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• Na/Mg mixing in aluminosilicates causes non-linear variations in their properties. • Entropy is not modelled by an ideal mixing law but by a sub-regular solution model. • Na and Mg are preferably arranged in sub-networks, reducing structural disorder. • Preferential role-sharing as network modifier and charge compensator between Na/Mg. • Extending sub-regular model and role-sharing to all alkaline/alkaline-earth mixing. This study documents the mixed modifier effect occurring between Na and Mg in aluminosilicate glasses. Rheological, thermodynamic, and structural data were acquired on compositions with 37.5 mol% Na 2 O+MgO, 12.5 mol% Al 2 O 3 and 50 mol% SiO 2. When Na substitutes Mg, melt viscosity decreases non-linearly, as does glass configurational entropy, showing large deviations from the ideal mixing law. This suggests ordering in the glass atomic structure upon Na/Mg mixing, as supported by structural data. Indeed, as Na 2 O/(MgO+Na 2 O) increases, Raman spectra reveal a shift of the 2Q3= Q2+Q4 equilibrium to the left-hand side, while 27Al MAS NMR spectra reveal that the Al coordination tends to 100 % Al[IV]. 23Na MAS NMR data further indicate that the network modifier/charge compensator role of metal cations also changes upon Na/Mg mixing. Those results therefore suggest important changes in melt/glass structure as alkali cations substitute calc-alkaline ones, with important rheological and thermodynamical impacts. [ABSTRACT FROM AUTHOR]

New Raman and NMR spectroscopy data on hydrous Ca aluminosilicate melts and glasses, with eutectic quartz-anorthite-wollastonite composition, are presented here. The glasses were obtained by rapid quench of melts equilibrated at high P and high T in a piston-cylinder apparatus. In situ Raman observations of the structure of the melts were also performed during hydrothermal diamond cell experiments. Using the intensities of the ~860 cm-1 and ~1630 cm-1 Raman signals, respectively assigned to vibrations of T-OH and H2Omol species, we determined the speciation of water in the glasses. T-OH and H2Omol values compare well with those determined from infrared (IR) spectra, except above ~5 wt% total water where IR determinations actually underestimate the proportion of hydroxyl groups. The analysis of the polarized Raman spectra and of the 29Si MAS NMR spectra of the hydrous glasses suggests limited changes in glass polymerization with variations in dissolved water content. However, at high temperatures, in situ Raman spectroscopy observations indicate that the hydrous melt structure differs very strongly from that of a glass containing a comparable concentration of dissolved water. Because of this, this study reinforces the fact that using glass data to try understanding high temperature processes in hydrous melts, like viscous flow or water diffusion toward bubbles during volcanic degassing, may not be very appropriate. [ABSTRACT FROM AUTHOR]

Following an unprecedented seismic activity that started in May 2018, a new volcanic edifice, now called Fani Maoré, was constructed on the ocean floor 50 km east of the island of Mayotte (Indian Ocean). This volcano is the latest addition to a volcanic chain characterized by an alkaline basanite-to-phonolite magmatic differentiation trend. Here, we performed viscosity measurements on five silicate melts representative of the East-Mayotte Volcanic Chain compositional trend: two basanites from Fani Maoré, one tephriphonolite and two phonolites from different parts of the volcanic chain. A concentric cylinder viscometer was employed at super-liquidus conditions between 1500 K and 1855 K and a creep apparatus was used for measuring the viscosity of the undercooled melts close to the glass transition temperature in the air. At super-liquidus temperatures, basanites have the lowest viscosity (0.11–0.34 to 0.99–1.16 log 10 Pa⸱s), phonolites the highest (1.75–1.91 to 3.10–3.89 log 10 Pa⸱s), while the viscosity of the tephriphonolite falls in between (0.89–1.97 log 10 Pa⸱s). Near the glass transition, viscosity measurements were performed for one phonolite melt because obtaining pure glass samples for the basanite and tephriphonolite compositions was unsuccessful. This is due to the formation of nanolites upon quench as evidenced by Raman spectroscopy. The phonolite viscosity ranges from of 10.19 log 10 Pa⸱s at 1058 K to 12.30 log 10 Pa⸱s at 986 K. Comparison with existing empirical models reveals an underestimation of 1.2 to 2.0 log units at super-liquidus and undercooled temperatures, respectively, for the phonolite. This emphasizes (i) the lack of data falling along the alkaline basanite-to-phonolite magmatic differentiation trend to calibrate empirical models, and (ii) the complexity of modeling viscosity variations as a function of temperature and chemical composition for alkaline compositions. The new measurements indicate that, at eruptive temperatures between 1050 °C and 1150 °C (1323–1423 K), the oxidized, anhydrous, crystal-free and bubble-free basanite melt have a viscosity around 2.6 log 10 Pa⸱s. In contrast, the anhydrous phonolite crystal- and bubble-free melt would have a viscosity around 6–10 log 10 Pa⸱s at expected eruptive temperatures, from 800 to 1000 °C (1073–1273 K). Considering that both basanite and phonolite lavas from the Mayotte submarine volcanic chain contain <6% crystals and a significant amount of water (1-2.3 wt% and 0.8-1.2 wt%, respectively), such viscosity values are probably upper limits. The new viscosity measurements are essential to define eruptive models and to better understand the storage and transport dynamics of Comoros Archipelago magmas, and of alkaline magmas in general, from the source to the surface. • Pure liquid viscosity measurements are performed on submarine melts from the basanite - phonolite trend. • Super-liquidus viscosity (log 10 Pas) is 0.11-1.16 for basanite, 0.89 –1.97 for tephriphonolite and 1.75-3.89 for phonolite. • A discrepancy is revealed by comparing experimental measurements and parametric viscosity models. • The effect of water at the eruptive temperatures for such compositions is explored. [ABSTRACT FROM AUTHOR]

