Is Iceland a wet spot?

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Glass transition temperatures of natural hydrous melts: a relationship with shear viscosity and implications for the welding process.

Glass transition temperatures (Tg) have been determined for natural multicomponent melts using differential scanning calorimetry. Trachytic, dacitic, phonolitic and basaltic base compositions have been analysed over a range of water contents up to 3.75 wt.%. For each sample Tg has been obtained over a range of cooling/heating rates using the extrapolated onset and the peak temperatures in heat capacity–temperature curves. Tg of all compositions are strongly reduced by increasing water content, particularly for the first 1 wt.% added. Base composition also has an effect, with the lowest Tg occurring in the peralkaline phonolite suite. For all samples a clear dependence on the cooling/heating rate has been recorded. These results have been compared with rheological investigations on the same samples. On the basis of the equivalence of the shear and enthalpic relaxation process timescales we provide a method to predict the shear viscosity at the glass transition for all the melts investigated in this study, both dry and hydrous. Our determinations of Tg provide a lower limit for the time–temperature envelope that gives rise to densely welded deposits and constraints on their emplacement temperature. Furthermore, by using the viscosity values predicted at the glass transition, we suggest that welding processes may occur over timescales on the order of tens of seconds to tens of minutes at Tg

Accounting for the species-dependence of the 3500 cm-1 H2Ot infrared molar absorptivity coefficient: implications for hydrated volcanic glasses

Fourier transform infrared (FTIR) spectroscopy can be used to determine the concentration and speciation of dissolved water in silicate glasses if the molar absorptivity coefficients (e) are known. Samples that are thin and/or water-poor typically require the use of the mid-IR 3500 cm–1 total water (H2Ot) and 1630 cm–1 molecular water (H2Om) absorbance bands, from which hydroxyl water (OH) must be determined by difference; however, accurate determination of H2Ot and OH is complicated because e3500 varies with water speciation, which is not usually known a priori. We derive an equation that uses end-member e3500 values to find accurate H2Ot and OH concentrations from the 3500 cm–1 absorbance for samples where only the H2Om concentration need be known (e.g., from the 1630 cm–1 band). We validate this new species-dependent e3500 method against published data sets and new analyses of glass standards. We use published data to calculate new end-member e3500 values of e3500OH = 79 ± 11 and e3500H2Om = 49 ± 6 L/mol∙cm for Fe-bearing andesite and e3500OH = 76 ± 12 and e3500H2Om = 62 ± 7 L/mol∙cm for Fe-free andesite. These supplement existing end-member values for rhyolite and albite compositions. We demonstrate that accounting for the species-dependence of e3500 is especially important for hydrated samples, which contain excess H2Om, and that accurate measurement of OH concentration, in conjunction with published speciation models, enables reconstruction of original pre-hydration water contents. Although previous studies of hydrous silicate glasses have suggested that values of e decrease with decreasing tetrahedral cation fraction of the glass, this trend is not seen in the four sets of end-member e3500 values presented here. We expect that future FTIR studies that derive end-member e3500 values for additional compositions will therefore not only enable the species-dependent e3500 method to be applied more widely, but will also offer additional insights into the relationship between values of e and glass composition

Viscosity of a molten mantle: insights from a combination of experimental techniques on liquid peridotite

Existence of molten peridotite in the early history of the Earth has long been the subject of debate and conjecture. Interest in the physical properties stems from a number of sources but was re-focussed in the wake of the proposal for the existence of a ¸Smagma ocean ¡T in the evolution of the moon, the Earth and other terrestrial planets. The application of phase equilibrium, buoyancy, thermodynamic and fluid dynamic constraints on the behaviour of molten mantle all rely on adequate characterisation ofthe properties of molten peridotite, largely lacking to date. The viscosity in particular, has received too little attention. A big experimental effort has been provided to obtain the dependence on temperature (T) of viscosity at ambient pressure (P) for the natural peridotite collected at Balmuccia, Italy. High-T measurements were performed by using concentric cylinder (CC). The high-T viscometry was started at 1600_C and proceeded at 10_C intervals, separated by cooling stages at 5_C/min, each one held for 1 hour. No measurements were possible below 1570 _C, because crystallization had occurred. All standard attempts to obtain a homogeneous glass failed. A new technique was therefore used. Small 1-2 mm chips were hung in Pt loops suspended from a long Pt wire and the loops lowered by hand into the high-T viscosity furnace until the chips fused into a bead of liquid held in the loop by surface tension. These samples were then left to quench and placed aside to be used in the splat-quenching device (SQD) (which allows quench rates on the order of 10exp4 _C/s) to finallyobtain a supercooled liquids by squeezing and rapidly quenching a falling liquid drop, through a joint action of a complex photoelectric-driven electromagnetic device. Electron microprobe analysis revealed that only a few vol% of the obtained glasses crystallized in isochemical crystals, whereas the homogeneity of the glassy matrix composition was found to be excellent. As the amount of glass obtained was too small to be used in the micropenetration technique we used differential scanning calorimetry (DSC) to derive the viscosity at low-T. DSC allowed us to unequivocally determine glass transition temperatures (Tg) for cooling/heating rates of 20, 15, 10, 8 and 5 K/min, as the peak of the Cp curves. At this point we used a recent method developed by [1] that, on the basis of the equivalence of the shear stress and the enthalpic relaxation time, allow to predict the low-T viscosity. The combined results obtained by using the different techniques above mentioned were fit by VFT equation with the high-T limiting value (viscosity value at infinite temperature) being fixed at a value of 10exp-4.31 Pas [2]. A comparison between the data obtained here with the recent model from [3] (calibrated with melts as basic as basanite), have shown that in the range 900 to 1600 _C, the viscosity calculated according to [3] is very similar to those measured or calculated by the VFT fit, if A = -4.31; the discrepancy becoming significant at T<900 _C. The very low T dependence of viscosity at superliquidus conditions obtained from the fitting here, indicates that at putative temperatures of the core-mantle boundary, near 5000 _C, the viscosity will decrease up to 10exp-3.5 Pa s. [1] J. Gottsmann et al. (2002), EPSL, 198, 417;. [2] J.K. Russell et al. (2003), Am. Mineral., 88, 1390;. [3] D. Giordano & D.B. Dingwell (2003), EPSL, 208, 337 and Errata Corrige, in press

