Francesco Vetere

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 and modeled chlorine solubilities in aluminosilicate melts at 1 to 7000 bars and 700 to 1250 °C: Applications to magmas of Augustine Volcano, Alaska

Abstract Hydrothermal experiments were conducted at ca. 1 to 7000 bars and 700 to 1250 °C in 121 rhyolitic to basaltic systems to determine Cl solubility in silicate melts, i.e., the maximum Cl concentration in melts that are saturated in a hydrosaline liquid with or without an aqueous or aqueous-carbonic vapor. The Cl concentration of melts increases with the Cl contents of the fluid unless the melt coexists with vapor plus hydrosaline liquid at fixed pressure and temperature; this phase assemblage buffers the Cl content of each phase with increasing Cl in the system. The Cl content of fluid(s)-saturated melts is independent of the CO2 concentration of the saline liquid ± vapor with up to 21 wt% CO2 in the fluid(s). The experiments show that Cl dissolution in aluminosilicate melts increases with temperature and pressure. Chlorine solubility is also a function of melt composition; it increases with the molar ([Al1/2+Ca1/2+Mg1/2+Na]/Si) of the melt. These experimental data have been integrated with results involving 41 other experiments (Webster and De Vivo 2002) to develop a broadly expanded model that supports calculation of Cl solubility in 163 aluminosilicate melts. This empirical model applies to Cl dissolution in melts of most silicate magmas at depths as great as 25 km. It determines the exsolution of hydrosaline liquid, with or without a coexisting vapor, as magmas ascend from depth, cool, crystallize, and differentiate from mafic to felsic compositions. In combination with H2O solubility models, our model supports determination of H2O-Cl solubility relations for most aluminosilicate magmas and is useful for barometric estimations based on silicate melt inclusions containing low CO2 and moderate to high-Cl concentrations. The model is applied to the phase relations of fluids in volatile-enriched magmas of Augustine volcano, Alaska. The Cl and H2O concentrations of melt inclusions from 14, basaltic to dacitic eruptive units are compared with modeled solubilities of Cl and H2O in Augustine melts. The majority of these eruptions involved magmas that first exsolved aqueous to aqueous-carbonic vapors when the melts were dacitic in composition (i.e., before the residual melts in these magmas had evolved to felsic compositions) and well prior to the eruptions. Hydrosaline liquid with or without a vapor phase exsolved from other, more-felsic fractions of Augustine melts at low, near-surface pressures of several tens of bars.

Elemental Imaging and Petro-Volcanological Applications of an Improved Laser Ablation Inductively Coupled Quadrupole Plasma Mass Spectrometry

We report on the performance of the a new LA-ICP-MS instrumentation installed at the Physics and Geology Department of Perugia University empathizing its capabilities in elemental imaging and the progresses in trace element in situ determination and U/Pb geochronology. The analytical device consists in a Thermo Fisher Scientific iCAP Q quadrupole mass spectrometer coupled with a Teledyne/Photon Machine ArF Excimer G2 laser ablation system. Results show that, in trace element configuration at 40 micron, precisions are better than 6.5% whereas accuracies are better than 10%. Results also show improved precisions with respect the X7 + UP213 instrumentation in U/Pb geochronological studies. On this regard, concordia ages for the Plesovice and R33 Zircons analyzed as unknowns are in close agreement with the accepted values for these reference materials highlighting the accuracy of the method. The potentials in 2D element imaging are also reported and successfully tested on a zoned plagioclase from the alkali basaltic Santa Venera lava Flow. Results evidences that expanding the analysis to the second dimension will lead to more reliable and accurate results and it is going to open new prospective for the modeling of igneous systems.

