Water Content Of Melt Research Articles

Crystal-poor, multiply saturated rhyolites (obsidians) from the Cascade and Mexican arcs: evidence of degassing-induced crystallization of phenocrysts

A detailed petrological study is presented for six phenocryst-poor obsidian samples (73–75 wt% SiO2) erupted as small volume, monogenetic domes in the Mexican and Cascade arcs. Despite low phenocryst (+microphenocryst) abundances (2–6 %), these rhyolites are each multiply saturated with five to eight mineral phases (plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± zircon ± hornblende ± clinopyroxene ± sanidine ± pyrrhotite). Plagioclase and orthopyroxene phenocrysts (identified using phase-equilibrium constraints) span ≤30 mol % An and ≤15 % Mg#, respectively. Eruptive temperatures (±1σ), on the basis of Fe–Ti two oxide thermometry, range from 779 (±25) to 940 (±18) °C. Oxygen fugacities (±1σ) range from −0.4 to 1.4 (±0.1) log units relative to those along the Ni–NiO buffer. With temperature known, the plagioclase-liquid hygrometer was applied; maximum water concentrations calculated for the most calcic plagioclase phenocryst in each sample range from 2.6 to 6.5 wt%. This requires that the rhyolites were fluid-saturated at depths ≥2–7 km. It is proposed that the wide compositional range in plagioclase and orthopyroxene phenocrysts, despite their low abundance, can be attributed to changing melt water concentrations owing to degassing during magma ascent. Phase-equilibrium experiments from the literature show that higher dissolved water concentrations lead to more Fe-rich orthopyroxene, as well as more calcic plagioclase. Loss of dissolved water leads to a progressive increase in melt viscosity, and phenocrysts often display diffusion-limited growth textures (e.g., dendritic and vermiform), consistent with large undercoolings caused by degassing. A kinetic barrier to microlite crystallization occurred at viscosities from 4.5 to 5.0 log10 Pa s for these rhyolites, presumably because the rate at which melt viscosity changed was high owing to rapid loss of dissolved water during magma ascent.

Behavior of trace elements in quartz from plutons of different geochemical signature: A case study from the Bohemian Massif, Czech Republic

In this study, the trace-element content in igneous quartz from granitoids of different geochemical types was investigated using the laser ablation ICP-MS technique. The Variscan granitoids in the Bohemian Massif provide an excellent opportunity to study the chemical composition of magmatic quartz from the following granite types: (1) geochemically primitive I-type tonalites and granodiorites, (2) peraluminous S-type two-mica granites, (3) moderately fractionated A-type volcano–plutonic complexes of the Teplice caldera, and (4) highly fractionated S- and A-type rare-metal granites. This diversity of granitoids permitted the study of the chemical composition of magmatic quartz as the result of (i) different magma protoliths and (ii) variable degrees of differentiation. There were only small differences in the quartz trace-element contents, ranging from weakly to moderately differentiated plutons of all geochemical types: Al (mostly in the range between 20 and 250ppm), Ti (mostly 20–110ppm), B (<13ppm), Be (<0.7ppm), Ge (<1ppm), Li (<30ppm), and Rb (<2ppm). Only the S-type granites from western Erzgebirge contain Al-enriched quartz (mostly 200–400ppm Al) since the beginning of its evolution. However, quartz from the highly fractionated granites (group 4) differs significantly: this quartz is generally poor in Ti (<20ppm Ti) and enriched in Al (up to 600ppm in A-type, and up to 1000ppm in S-type granites), Be (up to 3.2ppm), Ge (up to 5.7ppm), Li (up to 132ppm, particularly in the S-type granites), and Rb (up to 15ppm). The contents of the analyzed lithophile elements in the quartz from the highly fractionated granites are similar to the contents reported to be present in evolved complex pegmatites. Although the input of Ti into quartz is controlled mainly by the temperature and pressure of quartz crystallization, the entry of Al into quartz increases as a function of the water and fluorine content of the residual melt. The contents of Ge and Li increase significantly with the fractionation of parental melt. The concentrations of these elements in quartz from highly fractionated granites are controlled by the order of crystallization of the major minerals: comb quartz crystallizing before Li-mica is strongly Li-enriched, whereas groundmass and snowball quartz crystallizing after mica is relatively enriched in Ge.

