C. Romano

The Viscosities of dry and hydrous XAlSi3O8 (X=Li, Na, K, Ca0.5, Mg0.5) melts

The low-temperature viscosities of dry and hydrous X (X=Li, Na, K, Ca0.5, Mg0.5)AlSi3O8 melts have been investigated. The samples were hydrated via piston cylinder synthesis, and the water contents were subsequently determined by Karl-Fischer titration (KFT) and IR spectroscopy. Both the anhydrous and hydrous viscosities were measured using the micropenetration technique in the range of viscosities between 108.5 to 1011.9 Pa s, at 1 atm pressure and in the temperature ranges of 745–990°C and 400–790°C for the dry and wet melts, respectively. The range of water content varied for all of the samples from 0.70 to 3.13 wt.% H2O. The viscosities of dry melts vary, at fixed temperature, as a complex function of the identity of the cation in the order Li<Na<Ca≤Mg<K. This trend is interpreted as due to the combined effects of cation field strength and (Si, Al) distribution in these melts. With the introduction of water into these melts, the viscosity decreases for all of the compositions investigated. As water is further dissolved, the array of anhydrous viscosities converges into two distinct curves, for alkali-bearing and alkaline-earth-bearing aluminosilicate liquids, respectively. In contrast to the insensitivity of viscosity to alkali cation identity for hydrous melts, the alkali/aluminium ratio remains a sensitive control on viscosity. Thus, the viscosities of a slightly peralkaline albite glass (Naexc) are lower than all of the others, both for the dry and the hydrous systems. We suggest that, in the case of alkaline-earth-bearing melts, an aluminium pair must be closely related to a doubly charged cation, to maintain electrostatic neutrality. The increase in the size of smallest rearranging species, which participates in the viscous flow process, as well as clustering of silica-rich and alumina-rich domains on an “intermediate-range” scale, may be the factors resulting in the higher viscosities of Ca- and Mg-bearing compared to alkali-bearing liquids.

Extremely fluid behavior of Hydrous Peralkaline Rhyolites

The viscosities of a series of water-bearing peralkaline rhyolitic melts have been experimentally determined. The dry melt compositions are composed of a series of additions of Na2O to a metaluminous base composition. The melts, initially hydrated at high pressures and quenched isobarically, have been prepared by cutting and polishing, then reheating across the glass transition at 1 atm where they are annealed to a relaxed metastable state and then investigated dilatometrically using micropenetration methods. The measurements have been performed in the viscosity range of 108.5–1011.5 Pa s which corresponds to temperatures in the range of 675–220°C for these compositions. Despite the relatively low viscosities of dry peralkaline melts in comparison with metaluminous melts of similar SiO2 content, the viscosities of peralkaline rhyolitic melts also decrease strongly and non-linearly with the addition of water. The resulting viscosity–temperature relationships for water-bearing peralkaline rhyolitic melts are shifted to much lower temperatures such that glass transition temperatures for moderate cooling rates correspond to extraordinarily low temperatures. A model is presented for the calculation of melt viscosities in the range of 108.5–1011.5 Pa s for peralkaline rhyolites with up to 7 wt% H2O. The very fluid nature of these peralkaline rhyolites over a wide range of water contents may facilitate a very efficient degassing history of glassy peralkaline rhyolites in nature. Efficient degassing might explain the apparent contradiction of the presence of common water-rich melt inclusions in phenocryst phases hosted in water-free glassy rhyolites, versus the absence of vesicular layers or textural evidence for a vesicular past for the glassy rocks.

Compression Mechanisms in Aluminosilicate Melts: Raman and XANES Spectroscopy of Glasses Quenched from Pressures up to 10 Gpa

Raman and XANES spectroscopy were carried out on a series of glasses of composition 44CaO–12Al2O3–44SiO2, formed at pressures up to 10 GPa by isobaric quench from a temperature of 2200°C. The most significant changes in the Raman spectrum as a function of the synthesis pressure, or density, of the glass occur in the low-frequency region (300–700 cm−1), associated with T–O–T bending vibrations. With increasing density of the glass, the overall intensity at low frequencies decreases relative to the high-frequency portion of the spectrum. Relative intensities of bands within the low-frequency region of the Raman spectrum are also very sensitive to synthesis pressure, whereas there is little evidence that pressure influences Q-speciation as the high-frequency region of the spectrum remains virtually unchanged. With initial compression (V/V0=1–0.96), the severe loss in intensity near 500 cm−1 indicates coordination of bridging oxygen atoms to an additional cation, which inhibits the vibrational motion that gives rise to this band normally observed for silicate glasses formed at ambient pressure. At higher densities (V/V0<0.96), bands in the low-frequency region are shifted to higher frequencies, indicative of narrower T–O–T angles. No significant changes are observed in the Si and Ca K-edge XANES spectra with increasing densification of the glass. The Al K-edge spectra also show no significant changes among the lower density glasses (V/V0=1–0.96), but reveal a feature near 1570 eV that dramatically increases in relative intensity with increasing densification beyond V/V0=0.96. The observations from both Raman and XANES spectroscopy are consistent with two different compression mechanisms operating in different pressure ranges. At lower pressures, the spectroscopic data are characterized by features that we attribute to the presence of triclusters (OT3 units) in the quenched melt. At higher pressures, T–O–T angle reduction and also an increase in the average coordination number of Al are likely to occur to further reduce the volume of the melt. The complex response of the structure of aluminosilicate melts to compression suggests that their physical properties will also behave complexly as a function of pressure.

