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Papers by Wim Malfait

American Mineralogist, 2014

‡ Special collection papers can be found on GSW at http://ammin. geoscienceworld.org/site/misc/sp... more ‡ Special collection papers can be found on GSW at http://ammin. geoscienceworld.org/site/misc/specialissuelist.xhtml.

Canadian Journal of Chemistry, 2015

Recent O1s XPS studies suggest that significant free oxide (O 2or K-O-K) is present in potassium ... more Recent O1s XPS studies suggest that significant free oxide (O 2or K-O-K) is present in potassium silicate glasses, in contrast with what was concluded from 29 Si and 17 O solid-state NMR data. An alternative peak assignment of the Q n peaks in the 29 Si NMR spectra was proposed to bring the 29 Si NMR data in line with the O1s data, but this reassignment (i) is not supported by any evidence other than the resulting agreement between the NMR and XPS data, (ii) is at odds with the spectral properties of the bands (peak position and width), (iii) ignores the strong evidence for the original peak assignment, and (iv) results in highly implausible equilibrium constants for the Q n speciation reactions. More likely causes for the apparent discrepancy between the XPS and NMR data are the incorrect estimation of the precision of the bridging oxygen content from the O1s XPS data, the systematic overestimation of the bridging oxygen content from the O1s XPS data, and (or) the incorrect estimation of the accuracy of the compositional analysis, which was based on the precision rather than the accuracy of the electron microprobe analysis. Thus, the available evidence strongly suggests that the original assignment of the 29 Si NMR spectra is correct. Neither the 29 Si NMR nor the O1s XPS data support the presence of significant amounts of free oxide in potassium silicate glasses with K 2 O/SiO 2 Ͻ Ͻ 2.

The structures of high-pressure magmatic liquids have often been inferred from spectroscopic stud... more The structures of high-pressure magmatic liquids have often been inferred from spectroscopic studies on quenched and decompressed glasses. However, it has not been completely verified whether the structures of quenched and decompressed glasses are representative of the structure of their corresponding liquids at the glass transition temperature and synthesis pressure. Here, we provide quantitative evidence for the retention of pressure-induced configurational changes upon isobaric quench and isothermal decompression for synthesis pressures up to 3.5 GPa. We use the degree of densification and elastic compressibility of permanently densified glasses, together with thermo-elastic data from the literature, to calculate the density of the melt at the glass transition temperature and synthesis pressure. The derived densities agree with those derived directly from the thermal equations of state of the melts. This observation indicates that, at least up to 3.5 GPa, the densified structure of the melt is preserved in the glass upon quenching and decompression; this validates past and future structural studies of high-pressure melts based on studies of quenched and decompressed glasses.

Geochimica et Cosmochimica Acta, 2014

ABSTRACT The hydroxyl speciation of hydrous, metaluminous potassium and calcium aluminosilicate g... more ABSTRACT The hydroxyl speciation of hydrous, metaluminous potassium and calcium aluminosilicate glasses was investigated by 27Al–1H cross polarization and quantitative 1H MAS NMR spectroscopy. Al-OH is present in both the potassium and the calcium aluminosilicate glasses and its 1H NMR partial spectrum was derived from the 27Al–1H cross polarization data. For the calcium aluminosilicate glasses, the abundance of Al-OH could not be determined because of the low spectral resolution. For the potassium aluminosilicate glasses, the fraction of Al-OH was quantified by fitting its partial spectrum to the quantitative 1H NMR spectra. The degree of aluminum avoidance and the relative tendency for Si–O–Si, Si–O–Al and Al–O–Al bonds to hydrolyze were derived from the measured species abundances. Compared to the sodium, lithium and calcium systems, potassium aluminosilicate glasses display a much stronger degree of aluminum avoidance and a stronger tendency for the Al–O–Al linkages to hydrolyze. Combining our results with those for sodium aluminosilicate glasses (Malfait and Xue, 2010a), we predict that the hydroxyl groups in rhyolitic and phonolitic magmas are predominantly present as Si-OH (84–89% and 68–78%, respectively), but with a significant fraction of Al-OH (11–16% and 22–32%, respectively). For both rhyolitic and phonolitic melts, the AlOH/(AlOH + SiOH) ratio is likely smaller than the Al/(Al + Si) ratio for the lower end of the natural temperature range but may approach the Al/(Al + Si) ratio at higher temperatures.

ABSTRACT Silicate melts are essential components of igneous processes and are directly involved i... more ABSTRACT Silicate melts are essential components of igneous processes and are directly involved in differentiation processes and heat transfer within the Earth. Studies of the physical properties of magmas (e.g., density, viscosity, conductivity, etc) are however challenging and experimental data at geologically relevant pressure and temperature conditions remain scarce. For example, there is virtually no data on the density at high pressure of alkaline magmas (e.g., phonolites) typically found in continental rift zone settings. We present in situ density measurements of alkaline magmas at crustal and upper mantle conditions using synchrotron X-ray absorption. Measurements were conducted on ID27 beamline at ESRF using a panoramic Paris-Edinburgh Press (PE Press). The starting material is a synthetic haplo-phonolite glass similar in composition to the Plateau flood phonolites from the Kenya rift [1]. The glass was synthesized at 1673 K and 2.0 GPa in a piston-cylinder apparatus at ETH Zurich and characterized using EPMA, FTIR and density measurements. The sample contains less than 200 ppm water and is free of CO2. Single-crystal diamond cylinders (Øin = 0.5 mm, height = 1 mm) were used as sample containers and placed in an assembly formed by hBN spacers, a graphite heater and a boron epoxy gasket [2]. The density was determined as a function of pressure (1.0 to 3.1 GPa) and temperature (1630-1860 K) from the X-ray absorption contrast at 20 keV between the sample and the diamond capsule. The molten state of the sample during the data collection was confirmed by X-ray diffraction measurements. Pressure and temperature were determined simultaneously from the equation of state of hBN and platinum using the the double isochor method [3].The results are combined with available density data at room conditions to derive the first experimental equation of state (EOS) of phonolitic liquids at crustal and upper mantle conditions. We will compare our results with recent reports of the density of dry and hydrous rhyolite melts (Malfait et al., this meeting) and discuss compositional effects on the density of melts and the implications for magmatic processes in the lower crust and magma chambers. [1] Hay D.E., Wendlandt R.F., 1995, J.Geophys. Res. 100, 401-410. [2] van Kan Parker M. et al., 2010, High Pressure Research, 30: 2, 332 - 341. [3] W.A. Chrichton, M. Mezouar, 2002, High Temp.-High Press, 34, 235.