Bayesian inversion of data from effusive volcanic eruptions using physics‐based models: Application to Mount St. Helens 2004–2008

Physics‐based models of volcanic eruptions can directly link magmatic processes with diverse, time‐varying geophysical observations, and when used in an inverse procedure make it possible to bring all available information to bear on estimating properties of the volcanic system. We develop a technique for inverting geodetic, extrusive flux, and other types of data using a physics‐based model of an effusive silicic volcanic eruption to estimate the geometry, pressure, depth, and volatile content of a magma chamber, and properties of the conduit linking the chamber to the surface. A Bayesian inverse formulation makes it possible to easily incorporate independent information into the inversion, such as petrologic estimates of melt water content, and yields probabilistic estimates for model parameters and other properties of the volcano. Probability distributions are sampled using a Markov‐Chain Monte Carlo algorithm. We apply the technique using GPS and extrusion data from the 2004–2008 eruption of Mount St. Helens. In contrast to more traditional inversions such as those involving geodetic data alone in combination with kinematic forward models, this technique is able to provide constraint on properties of the magma, including its volatile content, and on the absolute volume and pressure of the magma chamber. Results suggest a large chamber of &gt;40 km3 with a centroid depth of 11–18 km and a dissolved water content at the top of the chamber of 2.6–4.9 wt%.

From circumpolar deep water to the glacial meltwater plume on the eastern Amundsen Shelf

The melting of Pine Island Ice Shelf (PIIS) has increased since the 1990s, which may have a large impact on ice sheet dynamics, sea-level rise, and changes in water mass properties of surrounding oceans. The reason for the PIIS melting is the relatively warm (∼1.2°C) Circumpolar Deep Water (CDW) that penetrates into the PIIS cavity through two submarine glacial troughs located on the Amundsen Sea continental shelf. In this study, we mainly analyze the hydrographic data obtained during ANTXXVI/3 in 2010 with the focus on pathways of the intruding CDW, PIIS melt rates, and the fate of glacial meltwater. We analyze the data by dividing CTD profiles into 6 groups according to intruding CDW properties and meltwater content. From this analysis, it is seen that CDW warmer than 1.23°C (colder than 1.23°C) intrudes via the eastern (central) trough. The temperature is controlled by the thickness of the intruding CDW layer. The eastern trough supports a denser CDW layer than the water mass in Pine Island Trough (PIT). The eastern intrusion is modified on the way into PIT through mixing with the lighter and colder CDW from the central trough. Using ocean transport and tracer transport calculations from the ice shelf front CTD section, the estimated melt rate in 2010 is ∼30myr−1, which is comparable to published values. From spatial distributions of meltwater content, meltwater flows along the bathymetry towards the west. When compared with earlier (2000) observations, a warmer and thicker CDW layer is observed in Pine Island Trough for the period 2007–2010, indicating a recent thickening of the CDW intrusion.

Three-dimensional CdS nanostructure for photoelectrochemical sensor

Cubic CdS nanoparticles and hexagonal CdS nanowalls were fabricated directly on Cd foils via a modified composite hydroxide mediated approach. The phase transition of CdS nanocrystals from cubic to hexagonal phase can be controlled by varying the water content in the hydroxide melts. The metal Cd foils serves as both Cd source and substrate in the formation of the CdS nanostructures. The surface morphology and phase structure were characterized by scanning electron microscope, energy dispersive X-ray spectrometer and X-ray diffraction. The obviously enhanced photoelectrochemical performances of the hexagonal nanowalls were found under the illumination of the simulated sunlight in comparison with that of the cubic CdS nanoparticles. The surface morphology plays a vital role in its photoelectrochemical behaviors due to the different specific surface area and charge transport, indicating such three-dimensional hexagonal CdS nanostructure has prominent advantages in photoelectrochemical applications.

