Benjamin Strozewski

Earth’s lower mantle is often considered to be ‘dry’ on the basis of hydrogen storage capacities of nominally anhydrous minerals. Recent investigations of dense oxyhydroxides in the (δ-AlOOH)-(MgSiO2(OH)2)-(ε-FeOOH) system have revealed that these phases may carry hydrogen into the lowermost mantle and contribute to heterogeneity of seismic velocities. To retain hydrogen at the high temperatures of the lower mantle, it is likely that hydrogen bonds must become symmetric, and consequently strong, at high pressure. In this work, we use Raman and infrared spectroscopy to assess the hydrogen bonding environment of (Al,Fe)-phase H (δ-Al0.84Fe3+ Mg0.02Si0.06OOH) and δ-Fe13 (δ-Al0.87Fe0.13OOH) at high pressure. We conclude that (Al,Fe)-phase H has occupationally disordered hydrogen atoms due to cation substitutions at ambient pressure. Hydrogen bonds in this phase become dynamically disordered at approximately 10 GPa and symmetric at 35 GPa. Our results are commensurate with previous observations of hydrogen-bond disorder in δ-(Al,Fe)OOH at 10 GPa.

The transport of hydrogen into Earth's deep interior may have an impact on lower mantle dynamics as well as on the seismic signature of subducted material. Due to the stability of the hydrous phases δ‐AlOOH (delta phase), MgSiO₂(OH)₂ (phase H), and ε‐FeOOH at high temperatures and pressures, their solid solutions may transport significant amounts of hydrogen as deep as the core‐mantle boundary. We have constrained the equation of state, including the effects of a spin crossover in the Fe³⁺ atoms, of (Al, Fe)‐phase H: Al_0.84Fe³⁺ _0.07Mg_0.02Si_0.06OOH, using powder X‐ray diffraction measurements to 125 GPa, supported by synchrotron Mössbauer spectroscopy measurements on (Al, Fe)‐phase H and δ‐(Al, Fe)OOH. The changes in spin state of Fe³⁺ in (Al, Fe)‐phase H results in a significant decrease in bulk sound velocity and occurs over a different pressure range (48–62 GPa) compared with δ‐(Al, Fe)OOH (32–40 GPa). Changes in axial compressibilities indicate a decrease in the compressibility of hydrogen bonds in (Al, Fe)‐phase H near 30 GPa, which may be associated with hydrogen bond symmetrization. The formation of (Al, Fe)‐phase H in subducted oceanic crust may contribute to scattering of seismic waves in the mid‐lower mantle (∼1,100–1,550 km). Accumulation of 1–4 wt.% (Al, Fe)‐phase H could reproduce some of the seismic signatures of large, low seismic‐velocity provinces. Our results suggest that changes in the electronic structure of phases in the (δ‐AlOOH)‐(MgSiO₂(OH)₂)‐(ε‐FeOOH) solid solution are sensitive to composition and that the presence of these phases in subducted oceanic crust could be seismically detectable throughout the lower mantle.

Quartz is an abundant mineral in Earth's crust whose mechanical behavior plays a significant role in the deformation of the continental lithosphere. However, the viscoplastic rheology of quartz is difficult to measure experimentally at low temperatures without high confining pressures due to the tendency of quartz (and other geologic materials) to fracture under these conditions. Instrumented nanoindentation experiments inhibit cracking even at ambient conditions, by imposing locally high mean stress, allowing for the measurement of the viscoplastic rheology of hard materials over a wide range of temperatures. Here we measure the indentation hardness of four synthetic quartz specimens and one natural quartz specimen with varying water contents over a temperature range of 23°C to 500°C. Yield stress, which is calculated from hardness but is model dependent, is fit to a constitutive flow law for low‐temperature plasticity to estimate the athermal Peierls stress of quartz. Below 500°C, the yield stresses presented here are lower than those obtained by extrapolating a flow law constrained by experiments at higher temperatures irrespective of the applied model. Indentation hardness and yield stress depend weakly on crystallographic orientation but show no dependence on water content.