The Journal of Physical Chemistry C, 2014

ABSTRACT Silica aerogels have exceptional physical and chemical properties related to their nanop... more ABSTRACT Silica aerogels have exceptional physical and chemical properties related to their nanoporous structure and high specific surface area. The hydrophobization of the silica surfaces, for example by modification with trimethylsilyl groups (TMS), is of central importance for silica aerogel production by ambient drying methods, particularly on an industrial scale. This study monitored the chemical modification of silica aerogels by quantitative and two-dimensional solid-state NMR spectroscopy. A series of two-step, acid/base-catalyzed silica alcogels were hydrophobized for different times in hexamethyldisiloxane (HMDSO) and subsequently dried at ambient pressure. Two-dimensional H-1-Si-29 heteronuclear correlation NMR spectroscopy confirms that both ethoxy and TMS groups are chemically bonded to the silica surfaces. For the single-pulse spectra, a procedure to calibrate the absolute H-1, C-13, and Si-29 NMR signal intensities with external references was developed. The quantification procedure is validated by the internal consistency between the H-1, C-13, and Si-29 results and the agreement between the measured sample mass and that predicted from the NMR data. The quantitative speciation data on the aerogel samples show that the silica surface is covered by a monolayer of ethoxy, TMS, and silanol groups. Silanol groups are progressively replaced by TMS groups with increasing modification time, and the TMS content has a strong effect on the density of the final aerogel.

Although halogens are minor volatiles in the Earth's crust, they are important tracers of... more Although halogens are minor volatiles in the Earth's crust, they are important tracers of magmatic and degassing processes and provide insights about subsurface magma movement and eruption likelihood in subduction-related volcanism [1]. Additionally, their ability to complex with other elements has also considerable implications on the formation and distribution of ore deposits [2]. Prerequisite for an efficient use of halogens

Solid State Nuclear Magnetic Resonance, 2010

The Component-Resolved methodology was applied to (1)H spin-echo and (27)Al-(1)H cross polarizati... more The Component-Resolved methodology was applied to (1)H spin-echo and (27)Al-(1)H cross polarization (CP) MAS NMR data of aluminosilicate glasses. The method was able to resolve two components with different T2 relaxation rates, hydroxyl groups (OH) and molecular water (H(2)O(mol)), from the spin-echo data and to determine partial spectra and the relative abundances of OH and H(2)O(mol). The algorithm resolved two to three components with different (27)Al-(1)H CP dynamics from the (27)Al-(1)H cross polarization data; the obtained partial NMR spectra for Al-OH are in excellent agreement with those obtained previously from the difference spectra between spectra with various contact times and confirm previous quantitative results and models for the Al-OH, Si-OH and H(2)O(mol) speciation (Malfait and Xue, 2010).

Journal of Non-Crystalline Solids, 2007

In situ, high-temperature Raman spectroscopy was used to study the Qn speciation in binary potass... more In situ, high-temperature Raman spectroscopy was used to study the Qn speciation in binary potassium silicate melts. Over 300 Raman spectra in the compositional range from 20 to 38mol% K2O were collected at temperatures between 800 and 1200K. Quantitative ...

Journal of Non-Crystalline Solids, 2007

A mathematical approach was developed to interpret Raman spectra of binary silicate glasses and m... more A mathematical approach was developed to interpret Raman spectra of binary silicate glasses and melts without the necessity of external calibration, e.g., from NMR spectroscopy. The developed approach is based on Principal Component Analysis (PCA), linear combinations of partial Raman spectra and a linear optimization technique. In order to apply and to test this approach, we developed an experimental method to collect a large number of Raman spectra efficiently. We applied the quantification and the experimental approaches to investigate potassium silicate glasses with compositions from 17.4 to 38 mol% K 2 O. The equilibrium constant for the reaction 2Q 3 ¢ Q 2 + Q 4 was found log K 3 = À2.37 ± 0.07, in excellent agreement with NMR studies for the same glasses.