Melting experiments on the Udachnaya kimberlite at 6.3–7.5 GPa: Implications for the role of H2O in magma generation and formation of hydrous olivine

Melting experiments on kimberlite from the Udachnaya pipe have been performed at 6.3–7.5GPa and 1300–1600°C using a split-sphere multianvil apparatus. The water content in kimberlite varied from 2.5 to 11.6wt.% and the CO2/(CO2+H2O) molar ratio was from 0.61 to 0.23. The samples were placed in a graphite container inside a Pt capsule. The oxygen fugacity (fO2) during the experiment was controlled by the equilibrium between graphite and water-bearing carbonate–silicate melt. At relatively low temperatures, fO2 was close to the EMOG/D equilibrium, at higher temperatures, it shifted for approximately 1 log unit to more reduced conditions. An olivine+garnet+clinopyroxene assemblage was present at ⩽100°C below the liquidus of the Udachnaya kimberlite, with 6–8wt.% H2O at the pressure 6.3GPa and 6–10wt.% H2O at 7.5GPa. At 2.5wt.% H2O the same assemblage appeared at ⩾150°C below liquidus, both at 6.3 and 7.5GPa. Orthopyroxene did not form at any temperature and pressure of the experiments. The presence of clinopyroxene near the liquidus was due to the calcic nature and a high degree of silica undersaturation in the Udachnaya kimberlite. At the supra-solidus conditions, garnet and clinopyroxene compositionally distinct from minerals of the megacryst/macrocryst suite crystallized in equilibrium with low-H2O carbonated melt. Near the liquidus of high-H2O kimberlite, the stable olivine composition was from Fo91 to Fo98. The garnet composition (CaO ∼8wt.%, TiO2 <1wt.% and Cr2O3 up to 2.2wt.%) approached that of Ti-bearing garnet typical of Cr-poor garnet megacrysts while the clinopyroxene was an analog of clinopyroxene megacrysts in kimberlite. Infrared absorption measurements showed that crystallized olivines contained water in the form of Ti-clinohumite-like and OH-clinohumite-like defects. The H2O content of olivine was found to depend mainly on water content in kimberlite melt and pressure. Olivine with 120–170ppm H2O crystallized at 6GPa in equilibrium with the melt compositionally close to the archetypical kimberlite rather than a carbonate one, and water as high as 6wt.%. Olivine with 270–300ppm H2O crystallized near kimberlite liquidus only at 7.5GPa and 9.5–10.1wt.% H2O. The experimental results imply that hydrous olivine, which is typical to kimberlites, including the Udachnaya pipe, very unlikely can crystallize from anhydrous carbonated magma. Judging by the conditions for multiphase saturation of the Udachnaya kimberlite melt, its generation was possible by partial melting of carbonated garnet wehrlite at pressures from 6 to 7.5GPa and temperatures within 1450–1600°C in the presence of aqueous fluids. Thus, water must have been a trigger for generation of the primary kimberlite magma.

Development of a subglacial drainage system and its effect on glacitectonism within the polydeformed Middle Pleistocene (Anglian) glacigenic sequence of north Norfolk, Eastern England

Abstract The efficiency of subglacial drainage is known to have a profound influence on subglacial deformation and glacier dynamics with, in particular, high meltwater contents and/or pressures aiding glacier motion. The complex sequence of Middle Pleistocene tills and glacial outwash sediments exposed along the north Norfolk coast (Eastern England) were deposited in the ice-marginal zone of the British Ice Sheet and contain widespread evidence for subglacial deformation during repeated phases of ice advance and retreat. During a phase of easterly directed ice advance, the glacial and pre-glacial sequences were pervasively deformed leading to the development of a thick unit of glacitectonic melange. Although the role of pressurised meltwater has been recognised in facilitating deformation and melange formation, this paper provides evidence for the subsequent development of a channelised subglacial drainage system beneath this part of the British Ice Sheet filled by a complex assemblage of sands, gravels and mass flow deposits. The channels are relatively undeformed when compared to the host melange, forming elongate, lenticular to U-shaped, flat-topped bodies (up to 20–30 m thick) located within the upper part of this highly deformed unit. This relatively stable channelised system led to an increase in the efficiency of subglacial drainage from beneath the British Ice Sheet and the collapse of the subglacial shear zone, potentially slowing or even arresting the easterly directed advance of the ice sheet.

