Valeria Misiti

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.

Experimental constraints on evolution of leucite-basanite magma at 1 and 10-4 GPa: implications for parental compositions of Roman high-potassium magmas

The separate effects of pressure (10 −4 and 1.0 GPa), water, CO 2 , oxygen fugacity and calcium doping on the liquid line of descent of a primitive leucite-basanite magma (SiO 2 = 47.06 wt%, MgO = 12.76 wt% and Mg# = 75.1) from the Montefiascone Volcanic Complex (Vulsini volcanoes, central Italy) were experimentally investigated in the 1350–1160 °C temperature range. Results indicate that low-pressure liquidus temperatures are ≤1280 °C and that the high-pressure T liquidus is 1350 °C under anhydrous conditions; the latter is lowered to ~ 1275 °C by the addition of 3 wt% water. Cr-spinel is always the liquidus phase. At comparable f O 2 values, high and low pressure runs produced the same phase assemblage (spinel + olivine + clinopyroxene) up to 50 % crystallization, although olivine was partially or totally replaced by phlogopite in hydrous experiments. An increase in oxygen fugacity and the addition of CaO determine an increase in both the degree of melt crystallization and the stability field of clinopyroxene. These determine contrasting effects on the composition of residual liquids: the former increases SiO 2 content, whereas the latter induces the desilication of melts. The replacement of olivine by phlogopite, induced by increasing amounts of water, leads to the production of glass with lower potassium contents. Comparison of the natural and experimental melts shows that many of major and trace element variations exhibited by high-K primitive ( i.e. , high Mg/Mg + Fe) magmas at Montefiascone, are consistent with their derivation from a single parental leucite-basanite melt by fractional crystallization of different proportions of mineral phases, plus carbonate assimilation. The changes in phases stability and melt composition caused by carbonate assimilation may also have fundamental implications for the origin of the calcic high-magnesium leucitites and melilitites. In particular, the complex metasomatic interactions that can develop at the interface between potassic magmas and carbonate wall rocks, may lead to melting of calcite. This low-viscosity melt readily mixes with the surrounding magma inducing the crystallization of Ca-Tschermak-rich pyroxene and hercynitic spinel, affecting significantly the SiO 2 , CaO and alumina composition of the resulting hybrid melt. A key finding of our study is that magmas such as the studied leucite-basanite may be considered parental to the wide spectrum of mafic high-K compositions in the Roman Province, which have been traditionally considered as representing near primary magmas reflecting distinct mantle source compositions and/or processes.

High-resolution geochemistry of volcanic ash highlights complex magma dynamics during the Eyjafjallajokull 2010 eruption

Abstract The April to May 2010 eruption of Eyjafjallajökull (Iceland) volcano was characterized by a large compositional variability of erupted products. To contribute to the understanding of the plumbing system dynamics of this volcano, we present new EMPA and LA-ICP-MS data on groundmass glasses of ash particles and minerals erupted between April 15 and 22. The occurrence of disequilibrium textures in minerals, such as resorption and inverse zoning, indicate that open system processes were involved in determining the observed compositional variability. The variation of major and trace element data of glasses corroborates this hypothesis indicating that mixing between magma batches with different compositions interacted throughout the whole duration of the eruption. In particular, the arrival of new basaltic magma into the plumbing system of the volcano destabilized and remobilized magma batches of trachyandesite and rhyolite compositions that, according to geophysical data, might have intruded as sills over the past 20 years beneath the Eyjafjallajökull edifice. Two mixing processes are envisaged to explain the time variation of the compositions recorded by the erupted tephra. The first occurred between basaltic and trachyandesitic end-members. The second occurred between trachyandesite and rhyolites. Least-squares modeling of major elements supports this hypothesis. Furthermore, investigation of compositional histograms of trace elements allows us to estimate the initial proportions of melts that interacted to generate the compositional variability triggered by mixing of trachyandesites and rhyolites.

Trace element behaviour during interaction between basalt and crustal rocks at 0.5–0.8 GPa: an experimental approach

We experimentally investigate the major and trace elements behavior during the interaction between two partially molten crustal rocks (meta-anorthosite and metapelite) and a basaltic melt at 0.5–0.8 GPa. Results show that a hybrid melt is formed at the basalt-crust contact, where plagioclase crystallizes. This contact layer is enriched in trace elements which are incompatible with plagioclase crystals. Under these conditions, the trace element diffusion coefficients are one order of magnitude larger than those expected. Moreover, the HFSE diffusivity in the hybrid melt is surprisingly higher than the REE one. Such a feature is related to the plagioclase crystallization that changes the trace elements liquid-liquid partitioning (i.e. diffusivity) over a transient equilibrium that will persist as long as the crystal growth proceeds.These experiments suggests that the behaviour of the trace elements is strongly dependent on the crystallization at the magma-crust interface. Diffusive processes like those investigated can be invoked to explain some unusual chemical features of contaminated magmatic suites.