Geochimica et Cosmochimica Acta, 2012

The coordination state of the network formers Si and Al is one of the primary parameters describi... more The coordination state of the network formers Si and Al is one of the primary parameters describing the structure of silicate glasses and melts. In this experimental study, we use 27Al MAS NMR spectroscopy to investigate the effect of temperature, pressure and composition on the coordination of Al in dry and hydrous rhyolite and andesite glasses, quenched from pressures of

Geochimica et Cosmochimica Acta, 2007

The structure of 21 binary potassium, rubidium and cesium silicate glasses (in the range 15-50 mo... more The structure of 21 binary potassium, rubidium and cesium silicate glasses (in the range 15-50 mol% alkali oxide) was analyzed by 29 Si single quantum and double quantum MAS NMR spectroscopy. Their glass transition temperatures (T g ) were measured by calorimetry. The chemical shifts and the relative abundance of Q n species correlate with the cationic field strength (Z/r) of the network modifier. A correlation is observed between T g and the inverse of the entropy of mixing of the different Q n species, which is explained in the framework of the Adam-Gibbs relaxation theory. At high alkali content, up to 44% of the SiO 4 tetrahedra are part of three-membered rings. At a given alkali content, the abundance of these rings increases with increasing cation size. The abundance of three-membered rings in K-silicate melts correlates with a temperature and a non-linear composition dependence of the heat capacity. It is also a possible cause for the anomalous volumetric behavior of potassium silicate glasses.

Geochimica et Cosmochimica Acta, 2010

ABSTRACT The combined results of 27Al–1H and 1H–29Si–1H cross polarization NMR experiments for hy... more ABSTRACT The combined results of 27Al–1H and 1H–29Si–1H cross polarization NMR experiments for hydrous glasses (containing 0.5–2wt% water) along the SiO2–NaAlSiO4 join confirm that the dissolution mechanism of water in aluminosilicate glasses is fundamentally the same as for Al-free systems, i.e. the dissolved water ruptures oxygen bridges and creates Si–OH and Al–OH groups, in addition to forming molecular water (H2Omol). The fraction of Al–OH increases non-linearly as the Al content increases with up to half of the OH groups as Al–OH for compositions close to NaAlSiO4. The relative abundances of the different species are controlled by the degree of Al-avoidance and the relative tendency of hydrolysis of the different types of oxygen bridges, Si–O–Si, Si–O–Al and Al–O–Al. A set of homogeneous reactions is derived to model the measured Al–OH/Si–OH speciation, and the obtained equilibrium constants are in agreement with literature data on the degree of Al-avoidance. With these equilibrium constants, the abundance of the different oxygen species, i.e. Si–O–Si, Si–O–Al, Al–O–Al, Si–OH, Al–OH and H2Omol, can be predicted for the entire range of water and Al contents.

Geochimica et Cosmochimica Acta, 2013

Field observations and solubility experiments show evidence for the efficient mobilization of nom... more Field observations and solubility experiments show evidence for the efficient mobilization of nominally insoluble HFSE (i.e., Ti, Zr, Nb and Hf) by high pressure fluids, probably via complexation with polymerized alkali-silica dissolved species and halogens (F and Cl). Here we investigate the complexation of Zr in subduction-related fluids (aqueous fluids and hydrous haplogranite melts) up to 800°C and 2.4 GPa using X-ray absorption spectroscopy (XANES and EXAFS) in a hydrothermal diamond anvil cell and provide evidence for the formation of Zr-O-Si/Na polymeric species in alkali-(alumino)silicate fluids at high pressure. Zr 4+ speciation in dilute fluids (2.5 wt% HCl) is dominated by 8-fold-coordinated [Zr(H 2 O) 8 ] 4+ hydrated complexes at room conditions and no evidence for extensive Zr-Cl complexation in the fluid was found up to 420°C, as confirmed by ab initio XANES calculations of various ZrO 8Àx Cl x clusters. The addition of Na and Si dissolved species (from 35 to 60 wt% dissolved Na 2 Si 2 O 5 , NS2) into the fluid favors the formation of alkali-zirconosilicate clusters Zr-O-Si/Na similar to those found in vlasovite (Na 2 ZrSi 4 O 11 ), with Zr 4+ in octahedral coordination with oxygen (Zr-O distance = 2.09 ± 0.04 Å ) and $6 Si (Na) second neighbors (Zr-Si/Na distance = 3.66 ± 0.06 Å ). This coordination environment also dominates Zr speciation in F-free and F-bearing NS2 and haplogranite glasses and high pressure hydrous haplogranite melts (15.5-33 wt% dissolved H 2 O) in the investigated pressure-temperature range. The XAS analyses, assisted by ab initio XANES calculations, are not conclusive concerning the extent of Zr-F complexation in hydrous granitic melts. Alkali-zirconosilicate Zr-O-Si/Na clusters such as those identified in this study may explain the enhanced solubility of zircon ZrSiO 4 (and other HFSE-bearing minerals) in alkali-aluminosilicate-bearing aqueous fluids produced by dehydration and melting of the slab and provide a favorable mechanism for the mobilization of HFSE in subduction zones. Fluid-rock interactions and/or P/T variations as fluids migrate through the mantle wedge could affect the stability of these complexes, triggering the precipitation of HFSE-bearing accessory phases that are eventually recycled into the mantle, contributing to the dispersion of HFSE. These processes provide a possible explanation for the characteristic HFSE depletion recorded in arc magmas.

