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  • GEOPHYSICS  (7)
  • 1
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    Molten (Mg0.88Fe0.12)2SiO4 at lower mantle conditions - Melting products and structure of quenched glasses (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Infrared spectra of quenched magnesium silicate glasses synthesized by fusing olivine at pressures in excess of 50 GPa and temperatures greater than 2500 K demonstrate that silicon is dominantly present in four-fold coordination with respect to oxygen within these quenched glasses. This low coordination is attributed, by analogy with the structural behavior of glasses compressed at 300 K, to the instability of higher coordinations in glasses of these compositions on decompression. Spectra of glasses formed in a hydrous environment document that water is extensively soluble in melts at these high pressures and temperatures. Also, these results are consistent with the melting of (Mg0.88Fe0.12)2SiO4 compositions to liquids near pyroxene in stoichiometry under these conditions, with iron-rich magnesiowuestite being the liquidus phase.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 17; 635-638
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  • 2
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    Carbonate stability in the earth's mantle - A vibrational spectroscopic study of aragonite and dolomite at high pressures and temperatures (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The structural changes of aragonite and dolomite taking place at high pressures and temperatures were investigated by measuring the Raman spectra of these materials to pressures of 23 and 28 GPa (generated in a diamond anvil cell), respectively; in addition, the IR spectra of aragonite were measured to 40 GPa. The spectroscopic data demonstrated that, at 300 K, dolomite and aragonite samples were stable to pressures of 28 and 41 GPa, respectively. No phase transitions were observed following heating of aragonite and dolomite to temperatures of 2000 K and 800 K, respectively. The mode Grueneisen parameters indicate that the carbonate group in these two minerals is relatively insensitive to pressure, with the dominant compaction mechanism being the compression of the Ca and Mg polyhedra.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 17
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  • 3
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    The high-pressure melting curve of iron - A technical discussion (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The melting curve of iron is reliably determined to 105 GPa using the laser-heated diamond cell, in close agreement with independent measurements using piston-cylinder and large-volume presses or shock-wave experiments. In order to obtain reliable melting data from the internally heated diamond cell, whether by laser or Joule heating, temperature gradients across the sample must be quantitatively measured; otherwise, such 'wire heating' experiments can lead to significant underestimates of the melting temperature and its pressure dependence. The best estimate of the high-pressure melting curve of iron, as derived from the laser-heated diamond cell and Hugoniot temperature measurements, yields melting temperatures of 4800 + or - 200 K and 6700 + or - 400 K at 133 GPa and 243 GPa, respectively.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 2171-218
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  • 4
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    Melting relations in the iron-sulfur system at ultra-high pressures - Implications for the thermal state of the earth (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The melting temperatures of FeS-troilite and of a 10-wt-pct sulfur iron alloy have been measured to pressures of 120 and 90 GPa, respectively. The results document that FeS melts at a temperature of 4100 (+ or - 300) K at the pressure of the core-mantle boundary. Eutecticlike behavior persists in the iron-sulfur system to the highest pressures of measurements, in marked contrast to the solid-solutionlike behavior observed at high pressures in the iron-iron oxide system. Iron with 10-wt-pct sulfur melts at a similar temperature as FeS at core-mantle boundary conditions. If the sole alloying elements of iron within the core are sulfur and oxygen and the outer core is entirely liquid, the minimum temperature at the top of the outer core is 4900 (+ or - 400) K. Calculations of mantle geotherms dictate that there must be a temperature increase of between 1000 and 2000 K across thermal boundary layers within the mantle. If D-double-prime is compositionally stratified, it could accommodate the bulk of this temperature jump.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 95; 19299-19
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  • 5
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    Waveform effects of a metastable olivine tongue in subducting slabs (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Velocity models of subducting slabs with a kinetically-depressed olivine to beta- and gamma-spinel transition are constructed, and the effect that such structures would have on teleseismic P waveforms are examined using a full-wave finite-difference method. These 2D calculations yielded waveforms at a range of distances in the downdip direction. The slab models included a wedge-shaped, low-velocity metastable olivine tongue (MOTO) to a depth of 670 km, as well as a plausible thermal anomaly; one model further included a 10-km-thick fast layer on the surface of the slab. The principal effect of MOTO is to produce grazing reflections at wide angles off the phase boundary, generating a secondary arrival 0 to 4 seconds after the initial arrival depending on the take-off angle. The amplitude and timing of this feature vary with the lateral location of the seismic source within the slab cross-section.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 18; 2201-220
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  • 6
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    The melting curve of iron to 250 gigapascals - A constraint on the temperature at earth's center (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The melting curve of iron, the primary constituent of earth's core, has been measured to pressures of 250 gigapascals with a combination of static and dynamic techniques. The melting temperature of iron at the pressure of the core-mantle boundary (136 GPa) is 4800 + or - 200 K, whereas at the inner core-outer core boundary (330 GPa), it is 7600 + or - 500 K. A melting temperature for iron-rich alloy of 6600 K at the inner core-outer core boundary and a maximum temperature of 6900 K at earth's center are inferred. This latter value is the first experimental upper bound on the temperature at earth's center, and these results imply that the temperature of the lower mantle is significantly less than that of the outer core.
    Keywords: GEOPHYSICS
    Type: Science (ISSN 0036-8075); 236; 181
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  • 7
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    Pressure-induced coordination changes in alkali-germanate melts - An in situ spectroscopic investigation (1992)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: The structure of liquid Na2Ge2O5-H2O, a silicate melt analog, has been studied with Raman spectroscopy to pressures of 2.2 gigapascals. Upon compression, a peak near more than 240 wavenumbers associated with octahedral GeO6 groups grows relative to a peak near 500 wavenumbers associated with tetrahedral GeO4 groups. This change corresponds to an increase in octahedral germanium in the liquid from near 0 percent at ambient pressures to more than 50 percent at a pressure of 2.2 gigapascals. Silicate liquids pausibly undergo similar coordination changes at depth in the earth. Such structural changes may generate decreases in the fusion slopes of silicates at high pressures as well as neutrally buoyant magmas within the transition zone of the earth's mantle.
    Keywords: GEOPHYSICS
    Type: Science (ISSN 0036-8075); 256; 5062,; 1427-143
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