Equilibrium temperatures, pressures, and oxygen fugacities of the equilibrated chondrites
Equilibrium temperatures, pressures and oxygen fugacities of equilibrated chondrites in meteorites
Williams, R. J.↗
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Restudy of pyroxene-pyroxene equilibration temperature for ordinary chondrite meteorites
Experimental verification of the conclusions of Van Schmus and Koffman (1967) that the equilibration temperatures of ordinary chondrites are roughly the same regardless of classification, and that these temperatures range from 770 to 860 C. The higher average relative equilibrium temperatures found are attributed to improvements in microprobe techniques, and better standards and correction procedures introduced over the last six years. Agreement is expressed with their conclusion that ordinary chondrites embody equilibria achieved within the metamorphic temperature regime and not under igneous temperatures as postulated earlier by Craig (1964) and Mueller (1964).
Bunch, T. E.↗
Double-resonance study of vibrational equilibration in optically pumped laser molecules
A CW frequency-locked CO2 laser is used in conjunction with a 1-J TEA CO2 laser to observe the vibrational equilibration of optically pumped gases. Velocity-dependent effects in the vibrational-deactivation rate are also observed. Experimental results are given for CH3F
Lawandy, N. M.↗
Equilibration temperatures of the ordinary chondrites - A new evaluation
The novel calibrations of the Ca-pyroxene thermometer accomplished by Kretz (1982) and Lindsley (1983) are presently applied to 24 analyzed pyroxenes from 24 H6, L6, and LL6 chondrites. Both thermometers indicate that the H group equilibrated to significantly lower temperature (820-830 C) than the L and LL groups, yielding values of 860 C (Lindsley) to 930 C (Kretz).
Olsen, E. J.↗
Observational study of ion-electron equilibration and of cloud evaporation in supernova remnants under the HEAO-2 guest investigator program
Observations of three selected supernovae remnants (Cygnus Loop, IC 443, and Puppis A) were made in the forbidden coronal iron lines (Fe X) lambda 6374 and (Fe XIV) lambda 5303. The resulting data was compared quantitatively with Einstein images of the same objects, and an attempt was made to determine (a) the process by which ion and electron energies are equilibrated behind the shock front in the ISM and (b) whether cloud evaporation occurs within the hot remnant interiors. Spatially-resolved X ray emission were modeled for Sedov-Taylor blast wave models of supernovae remnants (SNR) under conditions of non-equlibrium ionization. The computations are intended to provide results that can be directly compared with Einstein high resolution image (HRI) and imaging proportional counter (IPS) data. The computer program for predicting the spatial distribution of HRI and IPS counting rates was completed, and final testing of it had begun.
Teske, R. G.↗
Para hydrogen equilibration in the atmospheres of the outer planets
The thermodynamic behavior of the atmospheres of the Jovian planets is strongly dependent on the extent to which local thermal equilibration of the ortho and para states of molecular hydrogen is achieved. Voyager IRIS data from Jupiter imply substantial departures of the para hydrogen fraction from equilibrium in the upper troposphere at low latitudes, but with values approaching equilibrium at higher latitudes. Data from Saturn are less sensitive to the orth-para ratio, but suggest para hydrogen fractions near the equilibrium value. Above approximately the 200 K temperature level, para hydrogen conversion can enhance the efficiency of convection, resulting in a substantial increase in overturning times on all of the outer planets. Currently available data cannot definitively establish the ortho-para ratios in the atmospheres of Uranus and Neptune, but suggest values closer to local equilibrium than to the 3.1 normal ratio. Modeling of sub-millimeter wavelength measurements of these planets suggest thermal structures with frozen equilibrium lapse rates in their convective regions.
Conrath, Barney J.↗
Thermal equilibration of the earth following a giant impact
A thermal evolution model for the cooling of a terrestrial planet, a prescribed fraction of which is melted during a giant impact, is described. Two model geometries are considered, one having a laterally inhomogeneous distribution of melt and solid through a mega-crater geometry and the other having a global magma ocean. It is found that almost the whole planet may melt if the impact originally melted half of the planet and if the first melt temperature was double a typical planet melting temperature. For reasonable choices of parameter values it is determined that thermal equilibration of the earth occurs on a time-scale of 1 to ten million years.
Spohn, Tilman↗
Equilibration temperatures of EL chondrites: A major downward revision in the ferrosilite contents of enstatite
A study of enstatite grains separated from EL6 chondrites shows ferrosilite contents to be approx. 0.02-0.04 mol%, about an order of magnitude lower than reported in the literature. The higher values reported earlier can probably be attributed to the excitation of Fe atoms in Fe-Ni and FeS, in part from micrometer-sized blebs of FeS and Fe-Ni commonly present in enstatite grains but mainly from large neighboring opaque grains excited by bremsstrahlung or by doubly backscattered electrons. Our EL6 ferrosilite contents are similar to those observed in coarse enstatite grains in some aubrites. The equilibrium between ferrosilite and Si-bearing kamacite is highly temperature dependent. At constant Si concentration in kamacite, the equilibrium ferrosilite mole fraction drops by a factor of roughly 2.5 for each 100 K decrease in the equilibrium temperature. As a result, the compositions of these two most common EL6 phases can be used to estimate metamorphic equilibration temperatures. The inferred temperatures are near 1200 K.
