AbstractH2O transforms to two forms of superionic (SI) ice at high pressures and temperatures, which contain highly mobile protons within a solid oxygen sublattice. Yet the stability field of both phases remains debated. Here, we present the results of an ultrafast X-ray heating study utilizing MHz pulse trains produced by the European X-ray Free Electron Laser to create high temperature states of H2O, which were probed using X-ray diffraction during dynamic cooling. We confirm an isostructural transition during heating in the 26-69 GPa range, consistent with the formation of SI-bcc. In contrast to prior work, SI-fcc was observed exclusively above ~50 GPa, despite evidence of melting at lower pressures. The absence of SI-fcc in lower pressure runs is attributed to short heating timescales and the pressure-temperature path induced by the pump-probe heating scheme in which H2O was heated above its melting temperature before the observation of quenched crystalline states, based on the earlier theoretical prediction that SI-bcc nucleates more readily from the fluid than SI-fcc. Our results may have implications for the stability of SI phases in ice-rich planets, for example during dynamic freezing, where the preferential crystallization of SI-bcc may result in distinct physical properties across mantle ice layers.

The iron spin crossover in (Mg1-xFex)O ferropericlase causes changes to its physical properties that are expected to affect seismic velocities in Earth's lower mantle. We present new time-resolved pressure-volume measurements of iron-rich ferropericlase (xFe = 0.40, 0.59) and combine the results with literature data with xFe = 0.04-0.6 to investigate the dependence of ferropericlase elastic properties on iron content. We infer the relationship between unit-cell volume, pressure and iron content directly from the data by training Mixture Density Networks and derive bulk modulus, density and bulk sound velocity from the outputs. This allows us to constrain the effect of the spin crossover on these properties and estimate their uncertainties for different iron contents. Our findings indicate that the spin crossover may significantly alter the physical properties of ferropericlase in iron-enriched regions in the lowermost mantle, with implications for the interpretation of seismic heterogeneities observed near the core-mantle boundary.

Cubic CaSiO3 perovskite is a major phase in subducted oceanic crust, where it forms at a depth of about 550 kilometres from majoritic garnet1,2,28. However, its rheological properties at temperatures and pressures typical of the lower mantle are poorly known. Here we measured the plastic strength of cubic CaSiO3 perovskite at pressure and temperature conditions typical for a subducting slab up to a depth of about 1,200 kilometres. In contrast to tetragonal CaSiO3, previously investigated at room temperature3,4, we find that cubic CaSiO3 perovskite is a comparably weak phase at the temperatures of the lower mantle. We find that its strength and viscosity are substantially lower than that of bridgmanite and ferropericlase, possibly making cubic CaSiO3 perovskite the weakest lower-mantle phase. Our findings suggest that cubic CaSiO3 perovskite governs the dynamics of subducting slabs. Weak CaSiO3 perovskite further provides a mechanism to separate subducted oceanic crust from the underlying mantle. Depending on the depth of the separation, basaltic crust could accumulate at the boundary between the upper and lower mantle, where cubic CaSiO3 perovskite may contribute to the seismically observed regions of low shear-wave velocities in the uppermost lower mantle5,6, or sink to the core–mantle boundary and explain the seismic anomalies associated with large low-shear-velocity provinces beneath Africa and the Pacific7–9. At temperatures and pressures typical of the Earth’s lower mantle, cubic CaSiO3 perovskite is found to have lower strength and viscosity compared to bridgmanite and ferropericlase, providing clues to its role in subduction regions.

Seismic anisotropy in Earth's lowermost mantle, resulting from Crystallographic Preferred Orientation (CPO) of elastically anisotropic minerals, is among the most promising observables to map mantle flow patterns. A quantitative interpretation, however, is hampered by the limited understanding of CPO development in lower mantle minerals at simultaneously high pressures and temperatures. Here, we experimentally determine CPO formation in ferropericlase, one of the elastically most anisotropic deep mantle phases, at pressures of the lower mantle and temperatures of up to 1400 K using a novel experimental setup. Our data reveal a significant contribution of slip on {100} to ferropericlase CPO in the deep lower mantle, contradicting previous inferences based on experimental work at lower mantle pressures but room temperature. We use our results along with a geodynamic model to show that deformed ferropericlase produces strong shear wave anisotropy in the lowermost mantle, where horizontally polarized shear waves are faster than vertically polarized shear waves, consistent with seismic observations. We find that ferropericlase alone can produce the observed seismic shear wave splitting in D″ in regions of downwelling, which may be further enhanced by post-perovskite. Our model further shows that the interplay between ferropericlase (causing VSH > VSV) and bridgmanite (causing VSV > VSH) CPO can produce a more complex anisotropy patterns as observed in regions of upwelling at the margin of the African Large Low Shear Velocity Province.

We experimentally determined the bulk modulus of (Mg0.8Fe0.2)O ferropericlase across the iron spin transition and in the low-spin phase by employing a new experimental approach. In our measurements, we simulate the propagation of a compressional seismic wave (P wave) through our sample by employing a piezo-driven dynamic diamond anvil cell that allows to oscillate pressure at seismic frequencies. During pressure oscillations, X-ray diffraction images were continuously collected every 5-50 ms. The bulk modulus is directly calculated from these data at different pressures. Our experiments show a pronounced softening of the bulk modulus throughout the spin crossover, supporting previous single-crystal measurements at very high frequencies and computations. Comparison of our results to previous data collected on (Mg,Fe)O with lower iron contents shows that the magnitude of softening strongly depends on iron content. Our experiments at seismic frequencies confirm that the iron spin crossover markedly affects the ratio of seismic compressional to shear wave velocities in Earth's lower mantle. Plain Language Summary Information about the structure and composition of Earth's mantle can be derived from comparison of measured seismic wave speeds to those predicted from laboratory sound wave velocity measurements at high pressures. Ferropericlase, the second most abundant mineral in Earth's lower mantle, changes its electronic configuration at pressures corresponding to the lower mantle. Laboratory measurements carried out at very high frequencies (GHz) indicate that this so-called spin transition significantly decreases compressional wave velocities. If true, this effect can affect our interpretation of seismological observables. However, experimental results are partly in disagreement and no measurements have been conducted at typical seismic frequencies that are much lower as those typical for laboratory experiments. In this work. we directly measured the effect of the iron spin transition on the elastic response of ferropericlase at a frequency of 1 Hz. We find a significant softening effect on the bulk modulus that will lead to a decrease of compressional seismic velocities in Earth's lower mantle. Based on comparison of our results to previous work, we show that the effect strongly depends on iron content.

We present a method to perform improved measurements of the effects of chemical variability on physical properties of single-crystal samples in the diamond-anvil cell by employing a multi-sample approach. By customizing the sizes and shapes of the samples using a focused ion beam machine the simultaneous loading of relatively large crystals into a single sample chamber becomes feasible. To illustrate the potential of this approach, elastic properties of four single crystals of ringwoodite with different chemical compositions have been measured at high pressure. Our results suggest that the multi-sample approach allows for the quantification of small effects of chemical variations, such as iron and hydrogen incorporation, on physical properties. Furthermore, we discuss the possibility of using the multi-sample approach to load several crystals with different crystallographic orientations of the same material into one sample chamber in order to map out the direction dependence of anisotropic physical properties.