High-temperature, low-H2O Silicic Magmas of the Yellowstone Hotspot: an Experimental Study of Rhyolite from the Bruneau–Jarbidge Eruptive Center, Central Snake River Plain, USA

The phase relations have been investigated experimentally at 200 and 500 MPa as a function of water activity for one of the least evolved (Indian Batt Rhyolite) and of a more evolved rhyolite composition (Cougar Point Tuff XV) from the 12·8-8·1 Ma Bruneau-Jarbidge eruptive center of the Yellowstone hotspot. Particular priority was given to accurate determination of the water content of the quenched glasses using infrared spectroscopic techniques. Comparison of the composition of natural and experimentally synthesized phases confirms that high temperatures (>900°C) and extremely low melt water contents (<1·5 wt % H₂O) are required to reproduce the natural mineral assemblages. In melts containing 0·5-1·5 wt % H₂O, the liquidus phase is clinopyroxene (excluding Fe-Ti oxides, which are strongly dependent on fO₂), and the liquidus temperature of the more evolved Cougar Point Tuff sample (BJR; 940-1000°C) is at least 30°C lower than that of the Indian Batt Rhyolite lava sample (IBR2; 970-1030°C). For the composition BJR, the comparison of the compositions of the natural and experimental glasses indicates a pre-eruptive temperature of at least 900°C. The composition of clinopyroxene and pigeonite pairs can be reproduced only for water contents below 1·5 wt % H₂O at 900°C, or lower water contents if the temperature is higher. For the composition IBR2, a minimum temperature of 920°C is necessary to reproduce the main phases at 200 and 500 MPa. At 200 MPa, the pre-eruptive water content of the melt is constrained in the range 0·7-1·3 wt % at 950°C and 0·3-1·0 wt % at 1000°C. At 500 MPa, the pre-eruptive temperatures are slightly higher (by 30-50°C) for the same ranges of water concentration. The experimental results are used to explore possible proxies to constrain the depth of magma storage. The crystallization sequence of tectosilicates is strongly dependent on pressure between 200 and 500 MPa. In addition, the normative Qtz-Ab-Or contents of glasses quenched from melts coexisting with quartz, sanidine and plagioclase depend on pressure and melt water content, assuming that the normative Qtz and Ab/Or content of such melts is mainly dependent on pressure and water activity, respectively. The combination of results from the phase equilibria and from the composition of glasses indicates that the depth of magma storage for the IBR2 and BJR compositions may be in the range 300-400 MPa (13 km) and 200-300 MPa (10 km), respectively.

Lower crustal H2O controls on the formation of adakitic melts

At volcanic arcs, fluids released from the subducting slab lower the solidus of the mantle wedge and cause melting. Furthermore, slab melts may infiltrate the mantle wedge, and have been suggested to generate adakitic (residual garnet) signatures at some arc volcanoes. However, experimental work indicates that the garnet stability field will expand in the lower overriding crust in the presence of somewhat less hydrous melts, suggesting that such signatures may also develop at crustal levels. Here we use geothermometry and plagioclase hygrometry of mafic eruptives from southwest Japan to demonstrate that the adakitic compositions of associated intermediate magmas are of lower crustal origin due to a decrease in the water content of parental melts, and are not generated by partial melting of the eclogitic subducting slab at elevated temperatures. Lower crustal melt evolution at reduced water contents may represent an important process for generating adakitic signatures in all tectonic settings that have previously been considered to enhance slab melting. Our results demonstrate that magmatic water plays a key role in the differentiation of arc magmas in modern and ancient subduction settings.

Surface and bed morphology of Hansbreen, a tidewater glacier in Spitsbergen

Surface and bed morphology of Hansbreen, a tidewater glacier in SpitsbergenHansbreen, a medium size tidewater glacier in Southern Spitsbergen (Svalbard) is one of the most intensively studied glaciers in the Arctic. This work presents new digital elevation models of its surface and basal topography based on data collected during GPS/GPR campaigns conducted in the spring seasons of 2005 and 2008, as well as on other recent topographic/bathymetric sources. The mean thickness of the glacier is calculated as 171 m and its volume is estimated to be 9.6 (±0.1) km3. The main feature of the bedrock morphology is a vast depression that is overdeepened below sea level and extends as far as 11 km upstream from the glacier front. This depression is divided into four individual basins by distinct sills that are related to the main geological/tectonic features of the area. The bedrock morphology affects considerably the glacier's surface topography. The influence of bedrock and surface relief on the subglacial drainage system geometry is discussed. Vast depressions on the glacier surface favor concentration of meltwater and development of moulin systems.

