Carmela Freda

Amorphous Materials: Properties, structure, and Durability: Oxidation state of iron in hydrous phono-tephritic melts

Abstract The oxidation state of Fe in hydrous ultrapotassic (phono-tephritic) melts coexisting with mixed H2O-CO2 fluids was studied experimentally at 1200 and 1250 °C and pressures from 50 to 500 MPa. The oxygen fugacity (fO₂) varied from NNO-2.9 to NNO+2.6 in log fO₂, relative to the Ni-NiO oxygen buffer (NNO), as imposed by external redox conditions in experimental vessels and internal variations in water activity from 0.05 to 1 inside the capsules. The Fe redox state of the quenched melts was determined by colorimetric wet-chemical analysis. This analytical method was optimized to measure the Fe2+/ΣFe ratio of milligram-sized samples within ±0.03 (2σ). The accuracy and precision was tested with international reference materials and with standards analyzed by other methods. The Fe2+/ΣFe ratio of the experimental glasses covered a range of 0.41 to 0.85. A small negative effect of dissolved water on Fe2+/ΣFe at given fO₂ was found, consistent with the thermodynamic model of Moretti (2005). No effect of pressure and temperature on the redox state of Fe was resolvable in the investigated P-T range. Compared to hydrous ferrobasaltic melts that were studied previously under similar conditions, systematically lower Fe2+/ΣFe ratios were found for the phono-tephritic melts, in particular at low oxygen fugacities. This effect is attributed to the much higher K2O contents of the phono-tephrite (7.5 compared to 0.3 wt%), but the difference in FeOT (7.8 wt% in the phono-tephrite and 12.9 wt% in the ferrobasalt) may have an influence as well. Comparison of the experimentally obtained relationship between log fO₂ and Fe3+/Fe2+ for the studied hydrous ultrapotassic melts with commonly used empirical and thermodynamic models suggest that these models can be successfully applied to phono-tephritc melts, although such compositions were not implemented in the model calibrations. Furthermore, the new data can be used to improve the models with respect to the effects of compositional variables, such as H2O or K2O, on the redox state of Fe in silicate melts.

Solubility of H2O and CO2 in ultrapotassic melts at 1200 and 1250 °C and pressure from 50 to 500 MPa

Abstract The solubility of H2O-CO2 fluids in a synthetic analogue of a phono-tephritic lava composition from Alban Hills (Central Italy) was experimentally determined from 50 to 500 MPa, at 1200 and 1250 °C. Contents of H2O and CO2 in experimental glasses were determined by bulk-analytical methods and FTIR spectroscopy. For the quantification of volatile concentrations by IR spectroscopy, we calibrated the absorption coefficients of water-related and carbon-related bands for phono-tephritic compositions. The determined absorption coefficients are 0.62 ± 0.06 L/(mol·cm) for the band at ~4500 cm-1 (OH groups) and 1.02 ± 0.03 L/(mol·cm) for the band at ~5200 cm-1 (H2O molecules). The coefficient for the fundamental OH-stretching vibration at 3550 cm-1 is 63.9 ± 5.4 L/(mol·cm). CO2 is bound in the phono-tephritic glass as CO32- exclusively; its concentration was quantified by the peak height of the doublet near the 1500 cm-1 band with the calibrated absorption coefficient of 308 ± 110 L/(mol·cm). Quench crystals were observed in glasses with water contents exceeding 6 wt% even when using a rapid-quench device, limiting the application of IR spectroscopy for water-rich glasses. H2O solubility in the ultrapotassic melts (7.52 wt% K2O) as a function of pressure is similar to the solubility in basaltic melts up to 400 MPa (~8 wt%) but is higher at 500 MPa (up to 10.71 wt%). At 500 MPa and 1200 °C, the CO2 capacity of the phono-tephritic melt is about 0.82 wt%. The high CO2 capacity is probably related to the high K2O content of the melt. At both 200 and 500 MPa, the H2O solubility shows a non linear dependence on XfH₂O in the whole XfH₂O range. The variation of CO2 solubility with XfCO₂ displays a pronounced convex shape especially at 500 MPa, implying that dissolved H₂O promotes the solubility of CO2. Our experimental data on CO2 solubility indicate that the interaction between phono-tephritic magma and carbonate rocks occurring in the Alban Hills magmatic system may result in partial dissolution of CO2 from limestone into the magma. However, although the CO2 solubility in phono-tephritic melts is relatively high compared to that in silicic to basaltic melts, the capacity for assimilation of limestone without degassing is nevertheless limited to <1 wt% at the P-T conditions of the magma chamber below Alban Hills.

