Session V38: Materials in Extremes: Geophysics and Planetary Science

Formation of diamonds in laser-compressed hydrocarbons at planetary interior conditions

The effects of hydrocarbon reactions and diamond precipitation on the internal structure and evolution of icy giant planets such as Neptune and Uranus have been discussed for more than three decades. Inside these celestial bodies, simple hydrocarbons such as methane, which are highly abundant in the atmospheres, are believed to undergo structural transitions that release hydrogen from deeper layers and may lead to compact stratified cores. Indeed, from the surface towards the core, the isentropes of Uranus and Neptune intersect a temperature–pressure regime in which methane first transforms into a mixture of hydrocarbon polymers, whereas, in deeper layers, a phase separation into diamond and hydrogen may be possible. Here we show experimental evidence for this phase separation process obtained by in situ X-ray diffraction from polystyrene, PET ant PMMA samples dynamically compressed to conditions around 150 GPa and 5,000 K; these conditions resemble the environment around 10,000 km below the surfaces of Neptune and Uranus. Our findings demonstrate the necessity of high pressures for initiating carbon–hydrogen separation and imply that diamond precipitation may require pressures about ten times as high as previously indicated by static compression experiments. Our results will inform mass–radius relationships of carbon-bearing exoplanets, provide constraints for their internal layer structure and improve evolutionary models of Uranus and Neptune, in which carbon–hydrogen separation could influence the convective heat transport. In addition to their relevance for planetary modelling, by showing the formation of diamonds that are possibly a few nanometers in size from laser-irradiated plastic, our results may identify a new method to produce diamond nanoparticles for material science and industrial applications.   

Viscosity of the Inner Core

The Earth Core is mostly iron. The inner part of it is a solid inner core (IC). The attempts to explain the enigmatic properties of the IC by the stability of the hexagonal close-packed (hcp) phase were not very successful. Recently, it was demonstrated by first-principles molecular dynamics (FPMD) method that the stable phase of iron under pressure (330-360 GPa) and temperature (5500-8000 K) of the IC is the body-centered cubic (bcc) phase. This phase was actually already observed in experiments but some of the shockwave data was likely misinterpreted. There is also a very recent diamond anvil cell experiment that confirms the stability of the bcc phase. One of the enigmatic properties of the IC is high attenuation of seismic waves. The attenuation of the hcp phase is quite low. We computed, using FPMD, that the bcc phase possess the viscosity that explains the high attenuation in the IC. The explanation is in the unique feature of the bcc phase - iron atoms diffuse preserving the bcc lattice. This feature of the bcc phase provides a strong evidence in favor of the bcc stability in the IC.   

Self-Assembly of Prebiotic Organic Materials from Impact Events of Amino Acid Solutions

Proteinogenic amino acids can be produced on or delivered to a planet via abiotic sources and were consequently likely present before the emergence of life on early Earth. However, the role that these materials played in the in the emergence of life remains an open question, in part because little is known about the survivability and reactivity of astrophysical prebiotic compounds upon impact with a planetary surface. To this end, we have used a force matched semi-empirical quantum simulation method in development in our group to study oblique impacts of aqueous glycine solutions at conditions of up to 40 GPa and 3000 K. We find that these elevated conditions induce the formation of glycine-oligomeric structures with a number of different chemical moieties such as hydroxyl and amine groups diffusing on and off the C-N backbones. The C-N backbones of these structures generally remain stable during cooling and expansion, yielding relatively large three-dimensional molecules that contain a number of different functional groups and embedded bonded regions akin to oligo-peptides. Our results help determine the role of comets and other celestial bodies in both the delivery and synthesis of polypeptides and homochirality to early Earth.   

Ionic ammonia-rich hydrates at planetary conditions

The “ice giants” Uranus and Neptune, and exoplanets like them, contain large amounts of water, ammonia, and methane ices, as well as hydrogen in various forms. It is unknown how these compounds organize themselves under the extreme conditions of pressure and temperature in the interior of the planets - as a mixture, or as separated layers. Individual ices have been studied in detial under pressure, but properties of their mixtures are less explored. We computationally explore the binary ammonia–water mixtures as a function of pressure and temperature, using crystal structure prediction methods to find stable solid phases and explore superionicity and melting. At lower pressures the canonical mixtures are found to be stable, forming molecular compounds and ionic phases with increasing pressure. At higher pressures ammonia-rich hydrates dominate, due to a structural evolution involving fully ionic phases with completely deprotonated O²⁻ units and a new 4:1 hydrate. Above 500 GPa, close to the core-mantle boundary of Neptune, all mixtures are predicted to be unstable towards decomposition into ammonia and water.   