Earth and Planetary Science Letters, 2014

Available online xxxx Editor: L. Stixrude Keywords: density silicate melts partial molar volume h... more Available online xxxx Editor: L. Stixrude Keywords: density silicate melts partial molar volume high pressure experiments

Earth and Planetary Science Letters, 2013

Available online xxxx Editor: L. Stixrude Keywords: phonolitic magma density X-ray absorption met... more Available online xxxx Editor: L. Stixrude Keywords: phonolitic magma density X-ray absorption method crystal fractionation time scales

Contributions to Mineralogy and Petrology, 2005

A composition-independent quantitative determination of the water content in silicate glasses and... more A composition-independent quantitative determination of the water content in silicate glasses and silicate melt inclusions by confocal Raman spectroscopy Abstract A new approach was developed to measure the water content of silicate glasses using Raman spectroscopy, which is independent of the glass matrix composition and structure. Contrary to previous studies, the compositional range of our studied silicate glasses was not restricted to rhyolites, but included andesitic, basaltic and phonolitic glasses. We used 21 glasses with known water contents for calibration. To reduce the uncertainties caused by the baseline removal and correct for the influence of the glass composition on the spectra, we developed the following strategy: (1) application of a frequency-dependent intensity correction of the Raman spectra; (2) normalization of the water peak using the broad T-O and T-O-T vibration band at 850-1250 cm )1 wavenumbers (instead of the low wavenumber T-O-T broad band, which appeared to be highly sensitive to the FeO content and the degree of polymerization of the melt); (3) normalization of the integrated Si-O band area by the total number of tetrahedral cations and the position of the band maximum. The calibration line shows a ±0.4 wt% uncertainty at one relative standard deviation in the range of 0.8-9.5 wt% water and a wide range of natural melt compositions. This method provides a simple, quick, broadly available and cost-effective way for a quantitative determination of the water content of silicate glasses. Application to silicate melt inclusions yielded data in good agreement with SIMS data.

Chemical Geology, 2013

Keywords: CO 2 -bearing silicate glasses Partial molar volume Brillouin scattering Compressibilit... more Keywords: CO 2 -bearing silicate glasses Partial molar volume Brillouin scattering Compressibility Shear viscosity of melts Poisson's ratio Carbon dioxide is the second most abundant magmatic volatile and strongly affects the chemical and physical properties of melts. However, the volumetric properties of CO 2 -bearing silicate melts and glasses are still poorly constrained. In this study, the density, acoustic velocities and elastic properties of CO 2 -bearing basaltic, phonolitic and rhyolitic glasses have been determined by the sink/float method and Brillouin scattering spectroscopy to constrain the effect of magma composition on the partial molar volume and compressibility of dissolved CO 2 . The glasses were synthesized at elevated pressure to achieve high CO 2 contents, up to 3.9 wt.%, and subsequently annealed at room pressure to relax the pressure-induced densification. The relaxation times indicate that CO 2 lowers the viscosity of basaltic melts, while that of rhyolitic and phonolitic melts remains unchanged. The partial molar volume of CO 2 depends on the silicate composition with V CO2 = 26.6 ± 1.8, 22.1 ± 0.6 and 25.4 ± 0.9 cm 3 /mol, in basaltic, phonolitic and rhyolitic glasses, respectively, but is not a simple function of the CO 2 / CO 3 2− speciation. In addition, our data indicate a strong compositionally dependent partial molar adiabatic compressibility with βs CO2 = −10 ± 11, 6.7 ± 2.2 and 3.6 ± 0.9 10 −2 GPa −1 for basaltic, phonolitic and rhyolitic glasses, respectively. A plausible compositional dependence of the partial molar volume and compressibility of dissolved CO 2 in magmatic liquids may need to be considered in density models for CO 2 -bearing magmas.

Chemical Geology, 2013

The sound velocities and elastic properties of haplogranitic glasses with excess Na 2 O were dete... more The sound velocities and elastic properties of haplogranitic glasses with excess Na 2 O were determined by Brillouin scattering spectroscopy at ambient conditions. Qualitatively, excess Na 2 O has a similar effect on the elastic properties to that previously reported for water: both lead to an increase of the Poisson's ratio, indicative of a decrease in the degree of polymerization. Thus, as for Na 2 O, water can be considered a network modifier in haplogranitic glasses. A fit to the current and literature data allows the parameterization of the Poisson's ratio as a function of water content and degree of polymerization (NBO/T) for a wide range of geologically relevant composition. This model for the Poisson's ratio, combined with the observed correlation between the melt fragility and Poisson's ratio for our peralkaline granitic glasses, enables the quantitative prediction of the fragility for a broad spectrum of anhydrous melt compositions.

American Mineralogist, 2009

We used configurational entropy theory to model the viscosity (η) of hydrous melts of NaAlSi 3 O ... more We used configurational entropy theory to model the viscosity (η) of hydrous melts of NaAlSi 3 O 8 , haplogranite (SiO 2 -KAlSi 3 O 8 -NaAlSi 3 O 8 ), and complex (natural) granite composition from available measurements and recently published configurational heat-capacity data. The equation log η = A e + B e /TS conf (T), where S conf is configurational entropy, reproduces viscosity data for individual samples as well as or better than the empirical three-parameter TVF equation (defined below), and has the advantage of being based on thermodynamic theory. The variables A e , B e , and S conf (T g ), where T g is glass transition temperature, were parameterized as a function of water content for compilations of viscosity data for hydrous NaAlSi 3 O 8 , haplogranite, and peraluminous granite melts. With the simplest assumption of ideal mixing between silicate and water components, configurational entropy models with between 4 and 10 fitting parameters reproduce experimentally determined η-T-X H2O relationships significantly better than previous literature models based on empirical equations. Our preferred configurational entropy models have root-mean-square deviations of 0.26 log units for NaAlSi 3 O 8 (n = 77), 0.16 log units for haplogranite (n = 55), and 0.28 log units for peraluminous granites (n = 79). The best statistical fits to the data sometimes require thermodynamically unlikely variations in A e , B e , and S conf (T g ) as a function of water content, however, such that further calorimetry data are needed to extract accurate thermodynamic information from viscosity data sets for hydrous melts.

