Description: The adsorption of pure sulfur dioxide gas on volcanic ashes has been studied from 0.1 mbar to 1 bar and from –80 to +150 °C. Finely ground synthetic glasses of andesitic, dacitic, and rhyolitic composition served as proxies for fresh natural ash surfaces. Powders from two natural obsidian samples were also studied and yielded results broadly similar to the synthetic model systems. SO 2 adsorption on ash is partially irreversible; it appears that the first layer of SO 2 molecules absorbed on the surface cannot be removed. The pressure and temperature dependence of adsorption can be described by the equation ln c = A T –1 + B ln P + C, where c is the surface concentration of adsorbed SO 2 in mg/m 2 , T is temperature in Kelvin, and P is the partial pressure of SO 2 in mbar. A multiple regression analysis of the experimental data yielded A = 1645, B = 0.29, C = –7.43 for andesite, A = 2140, B = 0.29, C = –9.32 for dacite, and A = 910, B = 0.21, C = –4.48 for rhyolite. These data imply that adsorption strongly decreases with temperature, but only slowly decreases with decreasing partial pressure of SO 2 . Therefore, adsorption primarily occurs in the cool and diluted parts of the volcanic plume. Model calculations show that most of the SO 2 may be removed from the plume, if the initial SO 2 concentration in the volcanic gases is low and the surface area of the ash is high. For high initial SO 2 concentrations, the fraction of SO 2 that is lost by adsorption decreases, since the amount of SO 2 in the gas phase is proportional to the SO 2 partial pressure P , while adsorption is proportional only to P 0.2 – P 0.3 . Adsorption of SO 2 on ash particles may therefore be one reason why the climatic impact of explosive volcanic eruptions does not always scale with total sulfur yield.

Description: The distribution of sulfur in the system Na 2 O-K 2 O-CaO-Al 2 O 3 -SiO 2 -H 2 O-S was investigated at 2 kbar, 750–950 °C, and oxygen fugacities ranging from the Co-CoO to the Re-ReO 2 buffer. Anhydrite (CaSO 4 ) crystallized in all the experiments conducted at oxidizing conditions (Ni-NiO + 0.5 to 1 and above). Under otherwise equal conditions, an inverse relationship between sulfur and CaO concentration was observed in melts coexisting with anhydrite, with the solubility product K = [CaO][SO 3 ] being constant, where [CaO] and [SO 3 ] are the molar fraction of CaO and SO 3 in the quenched glasses. This suggests that anhydrite dissociates upon dissolution in the melt according to CaSO 4 anhydrite = Ca 2+ melt + SO 4 2– melt . The solubility product strongly depends on temperature, with lnK = – (28 573 ± 917)/ T + (11.26 ± 0.80). This corresponds to an enthalpy of dissolution of H R = 237.5 ± 7.6 kJ/mol. Under reducing conditions (Co-CoO and Ni-NiO buffer), CaO has no effect on the fluid/melt partition coefficient of sulfur D s fluid/melt . At 850 °C, 2 kbar partition coefficients were 519 ± 30 at the Ni-NiO buffer and 516 ± 11 at the Co-CoO buffer, for CaO contents in the melt up to 1 wt%. These data are virtually identical to those measured in the CaO-free haplogranite system under reducing conditions. However, under more oxidizing conditions, the fluid/melt partition coefficient of sulfur appeared to have increased somewhat in the presence of CaO. This increase may, however, also be related to the fact that the final melt compositions in these runs were distinctly peraluminous. Our data show that calcium has no effect on the degassing of sulfur at reducing conditions, but it greatly reduces the amount of sulfur available for rapid degassing under oxidizing conditions by stabilizing anhydrite.

Description: We present a new hydrothermal moissanite cell for in situ experiments at pressures up to 1000 bar and temperature to 850 °C. The original moissanite cell presented by Schiavi et al. (2010) was redesigned to allow precise control of fluid pressure. The new device consists of a cylindrical sample chamber drilled into a bulk piece of NIMONIC 105 super alloy, which is connected through a capillary to an external pressure control system. Sealing is provided by two gold gasket rings between the moissanite windows and the sample chamber. The new technique allows the direct observation of various phenomena, such as bubble nucleation, bubble growth, crystal growth, and crystal dissolution in silicate melts, at accurately controlled rates of heating, cooling, and compression or decompression. Several pilot experiments on bubble nucleation and growth at temperature of 715 °C and under variable pressure regimes (pressure oscillations between 500 and 1000 bar and decompression from 800 to 200 bar at variable decompression rates) were conducted using a haplogranitic glass as starting material. Bubble nucleation occurs in a short single event upon heating of the melt above the glass transition temperature and upon decompression, but only during the first 100 bar of decompression. New bubbles nucleate only at a distance from existing bubbles larger than the mean diffusive path of water in the melt. Bubbles expand and shrink instantaneously in response to any pressure change. The bubble-bubble contact induced during pressure cycling and decompression does not favor bubble coalescence, which is never observed at contact times shorter than 60 s. However, repeated pressure changes favor the diffusive coarsening of larger bubbles at the expense of the smaller ones (Ostwald ripening). Experiments with the haplogranite show that, under the most favorable conditions of volatile supersaturation (as imposed by the experiment), highly viscous melts are likely to maintain the packing of bubbles for longer time before fragmentation. In-situ observations with the new hydrothermal moissanite cell allow to carefully assess the conditions of bubble nucleation, eliminating the uncertainty given by the post mortem observation of samples run using conventional experimental techniques.

