arXiv subjects

James Badro

Publications and source records attributed to James Badro.

15 recordsLinked to original sources

arXiv2026

Low-spin ferrous iron suppresses mantle oxidation beyond Earth-like pressures

The Earth's mantle has elevated Fe$^{3+}$ relative to those of other rocky bodies, yet the oxidation- and electronic state of iron at extreme pressures is poorly known. We present in-situ energy-domain synchrotron M\"ossbauer spectra of $^{57}$Fe-enriched silicate glasses at 298 K from 1 bar to 174 GPa in a diamond anvil cell. Glasses were synthesised with Fe$^{3+}$/[Fe$^{3+}$ + Fe$^{2+}$] from 0.02 $\pm$ 0.02 to 1.00 $\pm$ 0.02, as determined by colourimetry. While pure Fe$^{3+}$-basaltic glass shows minimal changes up to 174 GPa, the spectra of Fe$^{2+}$-peridotitic and basaltic glasses are fit by two doublets, D$_1$ and D$_2$. At 1 bar, their relative intensities are $\sim$92 % and $\sim$8 %, respectively, but the integral area ratio, D$_2$/(D$_1$ + D$_2$), reaches 0.65 by 172 GPa. Because this transition is reversible with pressure and no metallic iron is detected, the D$_2$ feature is Fe$^{2+}$ low spin (LS), whereas D$_1$ is Fe$^{2+}$ high spin (HS). Consequently, the Fe$^{3+}$/[Fe$^{3+}$+Fe$^{2+}$] of planetary mantles reach a maximum near $\sim$40 GPa, before decreasing at higher pressures due to the stabilisation of Fe$^{2+}_{\mathrm{{LS}}}$. This peak coincides with estimated core-mantle equilibrium on Earth, implying that its uniquely oxidised mantle and habitable state may result from core formation within a Goldilocks pressure range. Secondary atmospheres are predicted to transition from H$_2$-rich for Moon-sized bodies, to CO-rich for Earth-like planets and H$_2$- and CH$_4$-bearing around super-Earths and sub-Neptunes.

astro-ph.EP↗

arXiv2025

Experiments reveal extreme water generation during planet formation

The most abundant type of planet discovered in the Galaxy has no analogue in our Solar System and is believed to consist of a rocky interior with an overlying thick H2 dominated envelope. Models have predicted that the reaction between the atmospheric hydrogen and the underlying magma ocean can lead to the production of significant amounts of water. The models suffer however from the current lack of experimental data on the reaction between hydrogen and silicate melt at high pressures and temperatures. Here we present novel experimental results designed to investigate this interaction. Laser heating diamond anvil cell experiments were conducted between 16 and 60 GPa at temperatures above 4000 K. We find that copious amounts of hydrogen dissolve into the silicate melt with a large dependence on temperature rather than pressure. We also find that the reduction of iron oxide leads to the production of significant amounts of water along with the formation of iron-enriched blebs. Altogether, the results predict that the typical processes attending planet formation will result in significant water production with repercussions for the chemistry and structure of the planetary interior as well as the atmosphere.

astro-ph.EP↗

arXiv2023

The role of interior dynamics and differentiation on the surface and atmosphere of lava planets

Lava planets are rocky exoplanets that orbit so close to their host star that their day-side is hot enough to melt silicate rock. Their short orbital periods ensure that lava planets are tidally locked into synchronous rotation, with permanent day and night hemispheres. Such asymmetric magma oceans have no analogs in the Solar System and will exhibit novel fluid dynamics. Here we report numerical simulations of lava planet interiors showing that solid-liquid fractionation in the planetary interior has a major impact on the compositional structure and evolution of the planet. We explored two styles of dynamics that depend primarily on the interior thermal state : 1) a hot fully molten interior, and 2) a mostly solid interior with a shallow day-side magma ocean. In the hot interior scenario, the atmosphere reflects the planet's bulk silicate composition and the night-side crust is gravitationally unstable and constantly replenished. In the cool interior scenario, the distilled atmosphere will lack Na, K and FeO, and the night-side mantle is entirely solid, with a cold surface. These two end-member cases can be distinguished with observations from the James Webb Space Telescope, offering an avenue to probe the diversity of terrestrial exoplanet evolutions.

