Compressional behavior and spin state of δ-(Al,Fe)OOH at high pressures

Abstract

Hydrogen transport from the surface to the deep interior and distribution in the mantle are important in the evolution and dynamics of the Earth. An aluminum oxy-hydroxide, δ-AlOOH, might influence hydrogen transport in the deep mantle because of its high stability extending to lower mantle conditions. The compressional behavior and spin states of δ-(Al,Fe3+)OOH phases were investigated with synchrotron X-ray diffraction and Mössbauer spectroscopy under high pressure and room temperature. Pressure-volume (P-V) profiles of the δ-(Al0.908(9)57Fe0.045(1))OOH1.14(3) [Fe/(Al+Fe) = 0.047(10), δ-Fe5] and the δ-(Al0.832(5)57Fe0.117(1))OOH1.15(3) [Fe/(Al+Fe) = 0.123(2), δ-Fe12] show that these hydrous phases undergo two distinct structural transitions involving changes in hydrogen bonding environments and a high- to low-spin crossover in Fe3+. A change of axial compressibility accompanied by a transition from an ordered (P21nm) to disordered hydrogen bond (Pnnm) occurs near 10 GPa for both δ-Fe5 and δ-Fe12 samples. Through this transition, the crystallographic a and b axes become stiffer, whereas the c axis does not show such a change, as observed in pure δ-AlOOH. A volume collapse due to a transition from high- to low-spin states in the Fe3+ ions is complete below 32–40 GPa in δ-Fe5 and δ-Fe12, which i ~10 GPa lower than that reported for pure ε-FeOOH. Evaluation of the Mössbauer spectra of δ-(Al0.824(10)57Fe0.126(4))OOH1.15(4) [Fe/(Al+Fe) = 0.133(3), δ-Fe13] also indicate a spin transition between 32–45 GPa. Phases in the δ-(Al,Fe)OOH solid solution with similar iron concentrations as those studied here could cause an anomalously high ρ/νΦ ratio (bulk sound velocity, defined as K/ρ at depths corresponding to the spin crossover region (~900 to ~1000 km depth), whereas outside the spin crossover region a low ρ/νΦ anomaly would be expected. These results suggest that the δ-(Al,Fe)OOH solid solution may play an important role in understanding the heterogeneous structure of the deep Earth.

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

“…As the δ‐(Al,Fe)OOH is subducted to the lowermost mantle where it remains stable (Duan et al, ; Ohira et al, ; Sano et al, ), the exceptionally low thermal conductivity could play a key role in affecting the route of water transportation and local thermo‐chemical structures at the base of the mantle. Again, assuming its thermal conductivity follows a typical T –1/2 dependence, at the lowermost mantle, the thermal conductivity of δ‐(Al,Fe)OOH with 12–15 mol % of FeOOH at T ~2400 K before decomposition is expected to be potentially as low as ~5 W·m −1 ·K −1 (Figure ), approximately half of the pyrolitic lowermost mantle (~8–10 W·m −1 ·K −1 , for temperatures in the range of 2500 K [~10 W·m −1 ·K −1 ]–3500 K [~8 W·m −1 ·K −1 ]) (Hsieh et al, ).…”

“…As the δ-(Al,Fe)OOH is subducted to the lowermost mantle where it remains stable (Duan et al, 2018;Ohira et al, 2019;, the exceptionally low thermal conductivity could play a key role in affecting the route of water transportation and local thermo-chemical structures at the base of the mantle. Again, assuming its thermal conductivity follows a typical T -1/2 dependence, at the lowermost mantle, the (Hsieh et al, 2018).…”

“…Polycrystals of δ-AlOOH and single crystals of δ-(Al,Fe)OOH with FeOOH content of 3, 12, and 15 mol.%, respectively, were synthesized by using a 1000-ton Kawai-type multi-anvil apparatus at Bayerisches Geoinstitut (see Kawazoe et al, 2017;Ohira et al, 2019 for more details). The polycrystalline samples were identified with a microfocused X-ray diffractometer (Bruker, D8 DISCOVER) with a two-dimensional solidstate detector (VÅNTEC500) and a microfocus source (IμS) of Co-Kα radiation operated at 40 kV and 500 μA.…”

“…Moreover, a very recent study further showed that iron (Fe)-bearing δ-AlOOH phase can coexist with bridgmanite in a model basaltic composition (Yuan et al, 2019). The presence of Fe 3+ in δ-AlOOH in the lower mantle induces profound impacts not only on its physical properties but also potentially on the fate of global cycles of water and iron in the deep mantle (Kawazoe et al, 2017;Ohira et al, 2019). A particularly intriguing property, also reported in Fe-bearing deep mantle minerals (Lin et al, 2013), is that when incorporated with iron, a pressure-induced spin transition of iron is observed in δ-(Al,Fe)OOH around 30-45 GPa (Ohira et al, 2019), through which its unit cell volume and sound velocities change drastically.…”

Spectroscopic evidence for the Fe3+ spin transition in iron-bearing δ-AlOOH at high pressure

“…The water cycling between the Earth's surface and interior plays a key role in the evolution and dynamics of Earth's interior (Mao and Mao, 2020;Ohira et al, 2019;Ohtani, 2005). Slab subduction and magmatism are the two key processes regulating the ingassing and outgassing rates of water and many other volatiles.…”