Aggregation-dominated ash settling from the Eyjafjallajökull volcanic cloud illuminated by field and laboratory high-speed imaging

The recent Eyjafjallajokull (Iceland) eruption strikingly underlined the vulnerability of a globalized society to the atmospheric dispersal of volcanic clouds from even moderate-size eruptions. Ash aggregation controls volcanic clouds dispersal by prematurely removing fine particles from the cloud and depositing them more proximally. Physical parameters of ash aggregates have been modeled and derived from ash fallout deposits of past eruptions, yet aggregate sedimentation has eluded direct measurement, limiting our ability to predict the dispersal of volcanic clouds. Here we use field-based, high-speed video analysis together with laboratory experiments to provide the first in situ investigation and parameterization of the physical features and settling dynamics of ash aggregates from a volcanic cloud. In May 2010, high-speed video footage was obtained of both ash particles and aggregates settling from the Eyjafjallajokull volcano eruption cloud at a distance of 7 km from the vent; fallout samples were collected simultaneously. Experimental laboratory determinations of the density, morphology, and settling velocity of individual ash particles enable their distinction from aggregates. The combination of field and experimental analyses allows a full characterization of the size, settling velocity, drag coefficient, and density distributions of ash aggregates as well as the size distribution of their component particles. We conclude that ash aggregation resulted in a tenfold increase in mass sedimentation rate from the cloud, aggravating the ash hazard locally and modifying cloud dispersal regionally. This study provides a valuable tool for monitoring explosive eruptions, capable of providing robust input parameters for models of cloud dispersal and consequent hazard forecast.

Clinopyroxene-liquid thermometers and barometers specific to alkaline differentiated magmas

We present new thermometers and barometers based on clinopyroxene–liquid equilibria specific to alkaline differentiated magmas. The new models were calibrated through the regression analyses of experimental datasets obtained by merging phase equilibria experiments from the literature with new experiments performed by using trachytic and phonolitic starting compositions. The regression strategy was twofold: (1) we have tested previous thermometric and barometric equations and recalibrated these models using the new datasets; (2) we have calibrated a new thermometer and a new barometer including only regression parameters that closely describe the compositional variability of the datasets. The new models yield more precise estimates than previous thermometers and barometers when used to predict temperatures and pressures of alkaline differentiated magmas. We have tested the reliability of the new equations by using clinopyroxene–liquid pairs from trachytes and phonolites erupted during major explosive eruptions at the Phlegrean Fields and Mt. Vesuvius (central Italy). The test yielded crystallization conditions comparable to those determined by means of melt and fluid inclusion analyses and phase equilibria studies; this validates the use of the proposed models for precise estimates of crystallization temperatures and pressures in differentiated alkaline magmas. Because these magmas feed some of the most voluminous, explosive, and threatening volcanic eruptions in the world, a better understanding of the environmental conditions of their reservoirs is mandatory and this is now possible with the new models provided here.

Water diffusion in natural potassic melts

Abstract Water diffusion experiments were performed on a trachytic melt from the Agnano-Monte Spina explosive eruption (Phlegrean Fields, South Italy). Experiments were run in a piston cylinder apparatus at 1 GPa pressure, at temperatures from 1373 to 1673 K and for durations of 0 to 255 s, using the diffusion-couple technique. Water concentration profiles were measured by Fourier transform infrared spectrometry. Water diffusion coefficients at different temperatures and water concentrations were calculated from the total water profiles, using the Boltzmann-Matano technique. Over the investigated range of temperatures and water concentrations, the diffusivity of water in potassic melts (Dwater), m2/s can be described by Arrhenius equations that can be generalized for water concentrations between 0.25 and 2 wt% as follows: Dwater =exp(−11.924−1.003lnCH2O)exp(−(exp(11.836−0.139lnCH2O))RT) where CH2O is the water concentration in wt%, R is 8.3145 (J K−1 mol.−1) and T is the temperature in Kelvin. Water diffusivities in trachytic melts were compared with water diffusivities in rhyolitic and basaltic melts. The activation energies for water diffusivity in trachyte and basalt are comparable, and higher than the haplogranitic melt. This results in a convergence of water diffusion coefficients in all melts at lower (magmatic) temperatures.