Dry and strong quartz during deformation of the lower crust in the presence of melt

[1] Granulite facies migmatitic gneisses from the Seiland Igneous Province (northern Norway) were deformed during deep crustal shearing in the presence of melt, which formed by dehydration melting of biotite. Partial melting and deformation occurred during the intrusion of large gabbroic plutons at the base of the lower crust at 570 to 520 Ma in an intracontinental rift setting. The migmatitic gneisses consist of high-aspect-ratio leucosome-rich domains and a leucosome-poor, restitic domain of quartzitic composition. According to thermodynamic modeling using synkinematic mineral assemblages, deformation occurred at T = 760°C–820°C, P = 0.75–0.95 GPa and in the presence of ≤5 vol % of residual melt. There is direct evidence from microstructural observations, Fourier transform infrared measurements, thermodynamic modeling, and titanium-in-quartz thermometry that dry quartz in the leucosome-poor domain deformed at high differential stress (50–100 MPa) by dislocation creep. High stresses are demonstrated by the small grain size (11–17 μm) of quartz in localized layers of recrystallized grains, where titanium-in-quartz thermometry yields 770°C–815°C. Dry and strong quartz forms a load-bearing framework in the migmatitic gneisses, where ∼5% melt is present, but does not control the mechanical behavior because it is located in isolated pockets. The high stress deformation of quartz overprints an earlier, lower stress deformation, which is preserved particularly in the vicinity of segregated melt pockets. The grain-scale melt distribution, water content and distribution, and the overprinting relationships of quartz microstructures indicate that biotite dehydration melting occurred during deformation by dislocation creep in quartz. The water partitioned into the segregated melt crystallizing in isolated pockets, in the vicinity of which quartz shows a higher intracrystalline water content and a large grain size. On the contrary, the leucosome-poor domain of the rock, from which melt was removed, became dry and thereby mechanically stronger. Melt removal at larger scale will result in a lower crust which is dry enough to be mechanically strong. The application of flow laws derived for wet quartz is not appropriate to estimate the behavior of such granulite facies parts of the lower crust.

Preeruptive magma viscosity: An important measure of magma eruptibility

[1] Using a compilation of melt compositions, meltwater contents, temperatures, and phenocryst contents, the preeruptive viscosities under magma reservoir conditions are calculated for 83 erupted magmas. The basaltic-to-rhyolitic magmas have preeruptive viscosities over the range 101 to 108 Pa s. Although bulk SiO2 content has often been used as a qualitative measure of preeruptive magma viscosity, the bulk SiO2 content shows a weak correlation with magma viscosity (correlation coefficient r = 0.5). Because of a wide range of phenocryst contents from 0 to ∼50 vol %, andesitic magmas have viscosities ranging from 102 to 107 Pa s, which are lower or higher than those of phenocryst-poor rhyolitic magmas with 105 to 106 Pa s. Focusing on andesitic to rhyolitic magmas, the r between bulk SiO2 contents and magma viscosities changes to −0.1. In contrast, the melt-only SiO2 content from a basaltic-to-rhyolitic melt shows a good linear correlation with melt-only viscosity (r = 0.9). Although most of the calculated viscosities of erupted magmas fall below ∼106 Pa s, as consistent with the previous compilation study, this paper describes 20 examples of highly viscous magmas with >106 Pa s, in most cases, composed of mixtures of high-silica rhyolitic melt (75–79 wt % SiO2) and abundant phenocrysts (30–55 vol %). In these highly viscous magmas, 9 examples have erupted following the precursory eruption of less viscous magma, suggesting that precursory dike propagation and conduit formation by the less viscous magma with <106 Pa s induced the following eruption of less eruptible, highly viscous magmas.

Iron speciation using electron microprobe techniques: application to glassy melt pockets within a spinel lherzolite xenolith

AbstractA quantitative determination of Fe3+/ΣFe ratios using the electron microprobe is presented for glasses. The measurements are based on the shift of the Fe-Lα peak position between Fe3+ and Fe2+, resulting from differences in self-absorption. We present new calibration curves for glasses with Fe contents as low as 2 wt.% Fe.This method was used for glasses from a metasomatized spinel Iherzolite (Massif Central, France) in which highly vesicular glass associated with secondary minerals occurs in reaction pockets around corroded Cr-spinels. The glass was found to be oxidized homogeneously with Fe3+/ΣFe ≈ 0.70 (measured with an uncertainty of ±0.06), in contrast with the value of ∼0.15 that would correspond to the former melt under fo2 conditions inferred from secondary minerals. The glass oxidation probably results from the dissociation of the melt-water content under degassing, prior to local hematite depositions (spinel rims and crack fillings).