Experimental constraints on the rheology, eruption and emplacement dynamics of analog lavas comparable to Mercury's northern volcanic plains

We present new viscosity measurements of a synthetic silicate system considered an analogue for the lava erupted on the surface of Mercury. In particular, we focus on the northern volcanic plains (NVP), which correspond to the largest lava flows on Mercury and possibly in the Solar System. High-temperature viscosity measurements were performed at both superliquidus (up to 1736 K) and subliquidus conditions (1569–1502 K) to constrain the viscosity variations as a function of crystallinity (from 0 to 28%) and shear rate (from 0.1 to 5 s-1). Melt viscosity shows moderate variations (4 –16 Pa s) in the temperature range 1736–1600 K. Experiments performed below the liquidus temperature show an increase in viscosity as shear rate increases from 0.1 to 5 s-1, resulting in a shear thinning behaviour, with a decrease in viscosity of ca. 1 log unit. The low viscosity of the studied composition may explain the ability of NVP lavas to cover long distances, on the order of hundreds of kilometres in a turbulent flow regime. Using our experimental data we estimate that lava flows with thickness of 1, 5 and 10 m are likely to have velocities of 4.8, 6.5 and 7.2 m/s respectively, on a 5° ground slope. Numerical modelling incorporating both the heat loss of the lavas and its possible crystallization during emplacement allows us to infer that high effusion rates (> 10000 m3/s) are necessary to cover the large distances indicated by satellite data from the MESSENGER spacecraft.

Crystallization from a melt and crystallization at subsolidus conditions: comparison from crystal size distribution study on Gennargentu Rocks (Sardinia, Italy)

Crystal size distributions (CSDs) has been constrained in plagioclase and cordierite in quartz-diorite and migmatites. Euhedral plagioclase crystals in a plutonic body preserve the crystallisation processes that occurred during the cooling history of the body. A crystallisation from a liquid (i.e. plagioclase crystals in quartz diorite and in migmatite leucosomes) has been compared with a crystallisation at subsolidus conditions (i.e. plagioclase and cordierite crystals in contact metamorphic rocks). At contact with the metamorphic basement the texture is migmatitic. Plagioclase CSDs have been investigated in a plutonic body (i.e. quartz diorite), in the leucosome of migmatites and in the melanosome of un-melted contact metamorphic rocks from Gennargentu Complex (Sardinia, Italy). Cordierite CSDs have been investigated only in the hybrid rocks of the same complex. CSDs indicate that, initially, plagioclase crystals in the quartz diorite nucleated and grew in a cooling system at a constant cooling rate, producing a straight-line crystal size distributions. The plagioclase crystallisation continued until the latent heat was available and the temperature was high enough to allow the plagioclase growing. When the temperature was low enough to reach the solidus temperature of plagioclase crystals, the nucleation was inhibited and the conditions were suitable for a textural coarsening (Ostwald ripening process). The small crystals, due to their high surface energy per unit volume dissolved and “fed” the growth of larger crystals. Moreover, the maintenance of temperature near the plagioclase liquidus may also inhibit the nucleation and growth of other phases. Cordierite crystals from contact metamorphic rocks show a similar behaviour, while the plagioclases from the migmatites show an accumulation process. From CSDs measurements we were able to calculate the different cooling ages for the different sample types as following: dark quartz diorite samples crystallise in an average of time of 5000 years; the light quartz diorite shows a slow cooling (average of 7300 years).

Nanomechanical characterization of K-basalt from Roman comagmatic province: A preliminary study

Based on atomic force microscopy (AFM), contact resonance AFM (CR-AFM) is a nondestructive technique that allows one to perform single point measurements as well as surface mapping of the indentation modulus of a material. In this work we exploit the possibility to use CR-AFM to study synthetic materials representative of K-basalt from Roman comagmatic Province. Having observed the presence of subsurface voids and inclusions at micrometer and sub-micrometer scale, a preliminary study has been conducted to verify the capability of CR-AFM nanomechanical mapping to nondestructively detect these features.Based on atomic force microscopy (AFM), contact resonance AFM (CR-AFM) is a nondestructive technique that allows one to perform single point measurements as well as surface mapping of the indentation modulus of a material. In this work we exploit the possibility to use CR-AFM to study synthetic materials representative of K-basalt from Roman comagmatic Province. Having observed the presence of subsurface voids and inclusions at micrometer and sub-micrometer scale, a preliminary study has been conducted to verify the capability of CR-AFM nanomechanical mapping to nondestructively detect these features.