The Adam and Gibbs theory depicts the viscous flow of silicate melts as governed by the cooperative re-arrangement of molecular sub-systems. Considering that such subsystems involve the silicate Q n units ( n = number of bridging oxygens), this study presents a model that links the Q n unit fractions to the melt configurational entropy at the glass transition temperature T g , S conf (T g ) , and finally, to its viscosity η . With 13 adjustable parameters, the model reproduces η and T g of melts in the Na 2 O-K 2 O-SiO 2 system (60 ≤ [SiO 2 ] ≤ 100 mol%) with 1σ standard deviations of 0.18 log unit and 10.6°, respectively. The model helps understanding the links between the melt chemical composition, structure, S conf and η . For instance, small compositional changes in highly polymerized melts generate important changes in their S conf (T g ) because of an excess of entropy generated by mixing Si between Q 4 and Q 3 units. Changing the melt silica concentration affects the Q n unit distribution, this resulting in non-linear changes in the topological contribution to S conf (T g ) . The model also indicates that, at [SiO 2 ] ≥ 60 mol%, the mixed alkali effect has negligible impact on the silicate glass Q n unit distribution, as corroborated by Raman spectroscopy data on mixed Na-K tri- and tetrasilicate glasses. Such model may be critical to link the melt structure to its physical and thermodynamic properties, but its refinement requires further high-quality quantitative structural data on silicate and aluminosilicate melts. [ABSTRACT FROM AUTHOR]

Abstract: Viscosity and structure of Na/K tectosilicate glasses containing 75 and 83mol% of SiO2 have been investigated. Viscosity increases non-linearly when K+ ions substitute Na+ ions in these melts. The viscosity variations depending on chemical composition cannot be reproduced using an ideal mixing model of the configurational entropy. Consequently, it appears that Na and K elements do not mix randomly in the studied aluminosilicate melts. Raman spectra of glasses show that, during the Na/K substitution, evolutions of both the ring distribution and the T-O-T mean angle, or force constant, occur. The proportion of small-membered (three, four) rings, compared to large-membered rings, is higher in K-rich glasses than in Na-rich glasses. Moreover, Raman spectra features suggest that two different TO2 environments exist and cohabit into the glass. They could represent two populations of six-membered rings differing by their force constant or their T-O-T angles. One of these environments evolves when K substitutes for Na, showing an increase of its mean T-O-T angle and force constant. The other environment remains unchanged. From the observations, we propose that Na/K mixed tectosilicate glasses contained two sub-networks: one composed of the Si, Al, O and K atoms, and another of the Si, Al, O and Na. We suggest that the different size of alkali elements combined to the charge balancing needs of Al3+ ions can be the source of the different clustering of alkali cations into different sub-networks. Furthermore, and as previously inferred by older studies, potassic tectosilicate glasses could present silica-like and leucite-rich regions, explaining notably the incongruent crystallization of the orthoclase liquid. [Copyright &y& Elsevier]

Raman spectroscopy can measure water concentrations of hydrous silicate glasses with several advantages such as: (1) high-spatial resolution of 1-2 µm²; (2) non-destructive character; and (3) easy access, without any specific sample preparation or mounting techniques. The latter reasons render Ra- man highly suitable for studying natural products, such as volcanic pumice and scoriae fragments. Two spectral regions can be distinguished in Raman spectra of hydrated silicate glasses: a low-wavenumber -1 region (15-1500 cm-1), which corresponds to vibrations of the silicate network, and a high-wavenumber -1 region (3100-3750 cm ), corresponding to the OH stretching vibrations of H2O molecules and OH groups. Behrens et al. (2006) have published empirical equations relating the area ratio between these two regions and the water content. However, the proposed internal calibrations depend on chemical composition of the glasses. In this paper, we reinvestigated the previous procedures to improve the background subtraction. Our results allow us to present a more general and linear calibration. Water concentrations up to 13 wt% can be measured for a broad range of natural silicate melts, from basalts to rhyolite (40 up to 80 wt% SiO2), using a single calibration curve with an absolute error of 0.2 wt%. [ABSTRACT FROM AUTHOR]