Wasson, John T.↗
Shock Re-equilibration of Fluid Inclusions
Fluid inclusions (microscopic volumes of fluid trapped within minerals as they precipitate) are extremely common in terrestrial minerals formed under a wide range of geological conditions from surface evaporite deposits to kimberlite pipes. While fluid inclusions in terrestrial rocks are nearly ubiquitous, only a few fluid inclusion-bearing meteorites have been documented. The scarcity of fluid inclusions in meteoritic materials may be a result of (a) the absence of fluids when the mineral was formed on the meteorite parent body or (b) the destruction of fluid inclusions originally contained in meteoritic materials by subsequent shock metamorphism. However, the effects of impact events on pre-existing fluid inclusions trapped in target and projectile rocks has received little study. Fluid inclusions trapped prior to the shock event may be altered (re-equilibrated) or destroyed due to the high pressures, temperatures, and strain rates associated with impact events. By examining the effects of shock deformation on fluid inclusion properties and textures we may be able to better constrain the pressure-temperature path experienced by terrestrial and meteoritic shocked materials and also gain a clearer understanding of why fluid inclusions are rarely found in meteorite samples.
Madden, M. E. Elwood↗
Oxygen Isotope Systematics of Chondrules from the Least Equilibrated H Chondrite
Oxygen isotope compositions of bulk chondrules and their mineral separates in type 3 ordinary chondrites (UOC) show several % variability in the oxygen three isotope diagram with slope of approx.0.7 [1]. In contrast, ion microprobe analyses of olivine and pyroxene phenocrysts in ferromagnesian chondrules from LL 3.0-3.1 chondrites show mass dependent isotopic fractionation as large as 5% among type I (FeO-poor) chondrules, while type II (FeO-rich) chondrules show a narrow range (less than or equal to 1%) of compositions [2]. The .Delta(exp 17)O (=delta(exp 17)O-0.52xdelta(exp 18)O) values of olivine and pyroxene in these chondrules show a peak at approx.0.7% that are systematically lower than those of bulk chondrule analyses as well as the bulk LL chondrites [2]. Further analyses of glass in Semarkona chondrules show .17O values as high as +5% with highly fractionated d18O (max +18%), implying O-16-poor glass in chondrules were altered as a result of hydration in the parent body at low temperature [3]. Thus, chondrules in LL3.0-3.1 chondrites do not provide any direct evidence of oxygen isotope exchange between solid precursor and O-16-depleted gas during chondrule melting events. To compare the difference and/or similarity between chondrules from LL and H chondrites, we initiated systematic investigations of oxygen isotopes in chondrules from Yamato 793408 (H3.2), one of the least equilibrated H chondrite [4]. In our preliminary study of 4 chondrules, we reported distinct oxygen isotope ratios from dusty olivine and refractory forsterite (RF) grains compared to their host chondrules and confirmed their relict origins [5].
Kita, N. T.↗
Equilibrium Climate Sensitivity Estimated by Equilibrating Climate Models
The methods to quantify equilibrium climate sensitivity are still debated. We collect millennial length simulations of coupled climate models and show that the global mean equilibrium warming is higher than those obtained using extrapolation methods from shorter simulations. Specifically, 27 simulations with 15 climate models forced with a range of CO2 concentrations show a median 17% larger equilibrium warming than estimated from the first 150 years of the simulations. The spatial patterns of radiative feedbacks change continuously, in most regions reducing their tendency to stabilizing the climate. In the equatorial Pacific, however, feedbacks become more stabilizing with time. The global feedback evolution is initially dominated by the tropics, with eventual substantial contributions from the midlatitudes. Time dependent feedbacks underscore the need of a measure of climate sensitivity that accounts for the degree of equilibration, so that models, observations, and paleo proxies can be adequately compared and aggregated to estimate future warming.
Maria Rugenstein↗
A Physical Relationship Between Electron-Proton Temperature Equilibration and Mach Number in Fast Collisionless Shocks
The analysis of Balmer-dominated optical spectra from nonradiative (adiabatic) SNRs has shown that the ratio of the electron to proton temperature at the blast wave is close to unity at v(s) approximate or smaller than 400 km s(-1) but declines sharply down to the minimum value of m(e) /m(p) dictated by the jump conditions at shock speeds exceeding 2000 km s(-1). We propose a physical model for the heating of electrons and ions in non-cosmic-ray-dominated, strong shocks (v(s) 400 km s(-1)) wherein the electrons are heated by lower hybrid waves immediately ahead of the shock front. These waves arise naturally from the cosmic ray pressure gradient upstream from the shock. Our model predicts a nearly constant level of electron heating over a wide range of shock speeds, producing a relationship (T(e)/T(p))sub 0 proportional to v(-2/s) (proportional to M(-2)) that is fully consistent with the observations.
