Simon C. Kohn

Structural controls on the solubility of CO2 in silicate melts: Part I: bulk solubility data

Abstract CO 2 solubility data are presented for a wide range of melt compositions in the following systems: SiO 2 –Na 2 O–Al 2 O 3 +CO 2 (SNA) at 2.0 GPa and 1600°C; SNA+CaO (SNAC) and SNA+MgO+CaO (MSNAC) at 1.5 GPa and 1275–1400°C; and for several “natural” magma compositions (Mg- and Ca-rich melilitites, andesite and phonolite) at 1.2–2.7 GPa and 1300–1600°C. At a given pressure and temperature, the solubility is found to be a strong function of the “non-bridging oxygen” (NBO) content of the melt, expressed as the NBO/T ratio, where T represents tetrahedral network-forming cations. The NBO/T ratio, calculated from the melt composition, thus provides a useful parameter for expressing and predicting the CO 2 solubility. In highly polymerised melts, other dissolution mechanisms involving bridging oxygens become important and NBO/T is no longer the exclusive control on solubility. In Fe-bearing systems, the best correlation between CO 2 solubility and NBO/T is found when both Fe 3+ and Fe 2+ are assumed to be tetrahedral (T), indicating that these cations should be considered in a polymerising role in the melt, with respect to CO 2 dissolution. There is also evidence that some fraction of the Mg 2+ in the melt should be assigned to a polymerising role.

Complete resolution of SiOSi and SiOAl fragments in an aluminosilicate glass by 17O multiple quantum magic angle spinning NMR spectroscopy

Abstract Complete NMR resolution of SiOSi and SiOAl species in an aluminosilicate glass (NaAlSi3O8) is shown using 17O multiple quantum magic angle spinning (MAS) NMR and indicates that no AlOAl is present at the detection limit of 0.5%.

Structural controls on the solubility of CO2 in silicate melts: Part II: IR characteristics of carbonate groups in silicate glasses

The characteristics of carbonate groups dissolved in silicate glasses have been investigated using FTIR spectroscopy. Glasses of natural melt compositions are compared with simple analogues. This approach allows systematic investigation of the role of each major oxide component found in the more complex compositions. Only Ca- and Fe2+-bearing systems display the characteristic spectral feature which is dominant in all carbonate-bearing natural compositions, although other cations are important in producing more subtle effects. Spectra of Mg-related carbonate groups suggest the associated Mg is in a network-forming role (T site) and this may explain the lower solubility for MgO-rich compositions noted in Part I. Several spectral features also favour the assignment of Fe2+ as well as Fe3+ to a network-forming role, consistent with the solubility data reported in Part I. As the solubility of CO2 is controlled largely by the availability of non-bridging oxygens (NBO) in depolymerised compositions, this implies that carbonate is more likely to be associated with Ca than with Fe2+ in natural melts.

Slab melting as a barrier to deep carbon subduction.

Interactions between crustal and mantle reservoirs dominate the surface inventory of volatile elements over geological time, moderating atmospheric composition and maintaining a life-supporting planet. While volcanoes expel volatile components into surface reservoirs, subduction of oceanic crust is responsible for replenishment of mantle reservoirs. Many natural, ‘superdeep’ diamonds originating in the deep upper mantle and transition zone host mineral inclusions, indicating an affinity to subducted oceanic crust. Here we show that the majority of slab geotherms will intersect a deep depression along the melting curve of carbonated oceanic crust at depths of approximately 300 to 700 kilometres, creating a barrier to direct carbonate recycling into the deep mantle. Low-degree partial melts are alkaline carbonatites that are highly reactive with reduced ambient mantle, producing diamond. Many inclusions in superdeep diamonds are best explained by carbonate melt–peridotite reaction. A deep carbon barrier may dominate the recycling of carbon in the mantle and contribute to chemical and isotopic heterogeneity of the mantle reservoir.