astro-ph.EP↗

arXiv2022

Solubility of water in peridotite liquids and the prevalence of steam atmospheres on rocky planets

Atmospheres are products of time-integrated mass exchange between the surface of a planet and its interior. On Earth and other planetary bodies, magma oceans likely marked significant atmosphere-forming events, during which both steam- and carbon-rich atmospheres may have been generated. However, the nature of Earth's early atmosphere, and those around other rocky planets, remains unclear for lack of constraints on the solubility of water in liquids of appropriate composition. Here we determine water solubility in 14 peridotite liquids, representative of Earth's mantle, synthesised in a laser-heated aerodynamic levitation furnace. We explore oxygen fugacities (fO$_2$) between -1.9 and +6.0 log units relative to the iron-w\"ustite buffer at constant temperature (2173$\pm$50 K) and total pressure (1 bar). The resulting fH$_2$O ranged from 0 to 0.027 bar and fH$_2$ from 0 to 0.064 bar. Total H$_2$O contents were determined by transmission FTIR spectroscopy from the absorption band at 3550cm$^{-1}$ and applying the Beer-Lambert law. The mole fraction of water in the liquid is $\propto$ (fH$_2$O)$^{0.5}$, attesting to its dissolution as OH. The data are fit by a solubility coefficient of 524$\pm$16 ppmw/bar$^{0.5}$, for a molar absorption coefficient, $\epsilon_{3550}$, of 6.3$\pm$0.3 m$^2$/mol in basaltic glasses or 647$\pm$25 ppmw/bar$^{0.5}$, with $\epsilon_{3550}$ = 5.1$\pm$0.3m$^2$/mol for peridotitic glasses. These solubility constants are 10-25 % lower than those for basaltic liquids at 1623 K and 1 bar. Higher temperature lowers water solubility, offsetting the greater depolymerisation of peridotite melts that would otherwise increase H$_2$O solubility relative to basaltic liquids. Because the solubility of water remains high relative to that of CO$_2$, steam atmospheres are rare, although they may form under oxidising conditions on telluric bodies, provided high H/C ratios prevail.

astro-ph.EP↗

arXiv2022

Deep two-phase, hemispherical magma oceans on lava planets

Astronomers have discovered a handful of exoplanets with rocky bulk compositions but orbiting so close to their host star that the surface of the planet must be at least partially molten. It is expected that the dayside of such "lava planets" harbors a rock vapor atmosphere that flows quickly towards the airless nightside -- this partial atmosphere is critical to the interpretation of lava planet observations, but transports negligible heat towards the nightside. As a result, the surface temperature of the magma ocean may range from 3000~K near the sub-stellar point down to 1500~K near the day-night terminator. We use simple models incorporating the thermodynamics and geochemistry of partial melt to predict the physical and chemical properties of the magma ocean as a function of the distance from the sub-stellar point. Our two principal findings are that 1) the dayside magma ocean is much deeper than previously thought, probably extending down to the core-mantle boundary in some locations, and 2) much of the dayside is only partially molten, leading to gradients in the surface chemistry of the magma ocean. These findings have important implications for the dynamics of the magma ocean as well as the composition and dynamics of the atmosphere.

astro-ph.EP↗

arXiv2020

Constraints on the composition and temperature of LLSVPs from seismic properties of lower mantle minerals

Here, we provide a reappraisal of potential LLSVPs compositions based on an improved mineralogical model including, for instance, the effects of alumina. We also systematically investigate the effects of six parameters: FeO and Al$_{2}$O$_{3}$ content, proportion of CaSiO$_{3}$ and bridgmanite (so that the proportion of ferropericlase is implicitly investigated), Fe$^{3+}$/$\sum$Fe and temperature contrast between far-field mantle and LLSVPs. From the 81 millions cases studied, only 79000 cases explain the seismic observations. Nevertheless, these successful cases involve a large range of parameters with, for instance, FeO content between 12--25~wt\% and Al$_{2}$O$_{3}$ content between 3--17~wt\%. We then apply a principal component analysis (PCA) to these cases and find two robust results: (i) the proportion of ferropericlase should be low ($<$6vol\%); (ii) the formation of Fe$^{3+}$-bearing bridgmanite is much more favored than other iron-bearing phases. Following these results, we identify two end-member compositions, Bm-rich and CaPv-rich, and discuss their characteristics. Finally, we discuss different scenarios for the formation of LLSVPs and propose that investigating the mineral proportion produced by each scenario is the best way to evaluate their relevance. For instance, the solidification of a primitive magma ocean may produce FeO and Al$_{2}$O$_{3}$ content similar to those suggested by our analysis. However, the mineral proportion of such reservoirs is not well-constrained and may contain a larger proportion of ferropericlase than what is allowed by our results.

