Solubility and solution mechanisms of C-O-H volatiles in silicate melt with variable redox conditions and melt composition at upper mantle temperatures and pressures     

Bjorn O. Mysen  Kathryn Kumamoto  Marilyn L. Fogel 《Geochimica et cosmochimica acta》2011,75(20):6183-6199

Solubility and solution mechanisms in silicate melts of oxidized and reduced C-bearing species in the C-O-H system have been determined experimentally at 1.5 GPa and 1400 °C with mass spectrometric, NMR, and Raman spectroscopic methods. The hydrogen fugacity, f_H2, was controlled in the range between that of the iron-wüstite-H₂O (IW) and the magnetite-hematite-H₂O (MH) buffers. The melt polymerization varied between those typical of tholeiitic and andesitic melts.The solubility of oxidized (on the order of 1-2 wt% as C) and reduced carbon (on the order of 0.15-0.35 wt% as C) is positively correlated with the NBO/Si (nonbridging oxygen per silicon) of the melt. At given NBO/Si-value, the solubility of oxidized carbon is 2-4 times greater than under reducing conditions. Oxidized carbon dioxide is dissolved as complexes, whereas the dominant reduced species in melts are CH₃-groups forming bonds with Si⁴⁺ together with molecular CH₄. Formation of complexes results in silicate melt polymerization (decreasing NBO/Si), whereas solution of reduced carbon results in depolymerization of melts (increasing NBO/Si).Redox melting in the Earth’s interior has been explained with the aid of the different solution mechanisms of oxidized and reduced carbon in silicate melts. Further, effects of oxidized and reduced carbon on melt viscosity and on element partitioning between melts and minerals have been evaluated from relationships between melt polymerization and dissolved carbon combined with existing experimental data that link melt properties and melt polymerization. With total carbon contents in the melts on the order of several mol%, mineral/melt element partition coefficients and melt viscosity can change by several tens to several hundred percent with variable redox conditions in the range of the Earth’s deep crust and upper mantle.  相似文献   

Resolution of bridging oxygen signals from O 1s spectra of silicate glasses using XPS: Implications for O and Si speciation     

Kim N. Dalby  H. Wayne Nesbitt  Penelope L. King 《Geochimica et cosmochimica acta》2007,71(17):4297-4313

We used an advanced charge compensation system on an X-ray Photoelectron Spectrometer to yield linewidths from O 1s, Si 2p and Pb 4f spectra of 1.22, 1.35, and 1.10 eV, respectively. These linewidths (eV) are the narrowest obtained for silicate glasses, on any X-ray Photoelectron Spectrometer, to date. The exceptional resolution reveals two O 1s peaks in the PbO-SiO₂ glasses studied. One clearly resolved, high binding energy O 1s peak represents the bridging oxygen signal and the second, lower energy peak represents both non-bridging oxygen and metal-bridging oxygen contributions. These data allow quantification of bridging oxygen contents without detailed deconvolution because both the peak width and intensity are determined solely by the spectral data. The intensity of the bridging oxygen signal decreases systematically with decreased SiO₂ content; however, the measured bridging oxygen abundance is greater than predicted if all Pb atoms in the glass are assumed to be associated with two non-bridging oxygen atoms (i.e., O-Pb-O units). There remains, for example, a significant quantity of bridging oxygen in the glass at the orthosilicate composition (Mol. frac.: 0.67 PbO, 0.33 SiO₂). We demonstrate that bridging oxygen, non-bridging oxygen and metal-bridging oxygen exist at this composition and at all glass compositions studied, including the 0.50 PbO, 0.50 SiO₂ glass. Equilibrium thermodynamic (speciation) calculations indicate that at least three silicate species dominate the glass: a network species (SiO₂), a () monomeric species and a trimeric ring-like species (). With these species, the bridging oxygen contents are accurately modeled in PbO-SiO₂ glasses over the compositional range 0.3 PbO, 0.70 SiO₂ to 0.67 PbO, 0.33 SiO₂, and there is a remarkable agreement between the modeled bridging oxygen and the measured bridging oxygen contents with this study and previous studies. However, we do not intend to imply that the SiO₂, () and () are the only species present in the glass structure. In addition, this study shows that the Si 2p spectrum consists of one peak, fitted with one doublet, which shifts systematically to higher binding energy with increased SiO₂ content. We propose that this shift results from a more intense signal from the networked (more siliceous) species that are located at higher binding energy.  相似文献   