The influence of H2O and CO2 on the glass transition temperature : insight into the effects of volatiles on magma viscosity

International audienc

Low degree melting under the Southwest Indian Ridge: the roles of mantle temperature, conductive cooling and wet melting

Both low mantle temperatures and conductive cooling have been suggested as the cause of the atypically thin oceanic crust and the incompatible element enrichment characteristic of very slow-spreading ridges. Here we present a model of melting under the Southwest Indian Ridge, which takes into account mantle temperature, conductive cooling, source composition and wet melting. The model parameters are constrained by oceanic crustal thickness, lava chemistry and isotopic composition and water content. The results suggest that conductive cooling to a depth of around 20 km, expected in areas with a full spreading rate of 15 mm/yr, is necessary to generate the Southwest Indian Ridge lava chemistry, but not that from faster spreading rate ridges at 23°N on the Mid Atlantic Ridge or 45°N on the Juan de Fuca Ridge. The mantle potential temperatures of ~1280°C, estimated for the Southwest Indian Ridge lavas are close to the global average of the upper mantle. Mantle water contents of 150-300 ppm can explain the observed melt water contents and allow sufficient melting at depth to explain the observed heavy rare earth element depletions in the melts

The influence of H2O and CO2 on the glass transition temperature : insight into the effects of volatiles on magma viscosity

International audienc

The influence of CO2 and H2O on the glass transition in synthetic phonolite and jadeite

International audienc

Heat capacity of hydrous trachybasalt from Mt Etna: comparison with CaAl2Si2O8 (An)–CaMgSi2O6 (Di) as basaltic proxy compositions

The specific heat capacity (Cp) of six variably hydrated (~3.5 wt% H2O) iron-bearing Etna trachybasaltic glasses and liquids has been measured using differential scanning calorimetry from room temperature across the glass transition region. These data are compared to heat capacity measurements on thirteen melt compositions in the iron-free anorthite (An)–diopside (Di) system over a similar range of H2O contents. These data extend considerably the published Cp measurements for hydrous melts and glasses. The results for the Etna trachybasalts show nonlinear variations in, both, the heat capacity of the glass at the onset of the glass transition (i.e., Cpg) and the fully relaxed liquid (i.e., Cpl) with increasing H2O content. Similarly, the “configurational heat capacity” (i.e., Cpc = Cpl − Cpg) varies nonlinearly with H2O content. The An–Di hydrous compositions investigated show similar trends, with Cp values varying as a function of melt composition and H2O content. The results show that values in hydrous Cpg, Cpl and Cpc in the depolymerized glasses and liquids are substantially different from those observed for more polymerized hydrous albitic, leucogranitic, trachytic and phonolitic multicomponent compositions previously investigated. Polymerized melts have lower Cpl and Cpc and higher Cpg with respect to more depolymerized compositions. The covariation between Cp values and the degree of polymerization in glasses and melts is well described in terms of SMhydrous and NBO/Thydrous. Values of Cpc increase sharply with increasing depolymerization up to SMhydrous ~ 30–35 mol% (NBO/Thydrous ~ 0.5) and then stabilize to an almost constant value. The partial molar heat capacity of H2O for both glasses (Cgp H2O) and liquids (C1p H2O) appears to be independent of composition and, assuming ideal mixing, we obtain a value for C1p H2O of 79 J mol−1 K−1. However, we note that a range of values for C1p H2O (i.e., ~78–87 J mol−1 K−1) proposed by previous workers will reproduce the extended data to within experimental uncertainty. Our analysis supgests that more data are required in order to ascribe a compositional dependence (i.e., nonideal mixing) to C1p H2Ol

Distribution of dissolved water in magmatic glass records growth and resorption of bubbles

Volcanic eruptions are driven by the growth of gas bubbles in magma. Bubbles grow when dissolved volatile species, principally water, diffuse through the silicate melt and exsolve at the bubble wall. On rapid cooling, the melt quenches to glass, preserving the spatial distribution of water concentration around the bubbles (now vesicles), offering a window into pre-eruptive conditions. We measure the water distribution around vesicles in experimentally-vesiculated samples, with high spatial resolution. We find that, contrary to expectation, water concentration increases towards vesicles, indicating that water is resorbed from bubbles during cooling; textural evidence suggests that resorption occurs largely before the melt solidifies. Speciation data indicate that the molecular water distribution records resorption, whilst the hydroxyl distribution records earlier decompressive growth. Our results challenge the emerging paradigm that resorption indicates fluctuating pressure conditions, and lay the foundations for a new tool for reconstructing the eruptive history of natural volcanic products