physics.geo-ph↗

arXiv2019

Blocked radiative heat transport in the hot pyrolitic lower mantle

The heat flux across the core-mantle boundary (QCMB) is the key parameter to understand the Earth/s thermal history and evolution. Mineralogical constraints of the QCMB require deciphering contributions of the lattice and radiative components to the thermal conductivity at high pressure and temperature in lower mantle phases with depth-dependent composition. Here we determine the radiative conductivity (krad) of a realistic lower mantle (pyrolite) in situ using an ultra-bright light probe and fast time-resolved spectroscopic techniques in laser-heated diamond anvil cells. We find that the mantle opacity increases critically upon heating to ~3000 K at 40-135 GPa, resulting in an unexpectedly low radiative conductivity decreasing with depth from ~0.8 W/m/K at 1000 km to ~0.35 W/m/K at the CMB, the latter being ~30 times smaller than the estimated lattice thermal conductivity at such conditions. Thus, radiative heat transport is blocked due to an increased optical absorption in the hot lower mantle resulting in a moderate CMB heat flow of ~8.5 TW, at odds with present estimates based on the mantle and core dynamics. This moderate rate of core cooling implies an inner core age of about 1 Gy and is compatible with both thermally- and compositionally-driven ancient geodynamo.

physics.geo-ph↗

arXiv2019

Opaque Lowermost Mantle

Earth/s lowermost mantle displays complex geological structures that likely result from heterogeneous thermal and electromagnetic interaction with the core. Geophysical models of the core-mantle boundary (CMB) region rely on the thermal and electrical conductivities of appropriate geomaterials which, however, have never been probed at representative pressure and temperature (P-T) conditions. Here we report on the opacity of single crystalline bridgmanite and ferropericlase, which is linked to both their radiative and electrical conductivity, measured in dynamically- and statically-heated diamond anvil cells as well as computed from first-principles at CMB conditions. Our results show that light absorption in the visible spectral range is enhanced upon heating in both minerals but the rate of change in opacity with temperature is a factor of six higher in ferropericlase. As a result, bridgmanite in the lowermost mantle is moderately transparent while ferropericlase is highly opaque. Our measurements suggest a very low (< 1 W/m/K) and largely temperature-independent radiative conductivity in the lowermost mantle, at odds with previous studies. This implies that the radiative mechanism has not contributed significantly to cooling the Earth/s core throughout the geologic time and points to a present-day CMB heat flow of 9-11 TW. Opaque ferropericlase is electrically conducting and mediates strong core-mantle electromagnetic coupling, explaining the intradecadal oscillations in the length of day, low secular geomagnetic variations in Central Pacific, and the preferred paths of geomagnetic pole reversals.

physics.geo-ph↗

arXiv1999

Pressure induced high-spin to low-spin transition in FeS evidenced by x-ray emission spectroscopy

We report the observation of the pressure-induced high-spin to low-spin transition in FeS using new high-pressure synchrotron x-ray emission spectroscopy techniques. The transition is evidenced by the disappearance of the low-energy satellite in the Fe K$β$ emission spectrum of FeS. Moreover, the phase transition is reversible and closely related to the structural phase transition from a manganese phosphide-like phase to a monoclinic phase. The study opens new opportunities for investigating the electronic properties of materials under pressure.

