Stability of Fe-bearing hydrous phases and element partitioning in the system MgO–Al2O3–Fe2O3–SiO2–H2O in Earth's lowermost mantle

Spin Transition of Iron in δ‐(Al,Fe)OOH Induces Thermal Anomalies in Earth's Lower Mantle

“…Moreover, the iron partitioning between the δ-(Al,Fe)OOH and bridgmanite is within the range of 1-3, indicating an Fe-rich composition of δ-(Al, Fe)OOH compared to the bridgmanite. If we assume that the δ-(Al,Fe) OOH contains approximately 12-15 mol % of FeOOH, similar to that observed by Yuan et al (2019), and its temperature dependence of the lattice thermal conductivity follows a typical T -1/2 dependence as many Febearing mantle minerals (Chang et al, 2017;Dalton et al, 2013;Deschamps & Hsieh, 2019;Hsieh et al, 2017Hsieh et al, , 2018Klemens et al, 1962;Xu et al, 2004;Zhang et al, 2019), the lattice thermal conductivity of δ-(Al,Fe)OOH at T~2000 K and depths of 900-1,100 km (~30-40 GPa) is expected to rapidly increase from~4 to 10 W·m −1 ·K −1 (red curve in Figure 2). (The temperature around the interface between a mantle and a subducting slab is expected to be similar.)…”

“…In addition, recent first-principles calculations (Muir & Brodholt, 2018) indicate that at the lowermost mantle, high concentration of water (>1,000 ppm) could be incorporated in Al-bearing bridgmanite, creating significant number of vacancies. The large amounts of water released from the enhanced dehydration of δ-(Al,Fe)OOH suggested by our results could serve as a local water source to form the hydrous Al-bearing bridgmanite and pyrite-type FeOOH x (Liu et al, 2017;Yuan et al, 2019), offering a novel scenario for the potential routes and fate of Earth's deep water cycle.…”

“…Moreover, due to the drastic increase in temperature above the CMB, release of water from the decomposition of δ-AlOOH or δ-(Al,Fe)OOH in this region could facilitate the formation of FeOOH x (Duan et al, 2018;Yuan et al, 2019) and trigger partial melting of minerals at the base of the mantle, both being proposed to be an origin of the ULVZs (Duan et al, 2018;Liu et al, 2017 ;Yuan et al, 2019). The local thermal insulating effect that additionally raises local temperature in δ-(Al,Fe)OOH due to its exceptionally low thermal conductivity at lowermost mantle would accelerate the dehydration of δ-(Al,Fe)OOH (occurring easier or at shallower depths) and creation of the seismic features for ULVZs (see illustration in Figure 2).…”

Dehydration of δ-AlOOH in Earth’s Deep Lower Mantle

“…We also note that the pressure conditions where we observed -FeOOH (between 100 and 110 GPa) are higher than previous reports for pure Fe end-member (below 80 GPa) [31]. We therefore hypothesize that the occurrence of -FeOOH above 100 GPa in our experiments might be caused by the presence of Al, which could stabilize the structure at higher pressures than its Al-free counterpart [32]. Although our measured unit-cell volumes of the phase is close to that of reported Al-free -FeOOH (Figure 4), we cannot rule out the possibility of a small amount of Al in the phase.…”

“…Previous high-pressure experiments have shown that when Al-bearing bridgmanite reacts with H 2 O, it can reduce the amount of Al in bridgmanite and form a separate AlOOH-MgSiO 2 (OH) 2 phase [8], also called phase H. These experiments also found that the presence of Al in the structure of phase H increases its thermal stability. Similarly, -FeOOH can form at least a partial solid solution with δ-AlOOH [32]. Last but not least, Chen et al [11] found that Al is separated from CaSiO 3 perovskite in the presence of H 2 O and forms a separate δ-AlOOH phase at high pressures.…”