Plagioclase–melt (dis)equilibrium due to cooling dynamics: Implications for thermometry, barometry and hygrometry

The compositional variation of plagioclase and the partitioning of major elements between plagioclase and melt have been experimentally measured as a function of the cooling rate. Crystals were grown from a basaltic melt at a pressure of 500MPa under (i) variable cooling rates of 0.5, 2.1, 3, 9.4, and 15°C/min from 1250°C down to 1000°C, (ii) quenching temperatures of 1025, 1050, 1075, 1090, and 1100°C at the fixed cooling rate of 0.5°C/min, and (iii) isothermal temperatures of 1000, 1025, 1050, 1075, 1090, and 1100°C. Our results show that euhedral, faceted plagioclases form during isothermal and slower cooling experiments exhibiting idiomorphic tabular shapes. In contrast, dendritic shapes are observed from faster cooled charges. As the cooling rate is increased, concentrations of Al+Ca+Fe+Mg increase and Si+Na+K decrease in plagioclase favoring higher An and lower Ab+Or contents. Significant variations of pl–liqKd are also observed by the comparison between isothermal and cooled charges; notably, pl–liqKdAb–An, pl–liqKdCa–Na and pl–liqKdFe–Mg progressively change with increasing cooling rate. Therefore, crystal–melt exchange reactions have the potential to reveal the departure from equilibrium for plagioclase-bearing cooling magmas. Finally, thermometers, barometers, and hygrometers derived through the plagioclase–liquid equilibria have been tested at these non-equilibrium experimental conditions. Since such models are based on assumption of equilibrium, any form of disequilibrium will yield errors. Results show that errors on estimates of temperature, pressure, and melt-water content increase systematically with increasing cooling rate (i.e. disequilibrium condition) depicting monotonic trends towards drastic overestimates. These trends are perfectly correlated with those of pl–liqKdCa–Na, pl–liqKdAb–An, and pl–liqKdFe–Mg, thus demonstrating their ability to test (dis)equilibrium conditions.

Phenocryst complexity in andesites and dacites from the Tequila volcanic field, Mexico: resolving the effects of degassing vs. magma mixing

The petrology of five phenocryst-poor (2–5%) andesites and dacites, all of which were erupted from different short-lived, monogenetic vents, is compared to that of phenocryst-rich (10–25%) andesites erupted from the adjacent stratovolcano, Volcan Tequila, in the Mexican arc. Despite differences in phenocryst abundances, these magmas have comparable phase assemblages (plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± augite ± hornblende), and similarly wide variations in phenocryst compositions, coupled to complex zoning patterns. For the phenocryst-poor lavas, equilibrium pairs of two Fe–Ti oxides lead to a narrow range of calculated temperatures for each sample that range from 934 (±24) to 1,073 (±6)°C and oxygen fugacities that range from +0.1 to +0.7 log units relative to the Ni–NiO buffer. Application of the plagioclase-liquid hygrometer to each sample at these calculated temperatures leads to maximum melt water concentrations of 4.6–3.1 wt% during plagioclase crystallization, indicating that the magmas were fluid saturated at depths ≥6.4–4.5 km. There is a wide, continuous range in the composition of plagioclase (≤44 mol% An) and orthopyroxene (≤16% Mg#) phenocrysts in each sample, which is consistent with a loss of dissolved water (≤2.8 wt%) from the melt phase during degassing as the magmas ascended rapidly to the surface. Evidence is presented that shows the effect of dissolved water is to reduce the activity of MgO relative to FeO in the melt phase, which indicates that degassing will also affect the Mg# of pyroxene phenocrysts, with higher melt water concentrations favoring Fe-rich pyroxene. Both plagioclase and orthopyroxene commonly display diffusion-limited growth textures (e.g., skeletal and hopper crystals, large interior melt hollows, and swallow tails), which are consistent with large undercoolings produced by degassing-induced crystallization. Therefore, degassing is proposed as a possible cause for the phenocryst compositional diversity documented in the phenocryst-poor andesite and dacite lavas erupted from peripheral vents, including the coexistence of normally zoned plagioclase and reversely zoned orthopyroxene. Degassing-induced crystallization may also explain some of the phenocryst complexity in crystal-rich andesites erupted from large stratovolcanoes, including Volcan Tequila.