Effect of aluminum on Ti-coordination in silicate glasses: A XANES study

Abstract The structure of glasses in the K2O-Al2O3-TiO2-SiO2 system was investigated using XANES spectroscopy. Glass samples, synthesized by quenching in air from high temperature fusions, represent the addition of Al2O3 to a base of composition K2TiSi4O11 in amounts corresponding to 0.25, 0.50, 0.75, 1.00, 1.25, and 1.50 mol p.f.u. In the Ti-free system, this range of alkali/aluminum ratios crosses the leucite stoichiometry at 1.0. Si K-edge and Al K-edge spectra indicate tetrahedral environments for these elements, and show no variations related to coordination change as a function of Al content. Changes in the relative intensities of peaks in the Al K-edge, however, suggest variation in the intertetrahedral (T-O-T) angle. We associate the decrease of this angle for the glasses of peraluminous composition with the presence of triclusters of tetrahedra. The pre-edge peak absorption features in the Ti K-edge XANES spectra indicate that the average Ti coordination decreases with the addition of Al2O3. We infer depletion of fivefold-coordinated titanium (possibly as alkali titanyl complexes), which are dominant in the Al-free glass, by the formation of fourfold coordinated Ti and alkali aluminate complexes (up to a concentration of 40% in the most peraluminous glass). Significant amounts of [V]Ti remain present, even at peraluminous compositions, in further support of tricluster formation as a mechanism for Al incorporation.

Hyaloclastite fragmentation below the glass transition: An example from El Barronal submarine volcanic complex (Spain)

This research has been funded by projects CGL2005-03511/BTE, HI2006-0073,PRIN2009 (PRIN 2009H37M59) and PRIN_2010-11 (2010TT22SC and 2010TT22SC_003).

Depth of formation of super-deep diamonds: Raman barometry of CaSiO3-walstromite inclusions

Abstract “Super-deep” diamonds are thought to have a sub-lithospheric origin (i.e., below ~300 km depth) because some of the mineral phases entrapped within them as inclusions are considered to be the products of retrograde transformation from lower-mantle or transition-zone precursors. CaSiO3-walstromite, the most abundant Ca-bearing mineral inclusion found in super-deep diamonds, is believed to derive from CaSiO3-perovskite, which is stable only below ~600 km depth, although its real depth of origin is controversial. The remnant pressure (Pinc) retained by an inclusion, combined with the thermoelastic parameters of the mineral inclusion and the diamond host, allows calculation of the entrapment pressure of the diamond-inclusion pair. Raman spectroscopy, together with X-ray diffraction, is the most commonly used method for measuring the Pinc without damaging the diamond host. In the present study we provide, for the first time, a calibration curve to determine the Pinc of a CaSiO3-walstromite inclusion by means of Raman spectroscopy without breaking the diamond. To do so, we performed high-pressure micro-Raman investigations on a CaSiO3-walstromite crystal under hydrostatic stress conditions within a diamond-anvil cell. We additionally calculated the Raman spectrum of CaSiO3-walstromite by ab initio methods both under hydrostatic and non-hydrostatic stress conditions to avoid misinterpretation of the results caused by the possible presence of deviatoric stresses causing anomalous shift of CaSiO3-walstromite Raman peaks. Last, we applied single-inclusion elastic barometry to estimate the minimum entrapment pressure of a CaSiO3-walstromite inclusion trapped in a natural diamond, which is ~9 GPa (~260 km) at 1800 K. These results suggest that the diamond investigated is certainly sub-lithospheric and endorse the hypothesis that the presence of CaSiO3-walstromite is a strong indication of super-deep origin.

Raman spectroscopy: and insight into the chemical short range order of kerogen for the assessment of thermal maturity

Abstract from 88th Congress of the Italian Geological Society, 2016-09-07 - 2016-09-09, NaplesAbstract from 88th Congress of the Italian Geological Society, 2016-09-07, 2016-09-09, Naplesbook Edited by D. Calcaterra, S. Mazzoli, F.M. Petti, B. Carmina & A. Zuccari doi: 10.3301/ROL.2016.79