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61.
62.
Carbonation and decarbonation of eclogites: the role of garnet 总被引:3,自引:0,他引:3
Ruth Knoche Russel J. Sweeney Robert W. Luth 《Contributions to Mineralogy and Petrology》1999,135(4):332-339
Carbonates are potentially significant hosts for primordial and subducted carbon in the Earth's mantle. In addition, the
coexistence of carbonate with silicates and reduced carbon (diamond or graphite), allows constraints to be placed on the oxidation
state of the mantle. Carbonate-silicate-vapor reactions control how carbonate + silicate assemblages may form from carbon-bearing
vapor + silicate assemblages with increasing pressure. In olivine-bearing rocks such as peridotite, considered the dominant
rock type in the upper mantle, the lowest-pressure carbonate-forming reactions involve olivine (±clinopyroxene) reacting with
CO2 (e.g., Wyllie et al. 1983). In eclogitic rocks, the essential mineral assemblage is omphacitic clinopyroxene + garnet, without
olivine. Therefore, alternative carbonate-forming reactions must be sought. The carbonation of clinopyroxene via the reaction
dolomite + 2 coesite = diopside + 2 CO2 was studied experimentally by Luth (1995). The alternative possibility that garnet reacts with CO2 is explored here by determining the location of the reaction 3 magnesite + kyanite + 2 coesite = pyrope + 3 CO2 between 5 and 11 GPa in multi-anvil apparatus. At the temperatures ≥1200 °C, carbonation of eclogitic rocks with increasing
pressure will proceed initially by reaction with clinopyroxene, because the pyrope-carbonation reaction lies at higher pressures
for a given temperature than does the diopside-carbonation reaction. Diluting the pyrope component of garnet and the diopside
component of clinopyroxene to levels appropriate for mantle eclogites does not change this conclusion. At lower temperatures,
appropriate for “cold” slabs, it is possible that the converse situation will hold, with initial carbonation proceeding via
reaction with garnet, but this possibility awaits experimental confirmation. Decarbonation of an eclogite under “normal mantle”
geothermal conditions by a decrease in pressure, as in an ascending limb of a mantle convection cell, would be governed by
the formation of clinopyroxene + CO2. At higher pressure than this reaction, any CO2 produced by the breakdown of magnesite reacting with kyanite and coesite would react with clinopyroxene to produce dolomite + coesite.
Release of CO2 from eclogite into mantle peridotite would form carbonate at sub-solidus conditions and produce a dolomitic carbonate melt
if temperatures are above the peridotite-CO2 solidus.
Received: 4 May 1998 / Accepted: 23 December 1998 相似文献
63.
Lenz Dawn D. DeLuca Edward E. Golub Leon Rosner Robert Bookbinder Jay A. Litwin Christof Reale Fabio Peres Giovanni 《Solar physics》1999,190(1-2):131-138
An initial study of long-lived loops observed with TRACE (Lenz et al., 1999) shows that they have no significant temperature stratification and that they are denser than the classic loop model
predicts. Models that agree better with the observations include a loop consisting of a bundle of filaments at different temperatures
and a loop with momentum input by MHD waves. Some implications for coronal heating models and mechanisms are discussed. 相似文献
64.
The compressibility of -Mns (alabandite) was determined by x-ray analysis using a Mao-Bell type diamond anvil cell. The zero pressure bulk modulus (K0) is 74±2 GPa with the pressure derivative of the bulk modulus (Ko) fixed at four. Allowing (Ko) to vary yielded a statistically better fit with K0 = 88±6 GPa and k0 = 2.2±0.6. Our data combined with the data of McCammon (1991) gave Ko = 73±1 GPa with ko fixed at four. A fit with ko allowed to vary yielded ko = 75±2 GPa and ko = 3.7±0.4. Alabandite transformed from the B1 structure (NaCl-type) to an unknown high-pressure phase at 26 GPa. The high-pressure phase has lower than hexagonal symmetry and it is stable to at least 46±4 GPa.Also affiliated with the James Franck Institute, University of Chicago 相似文献
65.
The changing territorial concentration of migration flows is of interest to many geographers, yet we still do not have a widely accepted index of spatial focus. The much used index of migration efficiency has been shown to be an inadequate index of such spatial concentration, and two candidates have been suggested to replace it: the Gini index and the coefficient of variation. Both are examined in this paper, and a comparative assessment is offered. Data from the 1970, 1980, and 1990 censuses are used to illustrate the two measures. An examination of the findings reveals that the coefficient of variation measure indicates higher levels of spatial focus than does the Gini index for states with highly concentrated flows. 相似文献
66.
Geotechnical properties of carbonate sediments result from several factors such as the particular constituents comprising
the sediment, the mineralogy, fabric, and effective stress. Investigating the effects of increasing effective stress by mechanical
consolidation using permeability, porosity, and porometry determinations reveals fabric changes that could not be determined
in scanning electron micrographs. Porometry changes, determined using an image analyzer, are a function of matrix composition
and grain support. Samples that were predominantly matrix-supported exhibited an increase in pore numbers per unit area and
a pore size decrease resulting from consolidation. Samples composed predominantly of aragonite needle matrices displayed opposite
behavior. 相似文献
67.
Natural abundances of 15N as a source indicator for near-shore marine sedimentary and dissolved nitrogen 总被引:1,自引:0,他引:1
Because organic matter originating in the euphotic zone of the ocean may have a distinctive nitrogen isotope composition (15N/14N), as compared to organic matter originating in terrestrial soils, it may be used to evaluate the relative nitrogen contribution to marine and estuarine sediment. The nitrogen isotope ratios of 42 sediment samples of total nitrogen and 38 dissolved pore-water ammonium samples from Santa Barbara Basin sediment cores were measured. The range of δ15N values for total nitrogen was with a mean of and for pore water ammonium, with a mean of 10.2‰.The results suggest that the dissolved ammonium in the pore water is produced from bacterial degradation of marine organic matter. The range of δ15N values for total nitrogen in the sediment is interpreted as resulting from an admixture of nitrogen derived from marine () and terrestrial (+2‰ marines. The marine component of this mixture, composed principally of calcium carbonate with smaller amounts of opal and organic matter, contains ~ 1.0% nitrogen. The terrestrial component, which comprises over 80% of the sediment, contains ~ 0.1% organically bound nitrogen and accounts for > 25% of the total nitrogen in Santa Barbara Basin sediment. 相似文献
68.
Kevin P. Stephens Peter Fleischer Dawn Lavoie Charlotte Brunner 《Geo-Marine Letters》1997,17(4):299-305
Spatial variability of shallow-water carbonate sediments near Dry Tortugas, Florida, is scale-dependent. Wet bulk density,
grain density, porosity, compressional wave velocity, and grain size variability generally increase down to 2.4 m vertically
and 850 m laterally. Grain size is most variable, followed by porosity, wet bulk density, compressional wave velocity, and
grain density bothvertically and laterally, consistent with Walther’s Law. Variability was empirically modeled by linear regression
analysis to predict variability based on scale, characterize sediment property variability, and quantify sedimentisotropy. 相似文献
69.
70.