Oliver Tschauner

Discovery of bridgmanite, the most abundant mineral in Earth, in a shocked meteorite

Meteorites exposed to high pressures and temperatures during impact-induced shock often contain minerals whose occurrence and stability normally confine them to the deeper portions of Earth’s mantle. One exception has been MgSiO3 in the perovskite structure, which is the most abundant solid phase in Earth. Here we report the discovery of this important phase as a mineral in the Tenham L6 chondrite and approved by the International Mineralogical Association (specimen IMA 2014-017). MgSiO3-perovskite is now called bridgmanite. The associated phase assemblage constrains peak shock conditions to ~ 24 gigapascals and 2300 kelvin. The discovery concludes a half century of efforts to find, identify, and characterize a natural specimen of this important mineral. X-ray analysis identifies magnesium silicate perovskite, now known as bridgmanite, in a heavily shocked meteorite. [Also see Perspective by Sharp] A mineral name for mantle perovskite A rock from outer space finally puts a name to Earths most abundant mineral, frequently referred to as perovskite. Mineral names are only bestowed on specimens that are found in nature and characterized. Tschauner et al. isolate a magnesium silicate in the perovskite structure, now called bridgmanite, in the Tenham L6 chondrite meteorite (see the Perspective by Sharp). Bridgmanite formed in this meteorite during a high-pressure and -temperature shock event. Other minerals associated with bridgmanite allow the pressure-temperature conditions to be narrowly bound, giving insight into the shock process. The long-sought-after specimen finally puts to rest a confusing nomenclature of this dense deep mantle silicate. Science, this issue p. 1100; see also p. 1057

High pressure single-crystal micro X-ray diffraction analysis with GSE_ADA/RSV software

GSE_ADA/RSV is a free software package for custom analysis of single-crystal micro X-ray diffraction (SCμXRD) data, developed with particular emphasis on data from samples enclosed in diamond anvil cells and subject to high pressure conditions. The package has been in extensive use at the high pressure beamlines of Advanced Photon Source (APS), Argonne National Laboratory and Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. The software is optimized for processing of wide-rotation images and includes a variety of peak intensity corrections and peak filtering features, which are custom-designed to make processing of high pressure SCμXRD easier and more reliable.

Melting of Cu under hydrostatic and shock wave loading to high pressures

Molecular dynamics simulations are performed to investigate hydrostatic melting and shock-induced melting of single crystal Cu described by an embedded-atom method potential. The thermodynamic (equilibrium) melting curve obtained from our simulations agrees with static experiments and independent simulations. The planar solid–liquid interfacial energy is found to increase with pressure. The amount of maximum superheating or supercooling is independent of pressure, and is 1.24 ± 0.01 and 0.68 ± 0.01 at a heating or cooling rate of 1 K ps−1, respectively. We explore shock loading along three main crystallographic directions: , and . Melting along the principal Hugoniot differs considerably from and , possibly due to different extents of solid state disordering. Along , the solid is superheated by about 20%, before it melts with a pronounced temperature drop. In contrast, melting along and is quasi-continuous, and premelting (~7%) is observed.

New transformations of CO(2) at high pressures and temperatures.

CO(2) laser heating of solid CO(2) at pressures between 30 and 80 GPa shows that this compound breaks down to oxygen and diamond along a boundary having a negative P-T slope. This decomposition occurs at temperatures much lower than predicted in theory or inferred from previous experiment. Raman spectroscopy and x-ray diffraction were used as structural probes. At pressures higher than 40 GPa the decomposition is preceded by the formation of a new CO(2) phase (CO(2)-VI). These findings limit the stability of nonmolecular CO(2) phases to moderate temperatures and provide a new topology of the CO(2) phase diagram.

In situ high-pressure x-ray diffraction study of H2O ice VII

Ice VII was examined over the entire range of its pressure stability by a suite of x-ray diffraction techniques in order to understand a number of unexplained characteristics of its high-pressure behavior. Axial and radial polycrystalline (diamond anvil cell) x-ray diffraction measurements reveal a splitting of diffraction lines accompanied by changes in sample texture and elastic anisotropy. In situ laser heating of polycrystalline samples resulted in the sharpening of diffraction peaks due to release of nonhydrostatic stresses but did not remove the splitting. Radial diffraction measurements indicate changes in strength of the material at this pressure. Taken together, these observations provide evidence for a transition in ice VII near 14 GPa involving changes in the character of the proton order/disorder. The results are consistent with previous reports of changes in phase boundaries and equation of state at this pressure. The transition can be interpreted as ferroelastic with the appearance of spontaneous strain that vanishes at the hydrogen bond symmetrization transition near 60 GPa.

