ALBERT

Description: Carbonates are the major hosts of carbon on Earth’s surface and their fate during subduction needs to be known to understand the deep carbon cycle. Magnesite (MgCO3) is thought to be an important phase participating in deep Earth processes, but its phase stability is still a matter of debate for the conditions prevalent in the lowest part of the mantle and at the core mantle boundary. Here, we have studied the phase relations and stabilities of MgCO3 at these P,T conditions, using Raman spectroscopy at high pressures (∼148GPa) and after heating to high temperatures (∼3600 K) in laser-heated diamond anvil cell experiments. The experimental Raman experiments were supplemented by x-ray powder diffraction data, obtained at a pressure of 110 GPa. Density-functional-theory-based model calculations were used to compute Raman spectra for several MgCO3 high-pressure polymorphs, thus allowing an unambiguous assignment of Raman modes. By combining the experimental observations with the density-functional-theory results, we constrain the phase stability field of MgCO3 with respect to the high-pressure polymorph, MgCO3-II. We further confirm that Fe-free MgCO3-II is a tetracarbonate with monoclinic symmetry (space group C2/m), which is stable over the entire P, T range of the Earth’s lowermost mantle geotherm.

Description: Experiments in laser-heated diamond anvil cells (LH DACs) are conducted to assess phase diagrams of planetary materials at high pressure-temperature (P-T) conditions; thus, reliable determination of temperature in LH DAC experiments is essential. Radiometric temperature determination in LH DACs relies on the assumption of sample's wavelength-independent optical properties (graybody assumption), which is not justified for major lower mantle materials. The result is that experimental phase diagrams contain systematic unconstrained errors. Here we estimate the systematic error in radiometric temperature of nongray polycrystalline bridgmanite (Bgm; Mg0.96Fe2+0.036Fe3+0.014Si0.99O3) in a LH DAC by modeling emission and absorption of thermal radiation in a sample with experimentally-constrained optical properties. A comparison to experimental data validates the models and reveals that thermal spectra measured in LH DAC experiments record the interaction of radiation with the hot nongray sample. The graybody assumption in the experiments on translucent Bgm (light extinction coefficient, k 〈 ∼250 cm-1 at 500–900 nm) yields temperatures ∼5% higher than the maximum temperature in the sample heated to ∼1900 K. In contrast, the graybody temperature of dark Bgm (k 〉 ∼1500 cm−1), such as that produced upon melt quenching in LH DACs, underestimates the maximum temperature by ∼10%. Our experimental results pose quantitative constraints on the effect of nongray optical properties on the uncertainty of radiometric temperature determination in Bgm in the LH DACs. Evaluating nongray temperature in the future would enable a revision of the Bgm to post-perovskite phase transition and the high-pressure melting curve of Bgm.

Description: Optical studies of materials at high pressure-temperature (P-T) conditions provide insights into their physical properties that may be inaccessible to direct determination at extreme conditions. Incandescent light sources, however, are insufficiently bright to optically probe samples with radiative temperatures above ~1000 K. Here we report on a system to perform optical absorption experiments in a laser-heated diamond anvil cell at T up to at least 4000 K. This setup is based on a pulsed supercontinuum (broadband) light probe and a gated CCD detector. Precise and tight synchronization of the detector gates (3 ns) to the bright probe pulses (1 ns) diminishes the recorded thermal background and preserves an excellent probe signal at high temperature. We demonstrate the efficiency of this spectroscopic setup by measuring the optical absorbance of solid and molten (Mg,Fe)SiO3, an important constituent of planetary mantles, at P~30 GPa and T~1200-4150 K. Optical absorbance of hot solid (Mg,Fe)SiO3 is moderately sensitive to temperature but increases abruptly upon melting and acquires a strong temperature-dependence. Our results enable quantitative estimates of the opacity of planetary mantles with implications to their thermal and electrical conductivity, all of which have never been constrained at representative P-T conditions, and call for an optical detection of melting in silicate-bearing systems to resolve the extant ambiguity in their high-pressure melting curves.