American Mineralogist, 2014

‡ Special collection papers can be found on GSW at http://ammin. geoscienceworld.org/site/misc/sp... more ‡ Special collection papers can be found on GSW at http://ammin. geoscienceworld.org/site/misc/specialissuelist.xhtml.

Canadian Journal of Chemistry, 2015

Recent O1s XPS studies suggest that significant free oxide (O 2or K-O-K) is present in potassium ... more Recent O1s XPS studies suggest that significant free oxide (O 2or K-O-K) is present in potassium silicate glasses, in contrast with what was concluded from 29 Si and 17 O solid-state NMR data. An alternative peak assignment of the Q n peaks in the 29 Si NMR spectra was proposed to bring the 29 Si NMR data in line with the O1s data, but this reassignment (i) is not supported by any evidence other than the resulting agreement between the NMR and XPS data, (ii) is at odds with the spectral properties of the bands (peak position and width), (iii) ignores the strong evidence for the original peak assignment, and (iv) results in highly implausible equilibrium constants for the Q n speciation reactions. More likely causes for the apparent discrepancy between the XPS and NMR data are the incorrect estimation of the precision of the bridging oxygen content from the O1s XPS data, the systematic overestimation of the bridging oxygen content from the O1s XPS data, and (or) the incorrect estimation of the accuracy of the compositional analysis, which was based on the precision rather than the accuracy of the electron microprobe analysis. Thus, the available evidence strongly suggests that the original assignment of the 29 Si NMR spectra is correct. Neither the 29 Si NMR nor the O1s XPS data support the presence of significant amounts of free oxide in potassium silicate glasses with K 2 O/SiO 2 Ͻ Ͻ 2.

The structures of high-pressure magmatic liquids have often been inferred from spectroscopic stud... more The structures of high-pressure magmatic liquids have often been inferred from spectroscopic studies on quenched and decompressed glasses. However, it has not been completely verified whether the structures of quenched and decompressed glasses are representative of the structure of their corresponding liquids at the glass transition temperature and synthesis pressure. Here, we provide quantitative evidence for the retention of pressure-induced configurational changes upon isobaric quench and isothermal decompression for synthesis pressures up to 3.5 GPa. We use the degree of densification and elastic compressibility of permanently densified glasses, together with thermo-elastic data from the literature, to calculate the density of the melt at the glass transition temperature and synthesis pressure. The derived densities agree with those derived directly from the thermal equations of state of the melts. This observation indicates that, at least up to 3.5 GPa, the densified structure of the melt is preserved in the glass upon quenching and decompression; this validates past and future structural studies of high-pressure melts based on studies of quenched and decompressed glasses.

Geochimica et Cosmochimica Acta, 2014

ABSTRACT The hydroxyl speciation of hydrous, metaluminous potassium and calcium aluminosilicate g... more ABSTRACT The hydroxyl speciation of hydrous, metaluminous potassium and calcium aluminosilicate glasses was investigated by 27Al–1H cross polarization and quantitative 1H MAS NMR spectroscopy. Al-OH is present in both the potassium and the calcium aluminosilicate glasses and its 1H NMR partial spectrum was derived from the 27Al–1H cross polarization data. For the calcium aluminosilicate glasses, the abundance of Al-OH could not be determined because of the low spectral resolution. For the potassium aluminosilicate glasses, the fraction of Al-OH was quantified by fitting its partial spectrum to the quantitative 1H NMR spectra. The degree of aluminum avoidance and the relative tendency for Si–O–Si, Si–O–Al and Al–O–Al bonds to hydrolyze were derived from the measured species abundances. Compared to the sodium, lithium and calcium systems, potassium aluminosilicate glasses display a much stronger degree of aluminum avoidance and a stronger tendency for the Al–O–Al linkages to hydrolyze. Combining our results with those for sodium aluminosilicate glasses (Malfait and Xue, 2010a), we predict that the hydroxyl groups in rhyolitic and phonolitic magmas are predominantly present as Si-OH (84–89% and 68–78%, respectively), but with a significant fraction of Al-OH (11–16% and 22–32%, respectively). For both rhyolitic and phonolitic melts, the AlOH/(AlOH + SiOH) ratio is likely smaller than the Al/(Al + Si) ratio for the lower end of the natural temperature range but may approach the Al/(Al + Si) ratio at higher temperatures.

ABSTRACT Silicate melts are essential components of igneous processes and are directly involved i... more ABSTRACT Silicate melts are essential components of igneous processes and are directly involved in differentiation processes and heat transfer within the Earth. Studies of the physical properties of magmas (e.g., density, viscosity, conductivity, etc) are however challenging and experimental data at geologically relevant pressure and temperature conditions remain scarce. For example, there is virtually no data on the density at high pressure of alkaline magmas (e.g., phonolites) typically found in continental rift zone settings. We present in situ density measurements of alkaline magmas at crustal and upper mantle conditions using synchrotron X-ray absorption. Measurements were conducted on ID27 beamline at ESRF using a panoramic Paris-Edinburgh Press (PE Press). The starting material is a synthetic haplo-phonolite glass similar in composition to the Plateau flood phonolites from the Kenya rift [1]. The glass was synthesized at 1673 K and 2.0 GPa in a piston-cylinder apparatus at ETH Zurich and characterized using EPMA, FTIR and density measurements. The sample contains less than 200 ppm water and is free of CO2. Single-crystal diamond cylinders (Øin = 0.5 mm, height = 1 mm) were used as sample containers and placed in an assembly formed by hBN spacers, a graphite heater and a boron epoxy gasket [2]. The density was determined as a function of pressure (1.0 to 3.1 GPa) and temperature (1630-1860 K) from the X-ray absorption contrast at 20 keV between the sample and the diamond capsule. The molten state of the sample during the data collection was confirmed by X-ray diffraction measurements. Pressure and temperature were determined simultaneously from the equation of state of hBN and platinum using the the double isochor method [3].The results are combined with available density data at room conditions to derive the first experimental equation of state (EOS) of phonolitic liquids at crustal and upper mantle conditions. We will compare our results with recent reports of the density of dry and hydrous rhyolite melts (Malfait et al., this meeting) and discuss compositional effects on the density of melts and the implications for magmatic processes in the lower crust and magma chambers. [1] Hay D.E., Wendlandt R.F., 1995, J.Geophys. Res. 100, 401-410. [2] van Kan Parker M. et al., 2010, High Pressure Research, 30: 2, 332 - 341. [3] W.A. Chrichton, M. Mezouar, 2002, High Temp.-High Press, 34, 235.