Carbon dioxide in silicate melts: A molecular dynamics simulation study     

Bertrand Guillot  Nicolas Sator 《Geochimica et cosmochimica acta》2011,75(7):1829-1857

We have performed a series of molecular dynamics simulations aimed at the evaluation of the solubility of CO₂ in silicate melts of natural composition (from felsic to ultramafic). In making in contact within the simulation cell a supercritical CO₂ phase with a silicate melt of a given composition, we have been able to evaluate (i) the solubility of CO₂ in the P-T range 1473-2273 K and 20-150 kbar, (ii) the density change experienced by the CO₂-bearing melt, (iii) the respective concentrations of CO₂ and species in the melt, (iv) the lifetime and the diffusivity of these species and (v) the structure of the melt around the carbonate groups. The main results are the following:(1) The solubility of CO₂ increases markedly with the pressure in the three investigated melts (a rhyolite, a mid-ocean ridge basalt and a kimberlite) from about ∼2 wt% CO₂ at 20 kbar to ∼25 wt% at 100 kbar and 2273 K. The solubility is found to be weakly dependent on the melt composition (as far as the present compositions are concerned) and it is only at very high pressure (above ∼100 kbar) that a clear hierarchy between solubilities occurs (rhyolite < MORB < kimberlite). Furthermore at a given pressure the calculated solubility is negatively correlated with the temperature.(2) In CO₂-saturated melts, the proportion of carbonate ions is positively correlated with the pressure at isothermal condition and is negatively correlated with the temperature at isobaric condition (and vice versa for molecular CO₂). Furthermore, at fixed (P, T) conditions the proportion of carbonate ions is higher in CO₂-undersaturated melts than in the CO₂-saturated melt. Although the proportion of molecular CO₂ decreases when the degree of depolymerization of the melt increases, it is still significant in CO₂-saturated basic and ultrabasic compositions at high temperatures. This finding is at variance with experimental data on CO₂-bearing glasses which show no evidence of molecular CO₂ as soon as the degree of depolymerization of the melt is high (e.g. basalt). These conflicting results can be reconciled with each other by noticing that a simple low temperature extrapolation of the simulation data predicts that the proportion of molecular CO₂ in basaltic melts might be negligible in the glass at room temperature.(3) The carbonate ions are found to be transient species in the liquid phase, with a lifetime increasing exponentially with the inverse of the temperature. Contrarily to a usual assumption, the diffusivity of carbonate ions into the liquid silicate is not vanishingly small with respect to that of CO₂ molecules: in MORB they differ from each other by a factor of ∼6 at 1473 K and only a factor of ∼2 at 2273 K. Although the bulk diffusivity of CO₂ is governed primarily by the diffusivity of CO₂ molecules, the carbonate ions contribute significantly to the diffusivity of CO₂ in depolymerized melts.(4) Concerning the structure of the CO₂-bearing silicate melt, the carbonate ions are found to be preferentially associated with NBO’s of the melt, with an affinity for NBOs which exceeds that for BOs by almost one order of magnitude. This result explains why the concentration in carbonate ions is positively correlated with the degree of depolymerization of the melt and diminishes drastically in fully polymerized melts where the number of NBO’s is close to zero. Furthermore, the network modifier cations are not randomly distributed in the close vicinity of carbonate groups but exhibit a preferential ordering which depends at once on the nature of the cation and on the melt composition. However at the high temperatures investigated here, there is no evidence of long lived complexes between carbonate groups and metal cations.  相似文献