cond-mat.mtrl-sci↗

arXiv1998

On the High-Pressure Phase Transition in GaPO4

X-ray diffraction (XRD) experiments have been carried out on quartz-like GaPO4 at high pressure and room temperature. A transition to a high pressure disordered crystalline form occurs at 13.5 GPa. Slight heating using a YAG infrared laser was applied at 17 GPa in order to crystallize the phase in its stability field. The structure of this phase is orthorhombic with space group Cmcm. The cell parameters at the pressure of transition are a=7.306 A, b=5.887 A and c=5.124 A.

cond-mat↗

arXiv1998

Melting and Pressure-Induced Amorphization of Quartz

It has recently been shown that amorphization and melting of ice were intimately linked. In this letter, we infer from molecular dynamics simulations on the SiO2 system that the extension of the quartz melting line in the metastable pressure-temperature domain is the pressure-induced amorphization line. It seems therefore likely that melting is the physical phenomenon responsible for pressure induced amorphization. Moreover, we show that the structure of a "pressure glass" is similar to that of a very rapidly (1e+13 to 1e+14 kelvins per second) quenched thermal glass.

cond-mat.mtrl-sci↗

arXiv1997

XAS Study of the High Pressure Behaviour of Quartzlike Compounds

EXAFS spectroscopy experiments have been carried out on quartz-like GaAsO4 and AlAsO4 at high pressure and room temperature. It has been shown that these materials exhibit two structural phase transitions; the first transition to a high pressure crystalline form occurs at 9 GPa and is reversible upon decompression, whereas the second transition occurs at higher pressures and is irreversible. In GaAsO4, EXAFS measurements agree with the predicted transition from four- to six-fold coordination of oxygen atoms around the cations, but the two local coordination transformations are not dissociated; in fact, both As and Ga atoms exhibit a coordination change at the onset of the first phase transition, the rate of transformation being significantly higher for Ga atoms. In both cases, the average bond length increases very rapidly with pressure thus yielding the first compression stage after the transition. In the second stage, the average bond lengths increase slowly, ultimately reaching six-fold coordination above 28 GPa and 24 GPa for As and Ga respectively. The behaviour of the As K-edge EXAFS is the same for both compounds, and enables us to link the behaviour of Ga and Al atoms. The local transformations are well described and a direct link with phosphate berlinites seems timely.

cond-mat.mtrl-sci↗

arXiv1997

A combined XAS and XRD Study of the High-Pressure Behaviour of GaAsO4 Berlinite

Combined X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) experiments have been carried out on GaAsO4 (berlinite structure) at high pressure and room temperature. XAS measurements indicate four-fold to six-fold coordination changes for both cations. The two local coordination transformations occur at different rates but appear to be coupled. A reversible transition to a high pressure crystalline form occurs around 8 GPa. At a pressure of about 12 GPa, the system mainly consists of octahedral gallium atoms and a mixture of arsenic in four-fold and six-fold coordinations. A second transition to a highly disordered material with both cations in six-fold coordination occurs at higher pressures and is irreversible.

cond-mat.mtrl-sci↗

arXiv1996

Strong to fragile transition in a model of liquid silica

The transport properties of an ionic model for liquid silica at high temperatures and pressure are investigated using molecular dynamics simulations. With increasing pressure, a clear change from "strong" to "fragile" behaviour (according to Angell's classification of glass-forming liquids) is observed, albeit only on the small viscosity range that can be explored in MD simulations.. This change is related to structural changes, from an almost perfect four-fold coordination to an imperfect five or six-fold coordination. (submitted to "Molecular Simulation")

cond-mat.mtrl-sci↗

arXiv1996

Numerical simulation of alpha-quartz under non-hydrostatic compression. Memory glass and five-coordinated crystalline phases

The behavior of $α$-quartz under hydrostatic and non-hydrostatic high-pressure conditions has been investigated in Molecular Dynamics simulations of silica in order to clarify the role of non-hydrostatic stresses in the amorphization process. It is shown that the amorphization threshold is not modified if the stress along the {\bf c} direction is lowered, so that the mean amorphization pressure can effectively be lowered under non-hydrostatic conditions. On the other hand, the application of a positive uniaxial stress along the {\bf c} axis, results in the appearance of a new crystalline phase, where all silicon atoms are in five-fold coordination.

cond-mat↗