The Journal of Physical Chemistry C, 2014

ABSTRACT Silica aerogels have exceptional physical and chemical properties related to their nanop... more ABSTRACT Silica aerogels have exceptional physical and chemical properties related to their nanoporous structure and high specific surface area. The hydrophobization of the silica surfaces, for example by modification with trimethylsilyl groups (TMS), is of central importance for silica aerogel production by ambient drying methods, particularly on an industrial scale. This study monitored the chemical modification of silica aerogels by quantitative and two-dimensional solid-state NMR spectroscopy. A series of two-step, acid/base-catalyzed silica alcogels were hydrophobized for different times in hexamethyldisiloxane (HMDSO) and subsequently dried at ambient pressure. Two-dimensional H-1-Si-29 heteronuclear correlation NMR spectroscopy confirms that both ethoxy and TMS groups are chemically bonded to the silica surfaces. For the single-pulse spectra, a procedure to calibrate the absolute H-1, C-13, and Si-29 NMR signal intensities with external references was developed. The quantification procedure is validated by the internal consistency between the H-1, C-13, and Si-29 results and the agreement between the measured sample mass and that predicted from the NMR data. The quantitative speciation data on the aerogel samples show that the silica surface is covered by a monolayer of ethoxy, TMS, and silanol groups. Silanol groups are progressively replaced by TMS groups with increasing modification time, and the TMS content has a strong effect on the density of the final aerogel.

Although halogens are minor volatiles in the Earth's crust, they are important tracers of... more Although halogens are minor volatiles in the Earth's crust, they are important tracers of magmatic and degassing processes and provide insights about subsurface magma movement and eruption likelihood in subduction-related volcanism [1]. Additionally, their ability to complex with other elements has also considerable implications on the formation and distribution of ore deposits [2]. Prerequisite for an efficient use of halogens

Solid State Nuclear Magnetic Resonance, 2010

The Component-Resolved methodology was applied to (1)H spin-echo and (27)Al-(1)H cross polarizati... more The Component-Resolved methodology was applied to (1)H spin-echo and (27)Al-(1)H cross polarization (CP) MAS NMR data of aluminosilicate glasses. The method was able to resolve two components with different T2 relaxation rates, hydroxyl groups (OH) and molecular water (H(2)O(mol)), from the spin-echo data and to determine partial spectra and the relative abundances of OH and H(2)O(mol). The algorithm resolved two to three components with different (27)Al-(1)H CP dynamics from the (27)Al-(1)H cross polarization data; the obtained partial NMR spectra for Al-OH are in excellent agreement with those obtained previously from the difference spectra between spectra with various contact times and confirm previous quantitative results and models for the Al-OH, Si-OH and H(2)O(mol) speciation (Malfait and Xue, 2010).

Journal of Non-Crystalline Solids, 2007

In situ, high-temperature Raman spectroscopy was used to study the Qn speciation in binary potass... more In situ, high-temperature Raman spectroscopy was used to study the Qn speciation in binary potassium silicate melts. Over 300 Raman spectra in the compositional range from 20 to 38mol% K2O were collected at temperatures between 800 and 1200K. Quantitative ...

Journal of Non-Crystalline Solids, 2007

A mathematical approach was developed to interpret Raman spectra of binary silicate glasses and m... more A mathematical approach was developed to interpret Raman spectra of binary silicate glasses and melts without the necessity of external calibration, e.g., from NMR spectroscopy. The developed approach is based on Principal Component Analysis (PCA), linear combinations of partial Raman spectra and a linear optimization technique. In order to apply and to test this approach, we developed an experimental method to collect a large number of Raman spectra efficiently. We applied the quantification and the experimental approaches to investigate potassium silicate glasses with compositions from 17.4 to 38 mol% K 2 O. The equilibrium constant for the reaction 2Q 3 ¢ Q 2 + Q 4 was found log K 3 = À2.37 ± 0.07, in excellent agreement with NMR studies for the same glasses.