Effect of defects on the Raman-active modes in zircon (ZrSiO4): a first-principles study

Zircon (ZrSiO₄) is a ubiquitous Earth’s crust mineral crucial for geochronology due to it’s natural inclusion of radioactive elements. For the same reason, zircon was suggested as a candidate host material for immobilization of radioactive waste. However, over the course of a zircon’s lifetime it’s crystal structure is being slowly destroyed due to internal radiation damage. This effect, known as metamictization, is focus of increased interest in the research community.
Here, we present a first-principles study on the effects of various substitutional defects, including radioactive elements like U and Th, on the Raman active modes in zircon. We find excellent quantitative agreement of the slopes ∂ω/∂c_U (where c_U is the molar concentration of uranium) with the experimental data of Geisler et al. (Eur. J. Mineral. 17, 883 (2005)) who performed Raman measurements in high-uranium zircon originating from the Chernobyl «lava». Based on this result, we predict the corresponding shift of Raman active modes in zircon when other extrinsic defects in the zircon lattice are considered. Our study provides the first quantum-mechanical atomistic understanding of the processes governing the changes in Raman signature in defective zircon crystals.
  

Post-perovskite transition in (Al,Fe)-bearing bridgmanite

The major mineral phase of the Earth’s lower mantle, (Al,Fe)-bearing bridgmanite, transitions to a seemingly layered structure known as post-perovskite at approximately P=125 GPa and T = 2500 K. This transition must produce seismic features in the Earth’s deep lower mantle. Despite extensive investigations by experiments and ab initio calculations, there are still important aspects of this transformation that need clarification. Here, we systematically address this question in (Al,Fe³⁺)-, (Fe²⁺)- and (Fe³⁺)-bearing bridgmanite using ab initio calculations. We particularly address the phase boundary dependence on the chemistry and acoustic velocity changes across this transformation. These results are important to constrain the nature of the D” layer, the deepest layer of the mantle that should be dominated by the post-perovskite phase. For instance, both the topography of and seismic velocity jumps at the D” discontinuity depend on the local composition and temperature. Knowledge of the effect of composition variation on the phase diagram allows us to validate hypotheses concerning the nature of the enigmatic D” layer.
  

Abstract Withdrawn

We employ large-scale molecular dynamics simulations to understand the physical and chemical behavior of the silicate part of the protolunar disk formed during the Giant Impact.
At high temperatures we identify the supercritical region characterized by one homogeneous fluid rich in ionic species. We show that the chemical speciation is very different from the one obtained at ambient pressure conditions.
At lower temperatures, in the 2000 – 4000 K, and low density, we capture the nucleation of bubbles. This marks the liquid side of the supercritical dome – the density region of space where gas and liquid coexist at a given temperature. The gas phase separating in the bubbles is formed mainly of isolated ionized small clusters of atoms. Their lifetimes vary, but usually does not extend beyond 100 fs, mainly due to the limited scale of the cavities, i.e. bubbles.
When volatiles are present they are the first one populating the bubbles. They enhance the bubble nucleation and favour the gas-liquid separation. They promote incongruent vaporization.   

Post-Aragonite Phases of CaCO3 at Lower Mantle Pressures

The properties of carbonate minerals at mantle conditions have significant impact on our understanding of the carbon cycle and the composition of the Earth interior. In recent years, there has been interest in the behavior of carbonates at lower mantle conditions, specifically in their C hybridization. Using high-pressure synchrotron X-ray diffraction in a diamond anvil cell coupled with direct laser heating using a CO₂ laser, we identify a crystalline phase of CaCO₃ above 40 GPa - corresponding to a lower mantle depth of ∼1,000 km - which is predicted by ab initio random structure search (AIRSS). The observed sp² C-hybridized species at 40 GPa is monoclinic (P2₁/c) and is stable up to 50 GPa, above which its structure cannot be indexed by existing CaCO₃ phases. We investigate with nudged elastic band calculations the reaction mechanisms between relevant phases of CaCO₃ and postulate that the mineral is capable of undergoing sp²-sp³ hybridization change purely in the P2₁/c structure - forgoing the accepted post-aragonite Pmmn structure.   