Fivefold-coordinated aluminum in tectosilicate glasses observed by triple quantum MAS NMR

Abstract Eight glasses with molar Mg/2Al ≈ 1 in the system MgO-Al2O3-SiO2 have been studied by magic angle spinning (MAS) NMR spectroscopy. Using triple quantum (3Q) NMR techniques we find evidence for significant concentrations of Al coordinated to five O atoms in all glasses, the proportion increasing with decreasing Mg/Al and decreasing silica content. In glasses with Mg/2Al = 1, up to 6% of the Al is estimated to be coordinated to five rather than four O atoms. Calculations of the polymerization state of these liquids made assuming that all aluminum is in tetrahedral coordination charge balanced by magnesium are thus seriously in error. Such errors may be of even greater importance at the high temperatures and pressures relevant to the Earth and materials sciences.

Sodium Environments in Dry and Hydrous Albite Glasses: Improved 23Na Solid State NMR Data and Their Implications for Water Dissolution Mechanisms

The sodium environments in albite glasses with water concentrations ranging from 0 to 60 mol% were studied using 23Na off-resonance quadrupole nutation and magic angle spinning (MAS) NMR spectroscopy. Crystalline albite was used as a model compound to demonstrate that off-resonance nutation is a suitable method for determination of the quadrupole coupling constant (Cq) for 23Na. Off-resonance nutation experiments gave a mean Cq = 1.75 ± 0.2 MHz for all the albite glasses studied here. MAS NMR experiments were performed at three magnetic fields, 7.05 T, 9.4 T, and 14.1 T in order to deduce the mean isotropic chemical shift, δiso, and to provide an independent measurement of the values of Cq. The mean isotropic chemical shift is a strong function of dissolved water concentration, but the mean Cq is essentially constant at 2.1-2.2 ± 0.2 MHz over the water concentration range studied. The distributions of both chemical shift and quadrupolar interactions decreases markedly with increasing water concentration, consistent with earlier suggestions that the hydrous glasses have a much more ordered structure. These new data using off-resonance nutation and faster MAS combined with higher applied magnetic fields supersede the 23Na NMR data of Kohn et al. 1989a and should be used in preference in devising or testing models for water dissolution mechanisms in albite melts and glasses. Our revised data provide no evidence for a change in water dissolution mechanism at 30 mol% H2O, but the other conclusions of Kohn et al. 1989a and the principal features of the dissolution mechanism developed by Kohn et al. 1989a, Kohn et al. 1992, Kohn et al. 1994 are essentially unchanged.

Solubility, speciation and dissolution mechanisms for CO2 in melts on the NaAlO2–SiO2 join

CO2 solubility and speciation in melts along the NaAlO2–SiO2 join from Ne (NaAlSiO4) to a model rhyolite (Ry) composition (NaAlSi6O14), have been investigated as a function of pressure (10 to 25 kb) and temperature (1450 to 1700°C). Quenched glasses have been analysed using LECO bulk carbon analysis in conjunction with Fourier transform infrared (FTIR), Raman, and 13C Magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopic measurements. In agreement with previous studies, the CO2 solubility was found to increase as a function of total pressure for the Ne, Jd (NaAlSi2O6), Ab (NaAlSi3O8), and “Eu” (NaAlSi4O10) compositions. The temperature dependence of the CO2 solubility was also investigated. For the Ne (15 kbar) composition, a slight increase in solubility was noted with increasing temperature, whereas a broad minimum in solubility was noted for the Jd composition (15 and 20 kbar) in the 1550 to 1650°C range. No obvious dependence of the solubility on run temperature was noted for the high silica compositions Eu to Ry at 15 kbar. Infrared (IR) and NMR spectra of the quenched glasses show that the type and relative amounts of carbon-bearing species change systematically as a function of composition. The relative and absolute abundance of carbonate (CO32−) groups increases rapidly with decreasing Si/(Na + Al) ratio, whereas the relative and absolute abundance of molecular CO2 decreases. The result is that, for a given pressure and temperature, the CO2 solubility remains approximately constant or decreases slightly with decreasing Si/(Na + Al) ratio between Ry and Jd compositions, but increases rapidly between Jd and Ne compositions, at 15 kbar and 1600°C. In the most silica-rich compositions nearly all the dissolved CO2 is in the form of molecular CO2. The IR and Raman spectra of dissolved molecular CO2 indicate some interaction with the silicate melt structure, which changes over the compositional range studied. Four different types of dissolved carbonate groups with differing degrees of distortion have been identified by NMR spectroscopy, the relative proportions changing systematically with glass composition. Two of these carbonate groups are dominant in silica-poor, carbonate-rich compositions and correlate with distinguishable features in the IR spectra. The structural changes in both the carbonate and the molecular CO2 species as a function of bulk composition along the join, result in changes in the IR extinction coefficients for these species. The degree to which the observed quenched glass species reflect the situation in the melt at run conditions are discussed. Dissolved carbon monoxide (CO) has been identified from the NMR and FTIR spectra of glasses for experiments carried out under (unintentionally) reducing conditions. The experimental results indicate that considerable caution is required in preparing nominally “CO2 -saturated” glasses. The absence of CO-related spectral features can be used to ensure that the experimental PCO2 is in fact equal to Ptot.