• Experimental stick-slip on SiO 2 glass surfaces causes localised frictional melting. • Systematic changes in molecular structure of glass recorded following slip. • Structural changes caused by high pressures and fast cooling rates on asperities. • Thermomechanical history alters future strength and behaviour of interface. New insights into the pressure and temperature conditions on fault surfaces during seismic slip are provided by Raman-active vibrational modes of SiO 2 glass. We performed triaxial stick-slip experiments at room temperature and high normal stresses on pre-ground, high-purity silica glass surfaces. During slip, velocities exceed 0.32 m s−1 over durations of less than one millisecond, generating frictional heat and locally melting the fault surfaces. Temperature increases permit structural rearrangement within the melt; these changes are preserved by rapid quenching. Using Raman spectroscopy, we analyse melt-welded regions and show that these areas exhibit systematic changes in the spectra of silica. Changes result from a decrease in the inter-tetrahedral Si-O-Si bond angle and are correlated to increasing silica glass density in the slip regions. Densification results from both rapid cooling rates and exposure to very high pressures at asperity contacts. We use data from other experiments to calibrate these effects, estimating quench temperatures up to 1800 K and pressures of ∼180 MPa. These results provide the first quantitative evidence for the effects of quench rates and high inter-asperity pressures on the physics of melting and quenching during seismic slip and its impact on fault behaviour. [ABSTRACT FROM AUTHOR]

CO 2 degassing of mafic silicate melts is an important part of the terrestrial carbon cycle, at mid-ocean ridges, oceanic hot spots, or in the middle of continents. Deeper CO 2 -bearing mafic magmas may also exist, such as those suspected in the D″ zone, and certainly existed in the magma ocean of the early Earth. Knowledge of the CO 2 solubility in mafic melts at high pressure is therefore important but unknown at present. Results from molecular dynamics simulation (e.g., Guillot and Sator, 2011) predict that CO 2 solubility in basalt may be much higher than previously thought at pressures and temperatures relevant to the upper mantle. But some recent models predict low solubility at high pressure (e.g., Eguchi and Dasgupta, 2018). The present study thus experimentally investigates the solubility of CO 2 in basalt and andesite in the pressure range 1.5–8.5 GPa at 1820–2130 K in oxidizing conditions. Up to 4 GPa, the quenched samples are essentially vitreous and CO 2 -saturated. Their CO 2 contents are measured using confocal micro-Raman spectroscopy, where we establish an internal calibration line relating CO 2 content to the area of the Raman band assigned to the ν 1 stretching vibration of carbonate groups. This calibration appears independent from the spectrometer, sample or experimentalist, thus enhancing confidence in this method. At 5 and 8.5 GPa, some of the quenched samples are found partially crystallized. Their CO 2 abundance is estimated at micro-scale from Raman mapping, and at large scale from image analysis and presence/absence of vesicles. Over the 1.5–8.5 GPa pressure range, obtained CO 2 concentrations vary between 1.8 and > 13.6 wt%. At 5 GPa and 1873 K, the CO 2 content in basalt and andesite are similar. These findings experimentally confirm the ability of mafic melts to accommodate large amounts of CO 2 at conditions prevailing in the deep Earth. A consequence is that magmas issued from partial melting of carbonate-bearing silicate rocks may ascend with significant quantities of CO 2 of several wt% and more: when reaching shallower depths, they may degas large quantities of CO 2. Present estimates of the global carbon flux to the atmosphere may thus be underestimated, and implications to early magma ocean degassing may be considered. Other consequences may concern the genesis of kimberlites and carbonatites. We finally speculate that, if silicate melts exist in the D″ zone, significant amounts of carbon may be stored there, and consequences may arise as to carbon sequestration in the core. [ABSTRACT FROM AUTHOR]

To better understand the solution of volatile species in a reduced magma ocean, we identify via Raman spectroscopy the nature of C-O-H-N volatile species dissolved in a series of reduced basaltic glasses. The oxygen fugacity (ƒ O2) during synthesis varied from highly reduced at two log units below the iron-wustite buffer (IW-2.1) to moderately reduced (IW-0.4), spanning much of the magmatic ƒ O2 conditions during late stages of terrestrial accretion. Raman vibrational modes for H 2 , NH 2 –, NH 3 , CH 4 , CO, CN–, N 2 , and OH– species are inferred from band assignments in all reduced glasses. The integrated area of Raman bands assigned to N 2 , CH 4 , NH 3 and H 2 vibrations in glasses increases with increasing molar volume of the melt, whereas that of CO decreases. Additionally, with increasing ƒ O2 , CO band areas increase while those of N 2 decrease, suggesting that the solubility of these neutral molecules is not solely determined by the melt molar volume under reduced conditions. Coexisting with these neutral molecules, other species as CN–, NH 2 – and OH– are chemically bonded within the silicate network. The observations indicate that, under reduced conditions, (1) H 2 , NH 2 –, NH 3 , CH 4 , CO, CN–, N 2 , and OH– species coexist in silicate glasses representative of silicate liquids in a magma ocean (2) their relative abundances dissolved in a magma ocean depend on melt composition, ƒ O2 and the availability of H and, (3) metal-silicate partitioning or degassing reactions of those magmatic volatile species must involve changes in melt and vapor speciation, which in turn may influence isotopic fractionation. [ABSTRACT FROM AUTHOR]