Ab Initio and Experimental Investigations on the Influence of Cation Ordering on the Elasticity of (Mg,Fe,Mn)Al2O4 Spinels

We explore the effect of cation disorder on the elastic properties of the (Mg,Fe,Mn)Al₂O₄ spinel systems using DFT and Brillouin scattering. The spinel structure (Fd-3m), [A_(1−x),B_x]^IV[A_xB_(2−x)]^VIO₄ (A=Mg,Mn,Fe; B =Al), is highly relevant in geosciences as it is adopted by some key oxide minerals in the Earth's upper mantle and transition zone, between 0-660km depth. In addition, due to the high-pressure/temperature stability of spinels, they also have industrial applications, e.g., as ceramic materials. The effect of disorder, quantified by the inversion parameter x, has been addressed for MgAl₂O₄-spinel. In our work, we extend these investigations to include the influence of magnetic cations, A =Mn or Fe. How disorder changes the physical properties in spinels is important to understand not only because it could have profound geophysical implications, as the degree of disorder is controlled by pressure, temperature, and perhaps local chemistry, but also to optimize their technological applications. Thus, within the LDA and GGA we study in a systematic way the impact of x in our spinels and compare the results to Brillouin scattering measurements to draw trends in their properties.   

Equation of State of Iron-Rich (Mg,Fe)O

Recent seismic observations of the core-mantle boundary (CMB) have provided increasing evidence for the presence of a chemically heterogeneous boundary layer, whose lateral variations in seismic velocities and densities may be explained by iron enrichment in lower mantle minerals like (Mg,Fe)O. Relatively little study has been directed toward iron-rich members of the (Mg,Fe)O solid solution despite the possibility for even low levels of iron enrichment to have significant impact on elastic properties. To that end, we present results from a powder synchrotron x-ray diffraction study on (Mg_0.06Fe_0.94)O up to 90 GPa at 300 K using helium as a pressure-transmitting medium. The diffraction data are used to determine the equations of state for the material’s B1 cubic and rhombohedral phases and constrain the transition pressure at ambient temperature. We combine our results with pressure-temperature-volume measurements on an identical composition [1] to produce a well-constrained thermal equation of state. Using these results, we report a thermal elasticity model for iron-rich (Mg,Fe)O at CMB conditions for use in dynamic modeling and comparison against seismic observations. [1] Wicks et al., PEPI, 249, 28 (2015).
  

Discovery of an ultradense hydrous phase in the deep lower mantle: Application of multigrain crystallography in megabar experiments

Water enters the Earth's interior through hydrated subducting slabs. The Al-rich hydrous phase is stable throughout the high pressure-temperature (P-T) conditions of the deep lower mantle (DLM), but with a density 10% less than the surrounding mantle (1); it cannot form as a sizeable water reservoir. Our experiments at 107-136 GPa and 2400 K showed that AlOOH could incorporate up to 75% FeOOH_x in a previously unknown hexagonal structure and the product is 17% denser than the mantle and 31% less dense than the core. Applying the multigrain crystallography, 27 individual crystallites—each with 50 to 120 reflections consistent with the HH-phase—were discovered, unequivocally confirming the 12-formula unit of the HH-phase. The hexagonal (Fe,Al)OOH_x (HH-phase) can transform to the cubic pyrite structure at low T with the same density, isostructural to the pyrite (Py) structured FeO₂ and FeOOH (2). The HH-phase can be formed when delta-AlOOH incorporates FeOOH_x produced by reaction between Fe and water at the DLM conditions, thus storing a substantial quantity of water at the CMB.

1. I. Ohira, et al., Earth and Planetary Science Letters 401, 12-17 (2014).
2. Q. Hu, et al., Nature 534, 241-244 (2016).