Eutectic crystallization in the undercooled Orthoclase-Quartz-H2O system experiments and simulations

Textures formed during crystallization of the eutectic composition in the system Orthoclase-Quartz-H2O at 500 MPa and 50, 100, and 200°C undercooling have been studied experimentally and simulated using a two- dimensional Ising model. The experiments performed at 50°C undercooling did not produce complex, interesting tex- tures but only very rare, isolated K-feldspar and quartz crystals. At 100°C undercooling the most common texture is a fine-grained, submicrometre to micrometre-scale, spherulitic quartz-K-feldspar intergrowth; set within this inter- growth are larger individual crystals of quartz or K-feldspar. Experiments performed at 200°C undercooling are remarkable for the occurrence of micrometre-scale graphic textures and millimetre scale spherulitic textures, charac- terized by quartz-K-feldspar intergrowths in the core and dominated by K-feldspar at the rims. Simulations of crys- tal growth, performed to complement and to interpret the experimental products, investigated what combination of growth (G) and diffusion (D) conditions can give rise to the crystal shapes and textures found in the experiments. These conditions correspond to growth rates between ~ 1 x 10-10 and 5 x 10-9 m s-1 and diffusion coefficients between 10-17 m2 s-1 in the melt phase and 10-8 m2 s-1 in the fluid phase. The simulations, despite their limitations, provide tex- tures similar to the experimental ones. In particular, simulations produced a quartz-K-feldspar intergrowth when G = D and singular, large quartz and K-feldspar crystals when G < D. These changes in the G:D ratio, in the experi- ments and in natural rocks, are attributed to a change in the growth of the crystal from a silicate melt to an aqueous fluid. The most interesting results of this study are that highly undercooled melts of a simplified pegmatite composi- tion produce textures remarkably similar to natural pegmatites and that simulations provide a powerful tool to under- stand what processes cause these textures in experimental run products and natural rocks.

Pyroclast Tracking Velocimetry illuminates bomb ejection and explosion dynamics at Stromboli (Italy) and Yasur (Vanuatu) volcanoes

A new image processing technique—Pyroclast Tracking Velocimetry—was used to analyze a set of 30 high-speed videos of Strombolian explosions from different vents at Stromboli (Italy) and Yasur (Vanuatu) volcanoes. The studied explosions invariably appear to result from the concatenation of up to a hundred individual pyroclast ejection pulses. All these pulses share a common evolution over time, including (1) a non-linear decrease of the pyroclast ejection velocity, (2) an increasing spread of ejection angle, and (3) an increasing size of the ejected pyroclasts. These features reflect the dynamic burst of short-lived gas pockets, in which the rupture area enlarges while pressure differential decreases. We estimated depth of pyroclast release to be approximately 1 and 8 m below the surface at Stromboli and Yasur, respectively. In addition, explosions featuring more frequent pulses also have higher average ejection velocities and larger total masses of pyroclasts. These explosions release a larger overall amount of energy stored in the pressurized gas by a combination of more frequent and stronger ejection pulses. In this context, the associated kinetic energy per explosion, ranging 103–109 J appears to be a good proxy for the explosion magnitude. Differences in the pulse-defining parameters among the different vents suggest that this general process is modulated by geometrical factors in the shallow conduit, as well as magma-specific rheology. Indeed, the more viscous melt of Yasur, compared to Stromboli, is associated with larger vents producing fewer pulses but larger pyroclasts.