Description: The sulfur compounds released by volcanic eruptions, generally believed to be in the form of SO 2 and H 2 S, may cause global cooling of the atmosphere. However, several recent field and experimental studies suggested that under moderately oxidized conditions hexavalent sulfur species may coexist with SO 2 in magmatic fluids and may later be directly emitted at volcanic vents, which contradicts some thermodynamic predictions. We have investigated sulfur speciation in magmatic-hydrothermal fluids by loading different amounts of dilute sulfuric acid into a hydrothermal diamond-anvil cell and performing in situ Raman spectroscopy at temperatures up to 700 °C. Upon heating SO 4 2– disappeared beyond 100 °C, and SO 2 formed at 〉250 °C probably due to reduction by the rhenium or iridium gasket. With high-fluid densities (such as 〉0.9 g/mL), the initial acid and air bubble homogenized into the liquid phase and most sulfur was present in the form of either HSO 4 – or H 2 SO 4 (the rest being SO 2 ) within investigated T – P conditions (with pressures up to 10 kb). With low-fluid densities (such as 〈0.2 g/mL), the system homogenized into the vapor phase and molecular H 2 SO 4 appeared to dominate (with pressures less than 1 kb). These observations strongly suggest that hexavalent sulfur is stabilized by hydration in magmatic fluids.

Description: The infrared spectra of hydroxyl in synthetic hydrous wadsleyite (β-Mg 2 SiO 4 ) and ringwoodite (-Mg 2 SiO 4 ) were measured at room temperature up to ~18.8 GPa for wadsleyite and up to ~21.5 GPa for ringwoodite. High-temperature spectra were measured in an externally heated diamond-anvil cell up to 650 °C at ~14.2 GPa for wadsleyite and up to 900 °C at ~18.4 GPa for ringwoodite. The synthetic samples reproduce nearly all the important OH bands previously observed at ambient conditions. Only subtle changes were observed in the infrared spectra of both minerals, both upon compression at room temperature and upon heating at high pressure. For wadsleyite, upon compression to ~18.8 GPa, the frequencies of the bands at ~3600 cm –1 remain almost unchanged, while the main bands at 3200–3400 cm –1 shift to lower frequencies. During heating at 14.2 GPa to 650 °C the bands at 3200–3400 cm –1 broaden and shift to slightly lower frequencies. For ringwoodite, upon compression to ~21.5 GPa, the main bands at 3115 cm –1 progressively shift to lower frequencies. During heating at 18.4 GPa to 900 °C, no frequency shift was observed for the band at ~3700 cm –1 , but the band initially at ~3115 cm –1 shifts very slightly to higher frequencies, which should yield almost the same band positions at 1300–1400 °C as those measured at ambient conditions. Our data suggest that water speciation in hydrous wadsleyite and ringwoodite at ambient conditions may be comparable to that under mantle conditions, except perhaps for subtle changes in hydrogen bonding. The low OH-stretching frequencies in wadsleyite and ringwoodite under transition zone conditions imply a large H/D fractionation during degassing of the deep mantle. This may explain the apparent disequilibrium between the hydrogen isotopic composition of the upper mantle and the ocean.

Description: We have studied the speciation of carbon monoxide in both Fe-bearing and Fe-free basaltic glasses using Raman, FTIR, and Mössbauer spectroscopy. We show that a band at 2110 cm –1 in the Raman spectrum and another band at 2210 cm –1 in the FTIR spectrum occur both in the Fe-bearing and Fe-free samples, implying that they cannot be due to any Fe-bearing species. This observation is consistent with 57 Fe Mössbauer spectra, which do not show any evidence for Fe species with zero isomer shift, as expected for carbonyls. Thermodynamic calculations show that iron carbonyl in basaltic melts under crustal and upper mantle conditions may only be a trace species. Rather than being due to distinct chemical species, the range of vibrational frequencies observed for carbon monoxide in silicate glasses appears to be due to rather subtle interactions of the CO molecule with the matrix. Similar effects are known from the extensive literature on carbon monoxide adsorption on oxides and other surfaces. In the melt at high temperature, there is likely little interaction of the CO molecule with the silicate matrix and solubility may be largely controlled by pressure, temperature, and the overall polymerization or ionic porosity of the melt.

Description: Melt inclusion data from primitive arc basalts from Mexico and Kamchatka show clear positive correlations of "fluid mobile element"/H 2 O ratios with the Cl/H 2 O ratio, suggesting that the trace element content of subduction zone fluids is strongly enhanced by complexing with chloride. This effect is observed for large-ion lithophile (LILE) elements, (e.g., Rb and Sr), but also for the light rare earth elements (REE, e.g., La and Ce) as well as for U. The correlations of these elements with Cl/H 2 O cannot be explained by the addition of sediment melts or slab melts to the mantle source, since Cl has no effect on the solubility or partitioning of these elements in silicate melt systems. On the other hand, the observed relationship of trace element abundance with Cl is consistent with a large body of experimental data showing greatly enhanced partitioning into aqueous fluid upon addition of chloride. Accordingly, it appears that a dilute, Cl-bearing aqueous fluid is the main carrier of LILE, light REE, and U from the slab to the source of melting in arcs. Moreover, elevated Ce/H 2 O ratios clearly correlate with fluid salinity and therefore are not suitable as a "slab geothermometer." From a synopsis of experimental and melt inclusion data, it is suggested that the importance of sediment or slab melting in the generation of arc magmas is likely overestimated, while the effects of trace element scavenging from the mantle wedge may be underestimated. Moreover, establishing reliable data sets for the fluid/mineral partition coefficients of trace elements as a function of pressure, temperature, and salinity requires additional efforts, since most of the commonly used experimental strategies have severe drawbacks and potential pitfalls.