Carbonation reactions for olivine-bearing ultramafic rocks at conditions appropriate for the Earth's mantle have been extensively studied over the past three decades (of. review by Luth et al., 1993, and references therein). In these assemblages, the carbonation reactions invariably involve olivine reacting with CO2. For example, carbonation in harzburgites (ol + opx) proceeds via: Enstatite + 2 Magnesite = 2 Forsterite + 2 CO2. In lherzolites (ol + opx + cpx), carbonation would occur initially via: Dolomite + 2 Enstatite = 2 Forsterite + Diopside + 2 CO2, with an exchange reaction between pyroxene and carbonate stabilizing magnesite at the expense of dolomite at higher pressure. Eelogite, a garnet + clinopyroxene rock that lacks olivine, is well-represented in suites of mantle-derived xenoliths, and is inferred to be a significant lithology in the mantle. The above carbonation reactions for olivine-bearing assemblages won't work for these olivine-frce assemblages, and little is known about possible carbonation reactions for these rocks. In a model (Fe-free) eclogite, an assemblage of epx + gar + coes + Mg, Ca carbonate is observed (Luth, unpub, data). A carbonation reaction consistent with this assemblage would be: Dolomite + Coesite = Diopside + CO2 (DCDV). This reaction is of interest aside from as a potential c a r b o n a t i o n react ion for eclogites. With decreasing Co2, DCDV intersects the carbonsilicate-carbonate reaction: Dolomite + Coesite = Diopside + 2 Diamond/Graphite + 02. This reaction may define the coexistence of elemental carbon and carbonate in eclogites and garnet pyroxenites (Luth, 1993). The lower-pressure, quartz-bearing analog of DCDV has been studied in CO2 and H20-CO2 fluids (Eggler et al., 1976; Wyllie et al., 1983; Slaughter et al., 1975; Eggert and Kerrick, 1981; Jacobs and Kerrick, 1981; LtRtge and Metz, 1991, 1993). The DCDV reaction has not been studied experimentally to date. It's location in pressuretemperature space may be calculated with the internally-consistent thermodynamic databases of Holland and Powell (1990) and Berman (1988, 1991). The two calculated curves are discrepant, markedly so at > 3 GPa, possibly resulting from the required extrapolation of the thermodynamic data to these conditions. Dalton and Wood (1993) found that these databases have difficulties reproducing reaction boundaries for carbonatebearing reactions for peridotites, which were attributed to uncertainties in the thermochemical data for carbonates. The experimental determination of the DCDV reaction serves to constrain the stability of carbonate in eclogitic assemblages, and in addition will provide an informative test of the currently-available databases for a carbonation reaction not involving magnesite.
We have studied short-range cation ordering across the diopside (CaMgSi2O6)-Ca-Tschermak pyroxene (CaAI2SiO6) (Di-CaTs) solid solution in samples synthesized at 1400 °C and 2 GPa, for 24 hours. Peak positions in 29Si MAS NMR spectra are sensitive to A1 substitution, both in the cornersharing NN tetrahedral sites on the single chain and in one of the three NN octahedral Ml sites. The substitution of A1 for Mg on Ml causes the 29Si chemical shift to be shielded by about the same magnitude as the deshielding caused by substitution of A1 for Si in NN tetrahedra, causing severe peak overlap among central peaks. Two pairs of the unique local environments have very similar chemical shifts, leaving only four peaks resolved in the spectrum, for which six site assignments have been made.
The melting of forsterite (Mg 2 SiO 4 ) in the presence of H 2 O was studied from 3 to 12 GPa in a multiple‐anvil apparatus to constrain the maximum temperature of the hydrous solidus in peridotitic systems relevant to the Earth's mantle. The solidus has a negative slope to 12 GPa, despite the stabilization of Phase E (a hydrous magnesium silicate) coexisting with forsterite and vapour at the solidus at > 9 GPa. Abundant quench vapour deposits in all the run products indicate extensive solubility of silicate in the vapour; however, there is no direct evidence for incongruent solution in the vapour to 9 GPa. At > 9 GPa, the coexistence of Fo + PhE + V at the solidus requires Mg/Si≠2 in the vapour. Silicate melts and hydrous vapours remain immiscible phases in this system to >12 GPa. Given current estimates of the temperature distribution in the mantle, a hydrous vapour can exist in the asthenospheric mantle only below ∼5 GPa (∼160 km depth). At greater depths, water would be present in a hydrous silicate melt rather than in a free vapour. This depth is a maximum, because solidus temperatures in hydrous peridotite systems would be lower than those in the endmember Mg 2 SiO 4 −H 2 O system. In cooler subducting slabs, a hydrous vapour could persist down to the transition zone.
Other| February 01, 1995 Spectroscopy of the cation distribution in the schorlomite species of garnet Andrew Locock; Andrew Locock University of Alberta, Department of Geology, Edmonton, AB, Canada Search for other works by this author on: GSW Google Scholar Robert W. Luth; Robert W. Luth Search for other works by this author on: GSW Google Scholar Ronald G. Cavell; Ronald G. Cavell Search for other works by this author on: GSW Google Scholar Dorian G. W. Smith; Dorian G. W. Smith Search for other works by this author on: GSW Google Scholar M. John M. Duke M. John M. Duke Search for other works by this author on: GSW Google Scholar American Mineralogist (1995) 80 (1-2): 27–38. https://doi.org/10.2138/am-1995-1-204 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Andrew Locock, Robert W. Luth, Ronald G. Cavell, Dorian G. W. Smith, M. John M. Duke; Spectroscopy of the cation distribution in the schorlomite species of garnet. American Mineralogist 1995;; 80 (1-2): 27–38. doi: https://doi.org/10.2138/am-1995-1-204 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract A homogeneous megacryst of schorlomite was investigated to determine the valence states of Fe and Ti and the crystallographic sites occupied by these elements. The chemical composition of the specimen was analyzed by electron microprobe, wet-chemical analysis, FTIR, and INAA. The results from X-ray absorption near-edge structure spectroscopy (XANES) are consistent with exclusively Ti4+ occupying the octahedral site only. The tetrahedral site is deficient in Si and the results of low-temperature 57Fe Mössbauer spectroscopy indicate that the remainder of the site is occupied by Fe3+ and substantial Fe2+. A spin-allowed intensified crystal-field transition of [4]Fe2+ is present in the near-infrared spectrum. The optical absorption spectrum is dominated by an intense band centered at 500 nm with a full width of 8000 cm−1 at half maximum peak height; this band is assigned to an Fe2+-Ti4+ intervalence charge transfer transition between [4]Fe2+ and [6]Ti. The cation site occupancies in this specimen of schorlomite can be expressed by the following formula: {Ca2.866Mg0.080Na0.038Mn0.019}Σ3.003[Ti1.0584+Fe0.6313+Al0.137Fe0.0572+Mg0.055Zr0.039V0.0143+Mn0.013]Σ2.004(Si2.348Fe0.3393+Fe0.3112+[4H]0.005)Σ3.003O12. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.
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