In situ bubble vesiculation in silicic magmas

Volatile degassing is a major process driving volcanic eruptions. Therefore, a full understanding of mechanisms ranging from bubble nucleation, growth, coalescence, to magma fragmentation is required. We have simulated magma degassing during ascent in the volcanic conduit by depressurizing hydrated haplogranite melts in high-pressure and high-temperature optical cells (a hydrothermal diamond-anvil cell and an internally heated pressure vessel fitted with sapphire windows). This allowed the whole process of bubble nucleation, growth, and coalescence to be directly observed in situ through images captured from the recording videos. Bubble nucleation pressures, number densities, growth laws, and characteristics of coalescence were estimated as a function of melt water content, decompression rate, and temperature. Melt/vapor surface tension during bubble nucleation and coalescence was calculated. Our data show good agreement with those previously obtained in classical vessels. Methodological improvements are proposed for the experimental simulation of magma degassing in volcanic conduits.

Hydrogen content in clinopyroxene phenocrysts from Salina mafic lavas (Aeolian arc, Italy)

Clinopyroxene phenocrysts from the mafic calc-alkaline lavas of Salina (Aeolian arc, southern Tyrrhenian Sea, Italy) have been analysed to determine the hydrogen content and iron oxidation state of this early crystallized phase. The volcanic activity of Salina, starting at 168 ka and developed in several centres up to 24 ka, was dominated by calc-alkaline and high-K calc-alkaline basalts and andesites, with minor dacites and rhyolites. The presence of OH vibrational bands was detected in the IR spectra of clinopyroxenes phenocrysts from Corvo, Rivi-Capo (168–87 ka), Fossa delle Felci (108–59 ka) and Monte dei Porri (57 ka) eruptions. Corvo-Rivi-Capo basalts have clinopyroxenes with the lowest water contents 75–97 ppm H2O by weight, whereas an increase in the hydrogen contents of clinopyroxenes from Fossa delle Felci centre, with 171–286 ppm H2O by weight, and Monte dei Porri with 343–390 ppm H2O by weight, was observed. Mossbauer spectroscopy showed only a limited variation on the Fe3+/Fetot ratio of the studied samples, and a very similar atomic Fe3+ content (0.042–0.047 a.p.f.u.) suggesting that only minor variation on fO2 occurred during the crystallization of these clinopyroxenes. The water content of parental melts, calculated by applying an IVAl-dependent partition coefficient to the measured hydrogen contents of clinopyroxenes, is 0.4–0.8 wt% of water in melt for the Rivi-Capo-Corvo basalts, 0.5–3.7 wt% water in melt for Fossa delle Felci lavas and 1.6–2.6 wt% of water in melt for Monte dei Porri lavas. An increase in the maximum hydrogen contents of clinopyroxenes can be recognized during the evolution of the Salina volcano, with the highest hydrogen content measured in clinopyroxenes from Monte dei Porri where the eruptions were characterized by a high degree of explosivity, suggesting a key role of volatiles.

Phase Equilibrium Constraints on Pre-eruptive Conditions of Recent Felsic Explosive Volcanism at Pantelleria Island, Italy

We present experimental phase equilibria carried out on a pantelleritic bulk-rock composition with a peralkalinity index [PI = molar (Na2O + K2O)/Al2O3] = 1*4, which is representative of the most energetic pumice fall eruption of the resurgent post-caldera volcanism on Pantelleria island. For the explored conditions (P = 25-150 MPa; T = 680-800°C; H2Omelt up to 6 wt %; fO2 ≤ NNO, where NNO is nickel-nickel oxide buffer) clinopyroxene is the liquidus phase followed by alkali feldspar and then quartz. The crystallization of amphibole is limited to temperatures below 700°C. Aenigmatite crystallizes near the liquidus for P ≥ 100 MPa. When clinopyroxene is the sole liquidus phase, the coexisting melt is characterized by a decrease in peralkalinity index to 1*2, if compared with the starting material, approaching comendite composition. In contrast, when alkali feldspar joins the liquidus phase assemblage, residual melts become extremely peralkaline (PI = 3*2), with FeO contents in excess of 14 wt %. Comparison of our experiments with natural phase abundances and compositions constrains the depth of the Fastuca pantellerite reservoir to be around 5 ± 1 km (or P = 120 ± 20 MPa) at T = 730 ± 10°C, with H2Omelt around 4 wt %, and fO2 1*5 log unit below NNO. Such conditions correspond to the dominant phase assemblage of feldspar, clinopyroxene and aenigmatite. Amphibole occurrence requires both higher melt water contents and lower temperatures (up to 6*5 wt % at 680°C), and may reflect provenance from a deeper and wetter portion of the tapped reservoir. In contrast, quartz is more probably related to crystallization during magma ascent, which explains the lower melt water content of its melt inclusions. Overall, the results confirm that pantellerites may be both water-rich and reduced magmas. These characteristics imply that any coexisting fluid phase in the reservoir will be dominated by water and hydrogen species, the latter amounting to a few mole per cent. Magma unrest at Pantelleria may thus be heralded by emissions of hydrous and reduced gases (i.e. CO2-poor and H2-rich), if the present-day reservoir is felsic in composition.

