No 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.

Extensive experimental studies on the structure and density of silicate glasses as laboratory analogs of natural silicate melts have attempted to address the nature of dense silicate melts that may be present at the base of the mantle. Previous ultrahigh-pressure experiments, however, have been performed on simple systems such as SiO 2 or MgSiO 3 , and experiments in more complex system have been conducted under relatively low-pressure conditions below 60 GPa. The effect of other metal cations on structural changes that occur in dense silicate glasses under ultrahigh pressures has been poorly understood. Here, we used a Brillouin scattering spectroscopic method up to pressures of 196.9 GPa to conduct in situ high-pressure acoustic wave velocity measurements of SiO 2 -Al 2 O 3 glasses in order to understand the effect of Al 2 O 3 on pressure-induced structural changes in the glasses as analogs of aluminosilicate melts. From 10 to 40 GPa, the transverse acoustic wave velocity (V S ) of Al 2 O 3 -rich glass (SiO 2 + 20. 5 mol% Al 2 O 3 ) was greater than that of Al 2 O 3 -poor glass (SiO 2 + 3.9 mol% Al 2 O 3 ). This result suggests that SiO 2 -Al 2 O 3 glasses with higher proportions of Al ions with large oxygen coordination numbers (5 and 6) become elastically stiffer up to 40 GPa, depending on the Al 2 O 3 content, but then soften above 40 GPa. At pressures from 40 to~100 GPa, the increase in V S with increasing pressure became less steep than below 40 GPa. Abovẽ 100 GPa, there were abrupt increases in the P-V S gradients (dV S /dP) at 130 GPa in Al 2 O 3 -poor glass and at 116 GPa in Al 2 O 3 -rich glass. These changes resemble previous experimental results on SiO 2 glass and MgSiO 3 glass. Given that changes of dV S /dP have commonly been related to changes in the Si-O coordination states in the glasses, our results, therefore, may indicate a drastic structural transformation in SiO 2 -Al 2 O 3 glasses above 116 GPa, possibly associated with an average Si-O coordination number change to higher than 6. Compared to previous acoustic wave velocity data on SiO 2 and MgSiO 3 glasses, Al 2 O 3 appears to promote a lowering of the pressure at which the abrupt increase of dV S /dP is observed. This suggests that the Al 2 O 3 in silicate melts may help to stabilize those melts gravitationally in the lower mantle.