the post-spinel and akimotoite-bridgmanite transition boundaries by multi-anvil experiments using in situ X-ray diffraction strictly based on the definition of phase equilibrium. The post-spinel boundary has almost no temperature dependence, whereas the akimotoite���bridgmanite transition has a very steep negative boundary at temperatures lower than ambient mantle geotherms. The large depressions of the 660-km discontinuity in cold subduction zones are thus interpreted as the akimotoite���bridgmanite transition. The steep negative boundary of the akimotoite-bridgmanite transition will cause slab stagnation due to significant upward buoyancy14,15., Published by Nature Publ. Group, London [u.a.]

The Clapeyron slope is the slope of a phase boundary in P–T space and is essential for understanding mantle dynamics and evolution. The phase boundary is delineating instead of balancing a phase transition’s normal and reverse reactions. Many previous high pressure–temperature experiments determining the phase boundaries of major mantle minerals experienced severe problems due to instantaneous pressure increase by thermal pressure, pressure drop during heating, and sluggish transition kinetics. These complex pressure changes underestimate the transition pressure, while the sluggish kinetics require excess pressures to initiate or proceed with the transition, misinterpreting the phase stability and preventing tight bracketing of the phase boundary. Our recent study developed a novel approach to strictly determine phase stability based on the phase equilibrium definition. Here, we explain the details of this technique, using the post-spinel transition in Mg2SiO4 determined by our recent work as an example. An essential technique is to observe the change in X-ray diffraction intensity between ringwoodite and bridgmanite + periclase during the spontaneous pressure drop at a constant temperature and press load with the coexistence of both phases. This observation removes the complicated pressure change upon heating and kinetic problem, providing an accurate and precise phase boundary.

Earth and planetary science letters 579, 117360 (2022). doi:10.1016/j.epsl.2021.117360, To constrain the effect of water on upper mantle dynamics, we measured the annihilation rate coefficients (k) of [100](010) and [001](100) dislocations in olivine, referred to as a-dislocations and c-dislocations, respectively, as a function of water content. Natural olivine single crystals were doped with 5–800 wt. ppm water and sheared in the [100] or [001] directions along the (010) and (100) planes to produce a- and c-dislocations, respectively, and then annealed under quasi-hydrostatic conditions at a constant pressure and temperature of 5 GPa and 1473 K. The obtained annihilation rate coefficients are fitted to a power-law equation, yielding astonishingly small water-content exponents of $0.0 \pm 0.1$ and $0.2 \pm 0.2$ for the a- and c-dislocations, respectively. The overall effect of water on dislocation mobility is therefore small because these two slip systems are considered to be the least and most sensitive to water, respectively. These results imply that water incorporation does not effectively increase the dislocation-creep rate and that a water-induced fabric transition is unlikely. The effects of water on asthenospheric dynamics may thus be limited, and the lateral seismic anisotropy changes observed in the asthenosphere may solely reflect changes of mantle flow geometry., Published by Elsevier, Amsterdam [u.a.]