176 research outputs found
How does viscosity contrast influence phase mixing and strain localization?
Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 125 (2020): e2020JB020323, doi: 10.1029/2020JB020323.Ultramylonitesâintensely deformed rocks with fine grain sizes and wellâmixed mineral phasesâare thought to be a key component of Earthâlike plate tectonics, because coupled phase mixing and grain boundary pinning enable rocks to deform by grainâsizeâsensitive, selfâsoftening creep mechanisms over long geologic timescales. In isoviscous twoâphase composites, âgeometricâ phase mixing occurs via the sequential formation, attenuation (stretching), and disaggregation of compositional layering. However, the effects of viscosity contrast on the mechanisms and timescales for geometric mixing are poorly understood. Here, we describe a series of highâstrain torsion experiments on nonisoviscous calciteâfluorite composites (viscosity contrast, ηca/ηfl â 200) at 500°C, 0.75 GPa confining pressure, and 10â6â10â4 sâ1 shear strain rate. At low to intermediate shear strains (Îł †10), polycrystalline domains of the individual phases become sheared and form compositional layering. As layering develops, strain localizes into the weaker phase, fluorite. Strain partitioning impedes mixing by reducing the rate at which the stronger (calcite) layers deform, attenuate, and disaggregate. Even at very large shear strains (Îł â„ 50), grainâscale mixing is limited, and thick compositional layers are preserved. Our experiments (1) demonstrate that viscosity contrasts impede mechanical phase mixing and (2) highlight the relative inefficiency of mechanical mixing. Nevertheless, by employing laboratory flow laws, we show that âidealâ conditions for mechanical phase mixing may be found in the wet middle to lower continental crust and in the dry mantle lithosphere, where quartzâfeldspar and olivineâpyroxene viscosity contrasts are minimized, respectively.This work was funded through a National Science Foundation grant (EARâ1352306) awarded to P. S., with additional support for A. J. C. provided by the McDonnell Center for the Space Sciences (Washington University in St. Louis), the J. Lamar Worzel Assistant Scientist Fund (WHOI), and the Penzance Endowed Fund in Support of Assistant Scientists (WHOI). Partial support for electron microscopy was provided by the Institute of Materials Science and Engineering (Washington University in St. Louis).2021-02-0
The First Spectrum of the Coldest Brown Dwarf
The recently discovered brown dwarf WISE 0855 presents our first opportunity
to directly study an object outside the Solar System that is nearly as cold as
our own gas giant planets. However the traditional methodology for
characterizing brown dwarfs---near infrared spectroscopy---is not currently
feasible as WISE 0855 is too cold and faint. To characterize this frozen
extrasolar world we obtained a 4.5-5.2 m spectrum, the same bandpass long
used to study Jupiter's deep thermal emission. Our spectrum reveals the
presence of atmospheric water vapor and clouds, with an absorption profile that
is strikingly similar to Jupiter. The spectrum is high enough quality to allow
the investigation of dynamical and chemical processes that have long been
studied in Jupiter's atmosphere, but now on an extrasolar world.Comment: submitted to ApJ
Sirius B Imaged in the Mid-Infrared: No Evidence for a Remnant Planetary System
Evidence is building that remnants of solar systems might orbit a large
percentage of white dwarfs, as the polluted atmospheres of DAZ and DBZ white
dwarfs indicate the very recent accretion of metal-rich material. (Zuckerman et
al. 2010). Some of these polluted white dwarfs are found to have large
mid-infrared excesses from close-in debris disks that are thought to be
reservoirs for the metal accretion. These systems are coined DAZd white dwarfs
(von Hippel et al. 2007). Here we investigate the claims of Bonnet-Bidaud &
Pantin (2008) that Sirius B, the nearest white dwarf to the Sun, might have an
infrared excess from a dusty debris disk. Sirius B's companion, Sirius A is
commonly observed as a mid-infrared photometric standard in the Southern
hemisphere. We combine several years of Gemini/T-ReCS photometric standard
observations to produce deep mid-infrared imaging in five ~10 micron filters
(broad N + 4 narrowband), which reveal the presence of Sirius B. Our photometry
is consistent with the expected photospheric emission such that we constrain
any mid-infrared excess to <10% of the photosphere. Thus we conclude that
Sirius B does not have a large dusty disk, as seen in DAZd white dwarfs.Comment: 13 pages, 3 figures, accepted to Ap
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