SNR(ONRADIATIVE SUPERNOVA REMNANTS)↗
Ultimate compressive strength and severe plastic deformation of equilibrated single-crystalline copper nanoparticles
Mechanical properties and deformation mechanisms of defect-free copper nanoparticles are investigated by combining experiments with atomistic simulations. The compressive strength of the particles increases with decreasing size and tends to saturate near the theoretical strength in the small-size limit. In this limit, the intrinsic size dependence of the strength is governed by the stochastic nature of dislocation nucleation near the particle surface. The particle deformation process evolves from the initial strain softening to strain hardening as the particle accumulates residual damage. The normalized strength-size relation for Cu is compared with those for Au, Ni, and Pt. The lack of universal behavior among the four FCC metals is discussed. Heavily deformed Cu nanoparticles develop polycrystalline structures and change the lattice orientation from [111] to [110]. The experiments and simulations reveal the twinning mechanism of the lattice rotation leading to the new grain formation.
36 MATERIALS SCIENCE↗
Diffusive equilibration of H[subscript 2]O and oxygen fugacity in natural olivine-hosted melt inclus
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Humphreys, Jennifer↗
Self-equilibrated backstresses induce compensation between hardening and softening: Micromechanical and microstructural features
For crystal plasticity formulation neglecting the strain gradients at the dislocation scale, backstresses are necessary to satisfy the mesoscopic stress equilibrium and strain compatibility requirements. Backstresses in a material point and those between neighboring points are treated separately. Physically backstresses can dynamically induce either hardening or softening effects to adjust the dislocation hardening of slip systems, whereas the sum of the stresses resulting in backstresses in the representative volume element (RVE) is zero. Backstress hardening suppresses certain slip activities, while backstress softening triggers excess dislocations. A two-phase brass is studied. Backstresses make the hard phase deform smaller and the soft phase deform more than those without employing backstresses in a model. In the former, more homogeneous lattice strain and higher yield stress are obtained. Backstress has limited effects on the evolution of the critical resolved shear stress and crystallographic texture. Furthermore, it is a major cause of the banded structures featuring sharp crystallographic contrasts which are not parallel to the slip planes. Tests indicate that backstress induces a serrated behavior at the early stages of deformation and induces the geometrically necessary bands (GNBs), which are parallel to the macroscopic axes.
36 MATERIALS SCIENCE↗
Equilibrated Gas and Carbonate Standard‐Derived Dual (Δ 47 and Δ 48 ) Clumped Isotope Values
Abstract Carbonate clumped isotope geochemistry has primarily focused on mass spectrometric determination of m/z 47 CO 2 for geothermometry, but theoretical calculations and recent experiments indicate paired analysis of the m/z 47 ( 13 C 18 O 16 O) and m/z 48 ( 12 C 18 O 18 O) isotopologues (referred to as Δ 47 and Δ 48 ) can be used to study non‐equilibrium isotope fractionations and refine temperature estimates. We utilize 5,448 Δ 47 and 3,400 Δ 48 replicate measurements of carbonate samples and standards, and 183 Δ 47 and 195 Δ 48 replicate measurements of gas standards from 2015 to 2021 from a multi‐year and multi‐instrument data set to constrain Δ 47 and Δ 48 values for 27 samples and standards, including Devils Hole cave calcite, and study equilibrium Δ 47 ‐Δ 48 , Δ 47 ‐temperature, and Δ 48 ‐temperature relationships. We compare results to previously published findings and calculate equilibrium regressions based on data from multiple laboratories. We report acid digestion fractionation factors, Δ* 63‐47 and Δ* 64‐48 , and account for their dependence on the initial clumped isotope values of the mineral.
58 GEOSCIENCES↗
Uncovering the spin ordering in magic-angle graphene via edge state equilibration
Abstract The flat bands in magic-angle twisted bilayer graphene (MATBG) provide an especially rich arena to investigate interaction-driven ground states. While progress has been made in identifying the correlated insulators and their excitations at commensurate moiré filling factors, the spin-valley polarizations of the topological states that emerge at high magnetic field remain unknown. Here we introduce a technique based on twist-decoupled van der Waals layers that enables measurement of their electronic band structure and–by studying the backscattering between counter-propagating edge states–the determination of the relative spin polarization of their edge modes. We find that the symmetry-broken quantum Hall states that extend from the charge neutrality point in MATBG are spin unpolarized at even integer filling factors. The measurements also indicate that the correlated Chern insulator emerging from half filling of the flat valence band is spin unpolarized and suggest that its conduction band counterpart may be spin polarized.
36 MATERIALS SCIENCE↗