Laboratory evidence for enhanced infiltration of ion load during snowmelt

Abstract. Meltwater ion concentration and infiltration rate into frozen soil both decline rapidly as snowmelt progresses. Their temporal association is highly non-linear and a covariance term must be added in order to use time-averaged values of snowmelt ion concentration and infiltration rate to calculate chemical infiltration. The covariance is labelled enhanced ion infiltration and represents the additional ion load that infiltrates due to the timing of high meltwater concentration and infiltration rate. Previous assessment of the impact of enhanced ion infiltration has been theoretical; thus, experiments were carried out to examine whether enhanced infiltration can be recognized in controlled laboratory settings and to what extent its magnitude varies with soil moisture. Three experiments were carried out: dry soil conditions, unsaturated soil conditions, and saturated soil conditions. Chloride solutions were added to the surface of frozen soil columns; the concentration decreased exponentially over time to simulate snow meltwater. Infiltration excess water was collected and its chloride concentration and volume determined. Ion load infiltrating the frozen soil was specified by mass conservation. Results showed that infiltrating ion load increased with decreasing soil moisture as expected; however, the impact of enhanced ion infiltration increased considerably with increasing soil moisture. Enhanced infiltration caused 2.5 times more ion load to infiltrate during saturated conditions than that estimated using time-averaged ion concentrations and infiltration rates alone. For unsaturated conditions, enhanced ion infiltration was reduced to 1.45 and for dry soils to 1.3. Reduction in infiltration excess ion load due to enhanced infiltration increased slightly (2–5%) over time, being greatest for the dry soil (45%) and least for the saturated soil (6%). The importance of timing between high ion concentrations and high infiltration rates was best illustrated in the unsaturated experiment, which showed large inter-column variation in enhanced ion infiltration due to variation in this temporal covariance.

Petrogenesis of the Fe-Ti intrusive complexes in the Sierra Leone region, Central Atlantic

The study of melt inclusions in Cr-spinels from melanocratic troctolites provided the first direct information on the physicochemical parameters of enriched magmatic systems that produced high-Fe and high-Ti intrusive complexes in the Sierra-Leone region (Central Atlantic, 6°N). These complexes are made up of predominating hornblende Fe-Ti oxide gabbronorites and gabbrodiorites with subordinate amount of ultramafics, diorites, quartz diorites, and trondhjemites. The study of melt inclusions and rocks showed that the majority of gabbroids of the Central Atlantic (Sierra Leone area and 15°20′ Fracture Zone) were derived from N-MORB-type melts, whereas differentiated Fe-Ti-oxide rocks were crystallized from other melts, which were preserved as inclusions in the Cr-spinels from the melanocratic troctolites of the Sierra Leone region. The ion-microprobe study of these inclusions yield direct evidence on the elevated water content (up to 1.24–1.77 wt %) in the parental melts of Fe-Ti oxide rocks. Data on trace and rare-earth element distribution together with high (La/Sm)N and (Ce/Yb)N ratios in the inclusions indicate the possible influence of deep plume source on the generation of these magmas. Simulation based on melt inclusion data testifies that high-Fe intrusions of the Sierra Leone area were crystallized from the water-saturated magmas at relatively low temperatures (1020–1240°C). It was shown that the geochemically enriched Fe-Ti melts were presumably formed regardless of N-MORB-type magmatism predominant in Central Atlantic, under the influence of new mantle plume that caused melting of hydrated oceanic lithosphere.