We present new thermometers and barometers based on clinopyroxene-liquid equilibria specific to 37 alkaline differentiated magmas. The new models were calibrated through regression analyses of 38 experimental datasets obtained by merging phase equilibria experiments from literature with new 39 experiments performed by using trachytic and phonolitic starting compositions. The regression 40 strategy was twofold: i) we have tested previous thermometric and barometric equations and 41 recalibrated these models using the new datasets; ii) we have calibrated a new thermometer and a 42 new barometer including only regression parameters that closely describe the compositional 43 variability of the datasets. The new models yield more precise estimates than previous 44 thermometers and barometers when used to predict temperatures and pressures of alkaline 45 differentiated magmas. We have tested the reliability of the new equations by using clinopyroxene-46 liquid pairs from trachytes and phonolites erupted during major explosive eruptions at the Phlegrean 47 Fields and Mt. Vesuvius (central Italy). The test yielded crystallization conditions comparable to 48 those determined by means of melt and fluid inclusion analyses and phase equilibria studies; this 49 validates the use of the proposed models for precise estimates of crystallization temperatures and 50 pressures in differentiated alkaline magmas. Because these magmas feed some of the most 51 voluminous, explosive, and threatening volcanic eruptions in the world, a better understanding of 52 the environmental conditions of their reservoirs is mandatory and this is now possible with the new 53 models provided here. 54 55 56 table 2 Click here to download table: Table 2.doc

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The intense explosive and effusive volcanic activity of the last 1000 years at La Fossa volcano (Vulcano Island, Italy) was characterized by the eruption of magmas ranging in composition from latites to trachytes and rhyolites, as well as K-rich trachytes. Evidence of syn-eruptive mixing among these magmas is frequently observed in the form of magmatic enclaves and bands in lava flows and pyroclastic products. The petrological and volcanological diversity of the erupted materials suggests that complex differentiation processes occurred in the shallow part of the plumbing system. With the aim to reconstruct the magmatic feeding system and to identify the differentiation processes behind such a petrologic complexity, we analysed lavas and pyroclastic products representative of the recent eruptive sequences at La Fossa and combined the petrochemical features with thermo-barometric calculations, geochemical modelling and temperature gradient experiments. Thermo-barometric calculations indicate that the K-rich trachytic magma crystallized at lower pressure (160 ± 54 MPa) compared to the latitic (307 ± 47 MPa) and trachytic (208 ± 30 MPa) magmas. Differentiation modelling suggests that both trachytic and rhyolitic compositions can be obtained through differentiation of a common latitic magma, essentially by varying the plagioclase/ sanidine ratio. Temperature gradient experiments, performed at the conditions inferred for the shallow plumbing system of La Fossa volcano (150 MPa and 1050-900 °C), indicate different paths of melt differentiation that overall produce an increase of the SiO 2 /K 2 O ratio with the increasing H 2 O in the system (from 0 to 4 wt.%). This is consistent with the origin of K-rich trachytes at lower pressure and lower H 2 O content. In turn, the formation of crystal-poor rhyolites is explained by the segregation of the interstitial melt formed in a latitic-trachytic crystal mush, favoured by the second boiling of the melt and consequent exsolution of a fluid phase.

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Bubble growth strongly affects the physical properties of degassing magmas and their eruption dynamics. Natural samples and products from quench experiments provide only a snapshot of the final state of volatile exsolution, leaving the processes occurring during its early stages unconstrained. In order to fill this gap, we present in situ high-temperature observations of bubble growth in magmas of different compositions (basalt, andesite and rhyodacite) at 1,100 to 1,240 °C and 0.1 MPa (1 bar), obtained using a moissanite cell apparatus. The data show that nucleation occurs at very small degrees of supersaturaturation (<60 MPa in basalt and andesite, 200 MPa in rhyodacite), probably due to heterogeneous nucleation of bubbles occurring simultaneously with the nucleation of crystals. During the early stages of exsolution, melt degassing is the driving mechanism of bubble growth, with coalescence becoming increasingly important as exsolution progresses. Ostwald ripening occurs only at the end of the process and only in basaltic melt. The average bubble growth rate (G R) ranges from 3.4 × 10-6 to 5.2 × 10-7 mm/s, with basalt and andesite showing faster growth rates than rhyodacite. The bubble number density (N B) at nucleation ranges from 7.9 × 104 mm-3 to 1.8 × 105 mm-3 and decreases exponentially over time. While the rhyodacite melt maintained a well-sorted bubble size distribution (BSD) through time, the BSDs of basalt and andesite are much more inhomogeneous. Our experimental observations demonstrate that bubble growth cannot be ascribed to a single mechanism but is rather a combination of many processes, which depend on the physical properties of the melt. Depending on coalescence rate, annealing of bubbles following a single nucleation event can produce complex bubble size distributions. In natural samples, such BSDs may be misinterpreted as resulting from several separate nucleation events. Incipient crystallization upon cooling of a magma may allow bubble nucleation already at very small degrees of supersaturation and could therefore be an important trigger for volatile release and explosive eruptions. © 2014 Springer-Verlag Berlin Heidelberg

To understand the solidification processes of natural magma and the texture evolution of igneous rocks, we have carried out in situ observation of the crystallization of a high-K basaltic melt cooling from ~1,240 °C in a moissanite cell. In a series of experiments with different thermal history, olivine or clinopyroxene (cpx) appeared as the liquidus phase before the formation of plagioclase. During cooling at 100 °C/h, the morphology of olivine and cpx transited from tabular to hopper habit. To first order approximation, crystal grow rate (2 × 10-9 to 7 × 10-9 m/s for olivine and 6 × 10-9 to 17 × 10-9 m/s for cpx), probably limited by chemical diffusion, is proportional to crystal size. In one experiment dominated by olivine crystallization, the good image quality allows the analysis of texture evolution over an extended period. Nucleation of olivine occurred only in a narrow temperature and time interval below the liquidus. Two-dimensional length- and area-based crystal size distributions (CSDs) show counterclockwise rotation around axes of 8 μm and 100 μm2, which is consistent with the proportionate crystal growth. Both CSDs and direct observation show the dissolution of small crystals and Ostwald ripening. These data suggest that conventional analyses of crystal size distributions of igneous rocks may be in error-the slope of the CSD cannot be interpreted in terms of a uniform growth rate, and the intercept with the vertical axis does not correspond to a nucleation density. © 2014 Springer-Verlag Berlin Heidelberg

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