Magmatic Evolution and plumbing system of ring-fault volcanism: the Vulcanello Peninsula (Aeolian Islands, Italy)

The Vulcanello peninsula is situated north of Vulcano, the southernmost island of the Aeolian Arc. It was built at the rim of La Fossa Caldera between 1000 and 1650 A.D. Erupted products are mafic to intermediate in composition, while the coeval products erupted inside the caldera are mainly rhyolitic. Therefore, Vulcanello’s activity represents an anomalous mafic post-caldera volcanism in a convergent setting. A petrographic and geochemical study was carried out on lavas and pyroclastic rocks representing the entire eruptive history of the volcanic centre. New data (major and trace elements and Sr isotope ratios on whole rocks, and major element compositions on mineral phases) and geochemical models were used to investigate shallow level differentiation processes ( i.e. , fractional crystallisation, fractional crystallisation plus crustal assimilation, degassing, magma mixing/recharge). The study suggests that the entire Vulcanello activity can be considered as the uninterrupted expulsion of a single deep magma batch of shoshonitic composition emitted from a NE–SW ring fault of La Fossa Caldera. The magma is genetically related to the shoshonitic basalts found as melt inclusions in the olivine crystals erupted in the products of the 1888–1890 “ La Fossa” activity. This points to a possible single deep plumbing system for both La Fossa Cone and Vulcanello centres, strongly controlled by NW–SE to N– S regional structures. The shoshonitic magma, undergoing fractional crystallisation, partly rose directly to the surface where two strombolian cones were constructed, while residual magma remained at depth, and, partially degassed and crystallised, it subsequently erupted both effusively to form a lava platform and explosively to form a third pyroclastic cone. The remaining magma evolved to latite by AFC process and was erupted both as a lava flow (Punta del Roveto) and in the form of pyroclastic products ( i.e. , the upper part of the third cone), controlled by shallow ring faults of La Fossa Caldera. Therefore the Vulcanello plumbing system is controlled by tectonic structures at depth and by shallower volcano-tectonic (caldera) fractures.

High-temperature apparatus for chaotic mixing of natural silicate melts

A unique high-temperature apparatus was developed to trigger chaotic mixing at high-temperature (up to 1800 °C). This new apparatus, which we term Chaotic Magma Mixing Apparatus (COMMA), is designed to carry out experiments with high-temperature and high-viscosity (up to 10(6) Pa s) natural silicate melts. This instrument allows us to follow in time and space the evolution of the mixing process and the associated modulation of chemical composition. This is essential to understand the dynamics of magma mixing and related chemical exchanges. The COMMA device is tested by mixing natural melts from Aeolian Islands (Italy). The experiment was performed at 1180 °C using shoshonite and rhyolite melts, resulting in a viscosity ratio of more than three orders of magnitude. This viscosity ratio is close to the maximum possible ratio of viscosity between high-temperature natural silicate melts. Results indicate that the generated mixing structures are topologically identical to those observed in natural volcanic rocks highlighting the enormous potential of the COMMA to replicate, as a first approximation, the same mixing patterns observed in the natural environment. COMMA can be used to investigate in detail the space and time development of magma mixing providing information about this fundamental petrological and volcanological process that would be impossible to investigate by direct observations. Among the potentials of this new experimental device is the construction of empirical relationships relating the mixing time, obtained through experimental time series, and chemical exchanges between the melts to constrain the mixing-to-eruption time of volcanic systems, a fundamental topic in volcanic hazard assessment.

Quantifying magma mixing with the Shannon entropy: Application to simulations and experiments

Abstract We introduce a new quantity to petrology, the Shannon entropy, as a tool for quantifying mixing as well as the rate of production of hybrid compositions in the mixing system. The Shannon entropy approach is applied to time series numerical simulations and high-temperature experiments performed with natural melts. We note that in both cases the Shannon entropy increases linearly during the initial stages of mixing and then saturates toward constant values. Furthermore, chemical elements with different mobilities display different rates of increase of the Shannon entropy. This indicates that the hybrid composition for the different elements is attained at different times generating a wide range of spatio-compositional domains which further increase the apparent complexity of the mixing process. Results from the application of the Shannon entropy analysis are compared with the concept of Relaxation of Concentration Variance (RCV), a measure recently introduced in petrology to quantify chemical exchanges during magma mixing. We derive a linear expression relating the change of concentration variance during mixing and the Shannon entropy. We show that the combined use of Shannon entropy and RCV provides the most complete information about the space and time complexity of magma mixing. As a consequence, detailed information about this fundamental petrogenetic and volcanic process can be gathered. In particular, the Shannon entropy can be used as complement to the RCV method to quantify the mobility of chemical elements in magma mixing systems, to obtain information about the rate of production of compositional heterogeneities, and to derive empirical relationships linking the rate of chemical exchanges between interacting magmas and mixing time.