Geochimica et Cosmochimica Acta, 2012

The coordination state of the network formers Si and Al is one of the primary parameters describi... more The coordination state of the network formers Si and Al is one of the primary parameters describing the structure of silicate glasses and melts. In this experimental study, we use 27Al MAS NMR spectroscopy to investigate the effect of temperature, pressure and composition on the coordination of Al in dry and hydrous rhyolite and andesite glasses, quenched from pressures of

Geochimica et Cosmochimica Acta, 2007

The structure of 21 binary potassium, rubidium and cesium silicate glasses (in the range 15-50 mo... more The structure of 21 binary potassium, rubidium and cesium silicate glasses (in the range 15-50 mol% alkali oxide) was analyzed by 29 Si single quantum and double quantum MAS NMR spectroscopy. Their glass transition temperatures (T g ) were measured by calorimetry. The chemical shifts and the relative abundance of Q n species correlate with the cationic field strength (Z/r) of the network modifier. A correlation is observed between T g and the inverse of the entropy of mixing of the different Q n species, which is explained in the framework of the Adam-Gibbs relaxation theory. At high alkali content, up to 44% of the SiO 4 tetrahedra are part of three-membered rings. At a given alkali content, the abundance of these rings increases with increasing cation size. The abundance of three-membered rings in K-silicate melts correlates with a temperature and a non-linear composition dependence of the heat capacity. It is also a possible cause for the anomalous volumetric behavior of potassium silicate glasses.

Geochimica et Cosmochimica Acta, 2010

ABSTRACT The combined results of 27Al–1H and 1H–29Si–1H cross polarization NMR experiments for hy... more ABSTRACT The combined results of 27Al–1H and 1H–29Si–1H cross polarization NMR experiments for hydrous glasses (containing 0.5–2wt% water) along the SiO2–NaAlSiO4 join confirm that the dissolution mechanism of water in aluminosilicate glasses is fundamentally the same as for Al-free systems, i.e. the dissolved water ruptures oxygen bridges and creates Si–OH and Al–OH groups, in addition to forming molecular water (H2Omol). The fraction of Al–OH increases non-linearly as the Al content increases with up to half of the OH groups as Al–OH for compositions close to NaAlSiO4. The relative abundances of the different species are controlled by the degree of Al-avoidance and the relative tendency of hydrolysis of the different types of oxygen bridges, Si–O–Si, Si–O–Al and Al–O–Al. A set of homogeneous reactions is derived to model the measured Al–OH/Si–OH speciation, and the obtained equilibrium constants are in agreement with literature data on the degree of Al-avoidance. With these equilibrium constants, the abundance of the different oxygen species, i.e. Si–O–Si, Si–O–Al, Al–O–Al, Si–OH, Al–OH and H2Omol, can be predicted for the entire range of water and Al contents.

Geochimica et Cosmochimica Acta, 2013

Field observations and solubility experiments show evidence for the efficient mobilization of nom... more Field observations and solubility experiments show evidence for the efficient mobilization of nominally insoluble HFSE (i.e., Ti, Zr, Nb and Hf) by high pressure fluids, probably via complexation with polymerized alkali-silica dissolved species and halogens (F and Cl). Here we investigate the complexation of Zr in subduction-related fluids (aqueous fluids and hydrous haplogranite melts) up to 800°C and 2.4 GPa using X-ray absorption spectroscopy (XANES and EXAFS) in a hydrothermal diamond anvil cell and provide evidence for the formation of Zr-O-Si/Na polymeric species in alkali-(alumino)silicate fluids at high pressure. Zr 4+ speciation in dilute fluids (2.5 wt% HCl) is dominated by 8-fold-coordinated [Zr(H 2 O) 8 ] 4+ hydrated complexes at room conditions and no evidence for extensive Zr-Cl complexation in the fluid was found up to 420°C, as confirmed by ab initio XANES calculations of various ZrO 8Àx Cl x clusters. The addition of Na and Si dissolved species (from 35 to 60 wt% dissolved Na 2 Si 2 O 5 , NS2) into the fluid favors the formation of alkali-zirconosilicate clusters Zr-O-Si/Na similar to those found in vlasovite (Na 2 ZrSi 4 O 11 ), with Zr 4+ in octahedral coordination with oxygen (Zr-O distance = 2.09 ± 0.04 Å ) and $6 Si (Na) second neighbors (Zr-Si/Na distance = 3.66 ± 0.06 Å ). This coordination environment also dominates Zr speciation in F-free and F-bearing NS2 and haplogranite glasses and high pressure hydrous haplogranite melts (15.5-33 wt% dissolved H 2 O) in the investigated pressure-temperature range. The XAS analyses, assisted by ab initio XANES calculations, are not conclusive concerning the extent of Zr-F complexation in hydrous granitic melts. Alkali-zirconosilicate Zr-O-Si/Na clusters such as those identified in this study may explain the enhanced solubility of zircon ZrSiO 4 (and other HFSE-bearing minerals) in alkali-aluminosilicate-bearing aqueous fluids produced by dehydration and melting of the slab and provide a favorable mechanism for the mobilization of HFSE in subduction zones. Fluid-rock interactions and/or P/T variations as fluids migrate through the mantle wedge could affect the stability of these complexes, triggering the precipitation of HFSE-bearing accessory phases that are eventually recycled into the mantle, contributing to the dispersion of HFSE. These processes provide a possible explanation for the characteristic HFSE depletion recorded in arc magmas.