Description: As theoretically hypothesized for several decades in group IV transition metals, we have discovered a dynamically stabilized body-centered cubic (bcc) intermediate state in Zr under uniaxial loading at sub-nanosecond timescales. Under ultrafast shock wave compression, rather than the transformation from a-Zr to the more disordered hex-3 equilibrium x-Zr phase, in its place we find the formation of a previously unobserved nonequilibrium bcc metastable in- termediate. We probe the compression-induced phase transition pathway in zirconium using time-resolved sub-picosecond x-ray diffraction analysis at the Linac Coherent Light Source. We also present molecular dynamics simula- tions using a potential derived from first-principles methods which indepen- dently predict this intermediate phase under ultrafast shock conditions. In contrast with experiments on longer timescale (〉 10 ns) where the phase diagram alone is an adequate predictor of the crystalline structure of a material, our recent study highlights the importance of metastability and time dependence in the kinetics of phase transformations.

Description: 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, on the lower end of previous QCMB 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.

Description: Earth’s core is composed of iron (Fe) alloyed with light elements, e.g., silicon (Si). Its thermal conductivity critically affects Earth’s thermal structure, evolution, and dynamics, as it controls the magnitude of thermal and compositional sources required to sustain a geodynamo over Earth’s history. Here we directly measured thermal conductivities of solid Fe and Fe–Si alloys up to 144 GPa and 3300 K. 15 at% Si alloyed in Fe substantially reduces its conductivity by about 2 folds at 132 GPa and 3000 K. An outer core with 15 at% Si would have a conductivity of about 20 W m−1 K−1, lower than pure Fe at similar pressure–temperature conditions. This suggests a lower minimum heat flow, around 3 TW, across the core–mantle boundary than previously expected, and thus less thermal energy needed to operate the geodynamo. Our results provide key constraints on inner core age that could be older than two billion-years.

Description: Ultrafast (130-fs) x-ray diffraction at the Linac Coherent Light Source has been applied to observe shockmelting, which is driven by a rapid (120-ps) laser pulse impinging on a thin (few micrometers) bilayer ofaluminum/zirconium. At a pressure of 100 GPa in the aluminum (130 GPa in the zirconium), there is rapidmelting of both metals and the recrystallization of zirconium into the bcc β phase. We observe the solidificationof the melt starting a few hundred picoseconds following the shock melting, out to 50 ns when the zirconiumis fully crystallized into the bcc β phase at a residual temperature of approximately 2000 K. The pressure isobtained directly from the early time x-ray data, whereas the additional information from the x-ray line widthand intensity at longer times inform a model of crystal nucleation and growth.

Description: Carbon‐bearing phases show a rich variety of structural transitions as an adaptation to pressure. Of particular interest is the crossover from sp 2 carbon to sp 3 carbon, as physical and chemical properties of carbon in these distinct electronic configurations are very different. In this chapter we review pressure‐induced sp 2 ‐sp 3 transitions in elemental carbon, carbonates, and hydrocarbons.

Description: The insulator‐to‐metal transition in dense fluid hydrogen is an essential phenomenon in the study of gas giant planetary interiors and the physical and chemical behavior of highly compressed condensed matter. Using direct fast laser spectroscopy techniques to probe hydrogen and deuterium precompressed in a diamond anvil cell and laser heated on microsecond timescales, an onset of metal‐like reflectance is observed in the visible spectral range at P 〉150 GPa and T ≥ 3000 K. The reflectance increases rapidly with decreasing photon energy indicating free‐electron metallic behavior with a plasma edge in the visible spectral range at high temperatures. The reflectance spectra also suggest much longer electronic collision time (≥1 fs) than previously inferred, implying that metallic hydrogen at the conditions studied is not in the regime of saturated conductivity (Mott–Ioffe–Regel limit). The results confirm the existence of a semiconducting intermediate fluid hydrogen state en route to metallization.