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.

Exponential decay of concentration variance during magma mixing: Robustness of a volcanic chronometer and implications for the homogenization of chemical heterogeneities in magmatic systems

Abstract The mixing of magmas is a fundamental process in the Earth system causing extreme compositional variations in igneous rocks. This process can develop with different intensities both in space and time, making the interpretation of compositional patterns in igneous rocks a petrological challenge. As a time-dependent process, magma mixing has been suggested to preserve information about the time elapsed between the injection of a new magma into sub-volcanic magma chambers and eruptions. This allowed the use of magma mixing as an additional volcanological tool to infer the mixing-to-eruption timescales. In spite of the potential of magma mixing processes to provide information about the timing of volcanic eruptions its statistical robustness is not yet established. This represents a prerequisite to apply reliably this conceptual model. Here, new chaotic magma mixing experiments were performed at different times using natural melts. The degree of reproducibility of experimental results was tested repeating one experiment at the same starting conditions and comparing the compositional variability. We further tested the robustness of the statistical analysis by randomly removing from the analysed dataset a progressively increasing number of samples. Results highlight the robustness of the method to derive empirical relationships linking the efficiency of chemical exchanges and mixing time. These empirical relationships remain valid by removing up to 80% of the analytical determinations. Experimental results were applied to constrain the homogenization time of chemical heterogeneities in natural magmatic system during mixing. The calculations show that, when the mixing dynamics generate millimetre thick filaments, homogenization timescales of the order of a few minutes are to be expected.

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).

Diffusive exchange of trace elements between alkaline melts: Implications for element fractionation and timescale estimations during magma mixing

Abstract The diffusive exchange of 30 trace elements (Cs, Rb, Ba, Sr, Co, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ta, V, Cr, Pb, Th, U, Zr, Hf, Sn and Nb) during the interaction of natural mafic and silicic alkaline melts was experimentally studied at conditions relevant to shallow magmatic systems. In detail, a set of 12 diffusion couple experiments have been performed between natural shoshonitic and rhyolitic melts from the Vulcano Island (Aeolian archipelago, Italy) at a temperature of 1200 °C, pressures from 50 to 500 MPa, and water contents ranging from nominally dry to ca. 2 wt.%. Concentration-distance profiles, measured by Laser Ablation ICP-MS, highlight different behaviours, and trace elements were divided into two groups: (1) elements with normal diffusion profiles (13 elements, mainly low field strength and transition elements), and (2) elements showing uphill diffusion (17 elements including Y, Zr, Nb, Pb and rare earth elements, except Eu). For the elements showing normal diffusion profiles, chemical diffusion coefficients were estimated using a concentration-dependent evaluation method, and values are given at four intermediate compositions (SiO2 equal to 58, 62, 66 and 70 wt.%, respectively). A general coupling of diffusion coefficients to silica diffusivity is observed, and variations in systematics are observed between mafic and silicic compositions. Results show that water plays a decisive role on diffusive rates in the studied conditions, producing an enhancement between 0.4 and 0.7 log units per 1 wt.% of added H2O. Particularly notable is the behaviour of the trivalent-only REEs (La to Nd and Gd to Lu), with strong uphill diffusion minima, diminishing from light to heavy REEs. Modelling of REE profiles by a modified effective binary diffusion model indicates that activity gradients induced by the SiO2 concentration contrast are responsible for their development, inducing a transient partitioning of REEs towards the shoshonitic melt. These results indicate that diffusive fractionation of trace elements is possible during magma mixing events, especially in the more silicic melts, and that the presence of water in such events can lead to enhanced chemical diffusive mixing efficiency, affecting also the estimation of mixing to eruption timescales.