Reduction of water loss from gold-palladium capsules during piston-cylinder experiments by use of pyrophyllite powder

Abstract Water loss was measured from H2O-undersaturated granitic melts in Au75Pd25 capsules during 6- 43 h piston-cylinder experiments using NaCl-pyrex glass-crushable alumina assemblies at 1050 to 1200 °C and 1.0 GPa. Experiments performed when capsules were surrounded only by alumina demonstrated severe water loss, in some cases more than 70% of the initial water added. It has long been known that surrounding capsules with pyrophyllite powder reduces water loss, but the efficacy of this technique has not been quantified previously. Our results confirm that by surrounding Au75Pd25 capsules with pyrophyllite in the assembly, the loss of water is significantly reduced at 1200 °C, 1.0 GPa. When ~5.6 wt% H2O is added to the sample, the loss of water decreases from ~60 to less than 20% relative by pyrophyllite addition, a value slightly higher than the uncertainty of SIMS analysis (10% relative). When about ~2 wt% H2O is added to a sample, the use of pyrophyllite in the assembly causes the loss of water to drop from more than 70 to 0% relative (no H2O loss within analytical uncertainty).

Ising models of undercooled binary system crystallization; comparison with experimental and pegmatite textures

Attempts to model crystal nucleation and growth from classic kinetic theory have been disappointing (Ohara and Reid 1973; Mullin 1974; Dowty 1980; Kirkpatrick 1981; Lasaga 1982). Population balance methods (Randolph and Larson 1988; Marsh 1988; Cashman and Marsh 1988; Cashman and Ferry 1988; Marsh 1998) used by chemical engineers and geologists to model growth often fail to predict accurately the shapes of crystal size distributions (CSDs; Larson et al. 1985), and have been criticized on theoretical grounds (Kerrick et al. 1991; Lasaga 1998). Until recently, there has been no theory, model or simulation method that accounts for all of the following aspects of crystal growth that are commonly observed in crystal growth experiments (Randolph and Larson 1988) and in some natural systems (Nordeng and Sibley 1996): (1) size dispersion, during which crystals initially having the same size may grow at different rates; (2) size dependent growth, during which larger crystals tend to grow faster; and (3) a lognormal shape for many CSDs. Recently, an approach was developed (Eberl et al. 1998) that simulates these three phenomena, which suggests that crystal growth mechanisms can be deduced from the shapes of CSDs and from the evolution of the parameters α and β during growth. Alpha is the mean of the natural logarithms of the crystal sizes, defined as: α = Σln(X)f(X), (1) ABSTRACT

Na-K interdiffusion in alkali feldspar melts

Abstract Na-K chemical interdiffusion between albite and orthoclase melts has been measured at 1.0 GPa between 800 and 1600°C and at 2.0 GPa, 1400°C, in anhydrous melts and in hydrous, 5.5 wt% H2O, melts at 1.0 GPa, 1200 and 1400°C. Anhydrous Na-K diffusivities at Xor = 0.5 display Arrhenian behavior even through the inferred glass transition. After correction for diffusion occurring during heating Na-K interdiffusion can be described by D=2.39(+1.7,−0.98)×10−5 exp(−145.8±6.8/RT) where D is the diffusion coefficient in m2s−1, 145.8 is the activation energy in kJ mol−1, R is the gas constant, and T is the temperature in kelvins. These diffusivities are four decades greater than estimated Si-Al diffusivities in the same melts due to the low alkali-oxygen bond valence, or cation field strength, and the abundance of appropriate charge-balanced locations for alkalis in alkali feldspar melts. Extrapolation of diffusion coefficients to 600–650°C for comparison with Na-K interdiffusion in alkali feldspar crystals demonstrates that melting increases diffusion by six orders of magnitude. The effect of pressure on Na-K diffusion at 1400°C is not measurable in this system and suggests that only minor dilation of the coordinating oxygen polyhedra which enclose alkalis in the melt is necessary for diffusion. A preliminary Arrhenius equation for alkali diffusion in silicic melts with 6 ± 0.5 wt% H2O was calculated using results of this and previous studies D=4.97(+0.85,−0.73)×10−6 exp(−115.8±1.8/RT). The effect of H2O only increases diffusivities by approximately a factor of four, distinctly different from the three orders of magnitude differences between Si-Al, Zr, and P diffusion in anhydrous and hydrous granitic melts. The differing behavior of alkalis and these other cations in hydrous silicic melts is associated with differences in the type and abundance of coordination polyhedra and the low bond valences and cation field strengths of alkalis.