CO2 in haplo-phonolite melt: solubility, speciation and carbonate complexation

CO2 solubility was measured in a synthetic iron-free phonolite (haplo-phonolite) by equilibrating melt with excess CO2 fluid in a piston cylinder apparatus for a range of pressures (1.0– 2.5 GPa) and temperatures (1300 to 1550°C). The quenched glasses were then analysed using a bulk carbon analytical method (LECO). The measured solubilities are between 0.65 and 2.77 wt.% for the range of conditions studied and show a negative correlation with temperature as reported for most other silicate melt compositions. A range of carbonate species are present within the glass, as well as minor amounts of molecular CO2. FTIR and NMR analyses suggest that carbonate is present as both ‘network’ and ‘depolymerised’ units as shown for relatively highly polymerised compositions in the model of Brooker et al. (2001b). The bulk CO2 analyses were used to calibrate the IR extinction coefficient for the carbonate groups. However, the results show that the values obtained for the glasses vary with the melt equilibration conditions, presumably because the ratio of the different carbonate species changes as a complex function of run pressure, temperature and quench rate. Thus the use of IR may not be a reliable method for the quantification of dissolved CO2 concentrations in natural glasses of ‘intermediate’ composition.

The dissolution mechanisms of water in silicate melts: a synthesis of recent data

Abstract Dissolved water has significant effects on the physical and chemical properties of silicate melts. Some of the different approaches towards understanding these effects are reviewed here. Spectroscopic measurements on hydrous glasses quenched from melts provide good models for the structure of the melts at the glass transition temperature (Tg). Such measurements suggest that the mechanism of dissolution of water in silicate melts varies strongly with the bulk composition. In particular framework aluminosilicate compositions seem to have very different dissolution mechanisms from Al-free compositions. The water speciation reactions are temperature dependent, with some of the molecular water which is present in glasses at room temperature being converted to hydroxyl at high temperatures. This conversion probably occurs only above Tg. Data on the kinetics of the speciation reaction and the dynamics of microscopic processes in hydrous silicate melts are also discussed. Finally some important aims of future work on hydrous silicate melts are suggested.

Solubility of H 2 O in nominally anhydrous mantle minerals using 1 H MAS NMR

An imaging-plate detector interfaced to a large-volume high-pressure device allows quantitative in situ powder X-ray diffraction measurements at the National Synchrotron Light Source (NSLS). High-quality diffraction patterns were recorded from an (Ni,Mg)2Si04 olivine solid-solution sample at 4 GPa and 800°C as a function of time using 3 min exposures at 40, 63, and 109 min. Refinement of the crystal structure indicated an increase in the ordering of Ni2+ and Mg2+ cations over the Ml and M2 sites at high pressure. The unit-cell volume was found to decrease with increasing ordering. Kinetic phenomena associated with the cation redistribution were observed on the time scale of tens of minutes. Extrapolation based on an exponential law of ordering relaxation toward equilibrium gave a distribution coefficient (KD) of 10.7(1) at 4 GPa and 800°C; the starting sample, which was annealed at 800 °c and room pressure, gave KD = 8.3(1).