Earth and Planetary Science Letters, 2014

Available online xxxx Editor: L. Stixrude Keywords: density silicate melts partial molar volume h... more Available online xxxx Editor: L. Stixrude Keywords: density silicate melts partial molar volume high pressure experiments

Earth and Planetary Science Letters, 2013

Available online xxxx Editor: L. Stixrude Keywords: phonolitic magma density X-ray absorption met... more Available online xxxx Editor: L. Stixrude Keywords: phonolitic magma density X-ray absorption method crystal fractionation time scales

Contributions to Mineralogy and Petrology, 2005

A composition-independent quantitative determination of the water content in silicate glasses and... more A composition-independent quantitative determination of the water content in silicate glasses and silicate melt inclusions by confocal Raman spectroscopy Abstract A new approach was developed to measure the water content of silicate glasses using Raman spectroscopy, which is independent of the glass matrix composition and structure. Contrary to previous studies, the compositional range of our studied silicate glasses was not restricted to rhyolites, but included andesitic, basaltic and phonolitic glasses. We used 21 glasses with known water contents for calibration. To reduce the uncertainties caused by the baseline removal and correct for the influence of the glass composition on the spectra, we developed the following strategy: (1) application of a frequency-dependent intensity correction of the Raman spectra; (2) normalization of the water peak using the broad T-O and T-O-T vibration band at 850-1250 cm )1 wavenumbers (instead of the low wavenumber T-O-T broad band, which appeared to be highly sensitive to the FeO content and the degree of polymerization of the melt); (3) normalization of the integrated Si-O band area by the total number of tetrahedral cations and the position of the band maximum. The calibration line shows a ±0.4 wt% uncertainty at one relative standard deviation in the range of 0.8-9.5 wt% water and a wide range of natural melt compositions. This method provides a simple, quick, broadly available and cost-effective way for a quantitative determination of the water content of silicate glasses. Application to silicate melt inclusions yielded data in good agreement with SIMS data.

Chemical Geology, 2013

Keywords: CO 2 -bearing silicate glasses Partial molar volume Brillouin scattering Compressibilit... more Keywords: CO 2 -bearing silicate glasses Partial molar volume Brillouin scattering Compressibility Shear viscosity of melts Poisson's ratio Carbon dioxide is the second most abundant magmatic volatile and strongly affects the chemical and physical properties of melts. However, the volumetric properties of CO 2 -bearing silicate melts and glasses are still poorly constrained. In this study, the density, acoustic velocities and elastic properties of CO 2 -bearing basaltic, phonolitic and rhyolitic glasses have been determined by the sink/float method and Brillouin scattering spectroscopy to constrain the effect of magma composition on the partial molar volume and compressibility of dissolved CO 2 . The glasses were synthesized at elevated pressure to achieve high CO 2 contents, up to 3.9 wt.%, and subsequently annealed at room pressure to relax the pressure-induced densification. The relaxation times indicate that CO 2 lowers the viscosity of basaltic melts, while that of rhyolitic and phonolitic melts remains unchanged. The partial molar volume of CO 2 depends on the silicate composition with V CO2 = 26.6 ± 1.8, 22.1 ± 0.6 and 25.4 ± 0.9 cm 3 /mol, in basaltic, phonolitic and rhyolitic glasses, respectively, but is not a simple function of the CO 2 / CO 3 2− speciation. In addition, our data indicate a strong compositionally dependent partial molar adiabatic compressibility with βs CO2 = −10 ± 11, 6.7 ± 2.2 and 3.6 ± 0.9 10 −2 GPa −1 for basaltic, phonolitic and rhyolitic glasses, respectively. A plausible compositional dependence of the partial molar volume and compressibility of dissolved CO 2 in magmatic liquids may need to be considered in density models for CO 2 -bearing magmas.

Chemical Geology, 2013

The sound velocities and elastic properties of haplogranitic glasses with excess Na 2 O were dete... more The sound velocities and elastic properties of haplogranitic glasses with excess Na 2 O were determined by Brillouin scattering spectroscopy at ambient conditions. Qualitatively, excess Na 2 O has a similar effect on the elastic properties to that previously reported for water: both lead to an increase of the Poisson's ratio, indicative of a decrease in the degree of polymerization. Thus, as for Na 2 O, water can be considered a network modifier in haplogranitic glasses. A fit to the current and literature data allows the parameterization of the Poisson's ratio as a function of water content and degree of polymerization (NBO/T) for a wide range of geologically relevant composition. This model for the Poisson's ratio, combined with the observed correlation between the melt fragility and Poisson's ratio for our peralkaline granitic glasses, enables the quantitative prediction of the fragility for a broad spectrum of anhydrous melt compositions.

American Mineralogist, 2009

We used configurational entropy theory to model the viscosity (η) of hydrous melts of NaAlSi 3 O ... more We used configurational entropy theory to model the viscosity (η) of hydrous melts of NaAlSi 3 O 8 , haplogranite (SiO 2 -KAlSi 3 O 8 -NaAlSi 3 O 8 ), and complex (natural) granite composition from available measurements and recently published configurational heat-capacity data. The equation log η = A e + B e /TS conf (T), where S conf is configurational entropy, reproduces viscosity data for individual samples as well as or better than the empirical three-parameter TVF equation (defined below), and has the advantage of being based on thermodynamic theory. The variables A e , B e , and S conf (T g ), where T g is glass transition temperature, were parameterized as a function of water content for compilations of viscosity data for hydrous NaAlSi 3 O 8 , haplogranite, and peraluminous granite melts. With the simplest assumption of ideal mixing between silicate and water components, configurational entropy models with between 4 and 10 fitting parameters reproduce experimentally determined η-T-X H2O relationships significantly better than previous literature models based on empirical equations. Our preferred configurational entropy models have root-mean-square deviations of 0.26 log units for NaAlSi 3 O 8 (n = 77), 0.16 log units for haplogranite (n = 55), and 0.28 log units for peraluminous granites (n = 79). The best statistical fits to the data sometimes require thermodynamically unlikely variations in A e , B e , and S conf (T g ) as a function of water content, however, such that further calorimetry data are needed to extract accurate thermodynamic information from viscosity data sets for hydrous melts.