P-V-T equation of state of platinum to 80GPa and 1900K from internal resistive heating/x-ray diffraction measurements

The P-V-T equation of state (EOS) of Pt has been determined to 80GPa and 1900K by in situ x-ray diffraction of a mixture of Pt and MgO using a modified internal resistive heating technique with a diamond anvil cell. The third-order Birch–Murnaghan EOS of Pt at room temperature can be fitted with K0=273.5±2.0GPa, K0′=4.70±0.06, with V0=60.38A3. High temperature data have been treated with both thermodynamic and Mie–Gruneisen-Debye methods for the thermal EOS inversion. The results are self-consistent and in excellent agreement with those obtained by the multianvil apparatus where the data overlap. MgO is taken as the standard because its thermal EOS is well established and based on a wealth of experimental and theoretical data, and because the EOS at room temperature has been determined by a primary method that is completely independent of any assumptions or measurements by other methods. Improvements to previous internal resistive heating methods were made by using a Re gasket that replaces the original g...

Laser-induced shock waves in condensed matter: some techniques and applications

Laser-induced shock waves in condensed matter have important applications in dynamic material studies and high pressure physics. We briefly review some techniques in laser-induced shock waves, including direct laser drive, laser-launched flyer plate, quasi-isentropic loading, point and line imaging velocity interferometry, transient X-ray diffraction, spectroscopy and shock recovery, and their applications to study of equation of state, spallation, and phase transitions.

Ultrafast growth of wadsleyite in shock-produced melts and its implications for early solar system impact processes

We observed micrometer-sized grains of wadsleyite, a high-pressure phase of (Mg,Fe)2SiO4, in the recovery products of a shock experiment. We infer these grains crystallized from shock-generated melt over a time interval of <1 μs, the maximum time over which our experiment reached and sustained pressure sufficient to stabilize this phase. This rapid crystal growth rate (≈1 m/s) suggests that, contrary to the conclusions of previous studies of the occurrence of high-pressure phases in shock-melt veins in strongly shocked meteorites, the growth of high-pressure phases from the melt during shock events is not diffusion-controlled. Another process, such as microturbulent transport, must be active in the crystal growth process. This result implies that the times necessary to crystallize the high-pressure phases in shocked meteorites may correspond to shock pressure durations achieved on impacts between objects 1–5 m in diameter and not, as previously inferred, ≈1–5 km in diameter. These results may also provide another pathway for syntheses, via shock recovery, of some high-value, high-pressure phases.

Behavior of iron in (Mg,Fe)SiO3 post-perovskite assemblages at Mbar pressures

The electronic environment of the iron sites in post-perovskite (PPv) structured (^(57)Fe,Mg)SiO_3 has been measured in-situ at 1.12 and 1.19 Mbar at room temperature using ^(57)Fe synchrotron Mossbauer spectroscopy. Evaluation of the time spectra reveals two distinct iron sites, which are well distinguished by their hyperfine fields. The dominant site is consistent with an Fe^(3+)-like site in a high spin state. The second site is characterized by a small negative isomer shift with respect to α-iron and no quadrupole splitting, consistent with a metallic iron phase. Combined with SEM/EDS analyses of the quenched assemblage, our results are consistent with the presence of a metallic iron phase co-existing with a ferric-rich PPv. Such a reaction pathway may aid in our understanding of the chemical evolution of Earths core-mantle-boundary region.

Effect of dilution on the spin pairing transition in rhombohedral carbonates

The compressibility of an iron-bearing magnesite was determined by means of single crystal diffraction up to 64 GPa. Up to 49 GPa the pressure-evolution of the unit cell volume of the solid solution with 12% of Fe2+ can be described by a third-order Birch–Murnaghan equation of state with parameters V 0=281.0(5) Å3, K 0=102.8(3) GPa, K . The spin pairing of the Fe2+ d-electrons occurs between 49 and 52 GPa, as evidenced by a discontinuous volume change. The transition pressure is increased by about 5 GPa compared with the iron end-member; an effect consistent with a cooperative contribution of adjacent clusters to the spin transition. The trend is, however, opposite in the periclase–wüstite solid solution. Differences among the two structures, in particular in the Fe–Fe interactions, that might explain the different behavior are discussed.