Thermal equation of state of cubic boron nitride: Implications for a high-temperature pressure scale

The equation of state of cubic boron nitride (cBN) has been determined to a maximum temperature of 3300 K at a simultaneous static pressure of up to more than 70 GPa. Ab initio calculations to 80 GPa and 2000 K have also been performed. Our experimental data can be reconciled with theoretical results and with the known thermal expansion at 1 bar if we assume a small increase in pressure during heating relative to that measured at ambient temperature. The present data combined with the Raman measurements we presented earlier form the basis of a high-temperature pressure scale that is good to at least 3300 K

Physical properties of liquid Fe alloys at high pressure and their bearings on the nature of metallic planetary cores

[1] Sulfur and silicon are among the expected alloying light elements in planetary liquid iron cores. Structural properties of Fe-27 wt % S and Fe-17 wt % Si liquid alloys at high pressure and high temperature (0-5 GPa/1400-2300 K) are measured by synchrotron X-ray diffraction. Sulfur strongly modifies the local structure of liquid iron whereas silicon has only small structural effects. Fe-27 wt % S melts are indeed poorly ordered which explains a higher compressibility compared to pure liquid Fe. These results point out the necessity to consider the strong effect of S on liquid Fe properties while modeling planetary interiors. They imply a low S content in the Earth's outer core, leaving Si as a strong candidate, and argue for a present-day Martian solid core when combined with previous global chemical models

Chemically-bound xenon in fibrous silica

High-level quantum chemical calculations reported here predict the existence and remarkable stability, of chemically-bound xenon atoms in fibrous silica. The results may support the suggestion of Sanloup and coworkers that chemically-bound xenon and silica account for the problem of "missing xenon" (by a factor of 20!) from the atmospheres of Earth and Mars. So far, the host silica was assumed to be quartz, which is in contradiction with theory. The xenon-fibrous silica molecule is computed to be stable well beyond room temperature. The calculated Raman spectra of the species agree well with the main features of the experiments by Sanloup et al. The results predict computationally the existence of a new family of noble-gas containing materials. The fibrous silica species are finite molecules, their laboratory preparation should be feasible, and potential applications are possible

Structure of (Fe_xCa_1-xO)_y(SiO₂)_1-y liquids and glasses from high-energy x-ray diffraction: Implications for the structure of natural basaltic magmas

International audienc

Deep Earth carbon reactions through time and space

The authors acknowledge partial support from the Sloan Foundation grant G-2016-7157.Reactions involving carbon in the deep Earth have limited manifestation on Earth’s surface, yet they have played a critical role in the evolution of our planet. The metal-silicate partitioning reaction promoted carbon capture during Earth’s accretion and may have sequestered substantial carbon in Earth’s core. The freezing reaction involving iron-carbon liquid could have contributed to the growth of Earth’s inner core and the geodynamo. The redox melting/freezing reaction largely controls the movement of carbon in the modern mantle, and reactions between carbonates and silicates in the deep mantle also promote carbon mobility. The ten-year activity of the Deep Carbon Observatory has made important contributions to our knowledge of how these reactions are involved in the cycling of carbon throughout our planet, both past and present, and helped to identify gaps in our understanding that motivate and give direction to future studies.Publisher PDFPeer reviewe

Lutetium incorporation in magmas at depth:changes in melt local environment and the influence on partitioning behaviour

International audienceThe structure of two Lu doped (4000 ppm) model end member silicate liquids, a highly polymerised haplogranite (Si-Al-Na-K-O) and a less polymerised anorthite-diopside (Si-Al-Mg-Ca-O), have been studied up to 8 GPa using in situ x-ray diffraction techniques. The results are the first to identify trace rare Earth element incorporation in silicate melts at high pressure. At pressures below 5 GPa, the bonding environment of Lu-O was found to be dependent on composition with coordination number CN Lu−O = 8 and bond distance r Lu−O = 2.36Å36Å in the haplogranite melt, decreasing to CN Lu−O = 6 and r Lu−O = 2.29Å29Å in the anorthite-diopside melt. This compositional variance in coordination number at low pressure is consistent with observations made for Y-O in glasses at ambient conditions and is coincident with a dramatic increase in the partition coefficients previously observed for rare Earth elements with increasing melt polymerisation. With increasing pressure we find that CN Lu−O and r Lu−O remain constant in the haplo-granite melt. However, an abrupt change in both Lu-O coordination and bond distance is observed at 5 GPa in the anorthite-diopside melt, with CN Lu−O increasing from 6 to 8-fold and r Lu−O from 2.29 to 2.39Å39Å. This occurs over a similar pressure range where a change in the P-dependence in the reported rare Earth element partition coefficients is observed for garnet-, clinopyroxene-, and olivine-melt systems. This work shows that standard models for predicting trace elements at depth must incorporate the effect of pressure-induced structural transformations in the melt in order to realistically predict partitioning behaviour

Geophysical tests for habitability in ice-covered ocean worlds

Geophysical measurements can reveal the structure of icy ocean worlds and cycling of volatiles. The associated density, temperature, sound speed, and electrical conductivity of such worlds thus characterizes their habitability. To explore the variability and correlation of these parameters, and to provide tools for planning and data analyses, we develop 1-D calculations of internal structure, which use available constraints on the thermodynamics of aqueous MgSO$_4$, NaCl (as seawater), and NH$_3$, water ices, and silicate content. Limits in available thermodynamic data narrow the parameter space that can be explored: insufficient coverage in pressure, temperature, and composition for end-member salinities of MgSO$_4$ and NaCl, and for relevant water ices; and a dearth of suitable data for aqueous mixtures of Na-Mg-Cl-SO$_4$-NH$_3$. For Europa, ocean compositions that are oxidized and dominated by MgSO$_4$, vs reduced (NaCl), illustrate these gaps, but also show the potential for diagnostic and measurable combinations of geophysical parameters. The low-density rocky core of Enceladus may comprise hydrated minerals, or anydrous minerals with high porosity comparable to Earth's upper mantle. Titan's ocean must be dense, but not necessarily saline, as previously noted, and may have little or no high-pressure ice at its base. Ganymede's silicious interior is deepest among all known ocean worlds, and may contain multiple phases of high-pressure ice, which will become buoyant if the ocean is sufficiently salty. Callisto's likely near-eutectic ocean cannot be adequately modeled using available data. Callisto may also lack high-pressure ices, but this cannot be confirmed due to uncertainty in its moment of inertia

On the relative importance of thermal and chemical buoyancy in regular and impact-induced melting in a Mars-like planet

We ran several series of two-dimensional numerical mantle convection simulations representing in idealized form the thermochemical evolution of a Mars-like planet. In order to study the importance of compositional buoyancy of melting mantle, the models were set up in pairs of one including all thermal and compositional contributions to buoyancy and one accounting only for the thermal contributions. In several of the model pairs, single large impacts were introduced as causes of additional strong local anomalies, and their evolution in the framework of the convecting mantle was tracked. The models confirm that the additional buoyancy provided by the depletion of the mantle by regular melting can establish a global stable stratification of the convecting mantle and throttle crust production. Furthermore, the compositional buoyancy is essential in the stabilization and preservation of local compositional anomalies directly beneath the lithosphere and offers a possible explanation for the existence of distinct, long-lived reservoirs in the martian mantle. The detection of such anomalies by geophysical means is probably difficult, however; they are expected to be detected by gravimetry rather than by seismic or heat flow measurements. The results further suggest that the crustal thickness can be locally overestimated by up to ~20 km if impact-induced density anomalies in the mantle are neglected.Comment: 29 pages, 10 figure