Optimizing pre-eruptive temperature estimates in thermally and chemically zoned magma chambers

We present a method to minimize the error of temperature estimate when multiple discrete populations of glass and clinopyroxene occur in a single heterogeneous eruptive unit. As test data we have used ~. 1100 clinopyroxene-melt pairs from isothermal and thermal gradient experiments. These latter are characterized by the crystallization of multiple modes of clinopyroxene as frequently documented for chemically and thermally zoned magma chambers. Equilibrium clinopyroxene-melt pairs are identified through the difference between predicted and measured components in clinopyroxene. The use of these equilibrium compositions as input data for one of the most recent clinopyroxene-based thermometers demonstrates that the error of temperature estimate is minimized and approaches to the calibration error of the thermometric model. To emphasize the paramount importance of this method for predicting the crystallization temperature of heterogeneous magmas, we have tested for equilibrium ~. 480 and ~. 150 clinopyroxene-melt pairs from the explosive eruptions of the Sabatini Volcanic District (Latium Region, Central Italy) and the Campi Flegrei Volcanic Field (Campanian Region, Southern Italy), respectively. These eruptions were fed by zoned magma chambers, as indicated by the occurrence of multiple modes of clinopyroxene in the eruptive units. Results from calculations demonstrate that clinopyroxene-melt pairs in equilibrium at the time of eruption are effectively captured by our method and, consequently, the error of temperature estimate is significantly reduced. © 2014 Elsevier B.V

In situ observations of bubble growth in basaltic, andesitic and rhyodacitic melts

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

Fluid-melt partitioning of sulfur in differentiated arc magmas and the sulfur yield of explosive volcanic eruptions

The fluid-melt partitioning of sulfur (DSfluid/melt) in differentiated arc magmas has been experimentally investigated under oxidizing conditions (Re-ReO2 buffer) from 800 to 950°C at 200MPa. The starting glasses ranged in composition from trachyte to rhyolite and were synthesized targeting the composition of the residual melt formed after 10-60% crystallization of originally trachy-andesitic, dacitic and rhyodacitic magmas (Masotta and Keppler, 2015). Fluid compositions were determined both by mass balance and by Raman spectroscopy of fluid inclusions. DSfluid/melt increases exponentially with increasing melt differentiation, ranging from 2 to 15 in the trachytic melt, from 20 to 100 in the dacitic and rhyodacitic melts and from 100 to 120 in the rhyolitic melt. The variation of the DSfluid/melt is entirely controlled by the compositional variation of the silicate melt, with temperature having at most a minor effect within the range investigated. Experiments from this study were used together with data from the literature to calibrate the following model that allows predicting DSfluid/melt for oxidized arc magmas: lnDSfluid/melt=9.2-31.4·nbot-1.8·ASI-29.5·Al#+4.2·Ca#where nbot is the non-bridging oxygen atoms per tetrahedron, ASI is the alumina saturation index, Al# and Ca# are two empirical compositional parameters calculated in molar units (Al#=XAl2O3XSiO2+XTiO2+XAl2O3 and Ca#=XCaOXNa2O+XK2O).The interplay between fluid-melt partitioning and anhydrite solubility determines the sulfur distribution among anhydrite, melt and fluid. At increasing melt polymerization, the exponential increase of the partition coefficient and the decrease of anhydrite solubility favor the accumulation of sulfur either in the fluid phase or as anhydrite. On the other hand, the higher anhydrite solubility and lower partition coefficient for less polymerized melts favor the retention of sulfur in the melt. At equilibrium conditions, these effects yield a maximum of the sulfur fraction in the fluid phase for slightly depolymerized melts (nbot= 0.05-0.15). Our data allow quantitative predictions of the sulfur yield of explosive volcanic eruptions over a wide range of magma compositions

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. © 2013 Springer-Verlag Berlin Heidelberg

Major explosive activity in the Monti Sabatini Volcanic District(central Italy) over the 800-390ka interval: Geochronological-geochemical overview and tephrostratigraphic implications

A review of the existing chronological, stratigraphic and chemo-petrologic data of the major eruptive units from the early phase of activity (800-390ka) in the Monti Sabatini Volcanic District (MSVD), belonging to the ultra-potassic magmatic region of central Italy, is presented along with new radioisotopic age determinations and geochemical analyses. Through the combined use of electron microprobe glass compositions, selected trace-element compositions, and single-crystal 40Ar/39Ar age determinations, we provide a new chrono- and chemo-stratigraphic classification of the products emplaced in the 800-390ka time interval. Besides giving insights on the petrologic evolution of the Roman Comagmatic Region, the large dataset provides fundamental information that is applicable to tephrostratigraphic studies in the wide region encompassing the Tyrrhenian Sea margin to the Adriatic Sea basin. Distal tephras from this volcanic activity also act as important geochronologic markers for the coastal sedimentary successions deposited in response to glacio-eustatic fluctuations, as well as for successions in the Quaternary tectonic basins of the Central and Southern Apennines. An innovative approach based on the use of discrimination diagrams of Zr/Y vs Nb/Y ratios for fingerprinting altered volcanic rocks - recently developed and successfully employed in archaeometric studies - is here combined to the glass compositions for classifying the MSVD deposits and tested on two distal tephra layers, showing its potentiality for tephrostratigraphic correlation. © 2014 Elsevier Ltd

CO2 bubble generation and migration during magma–carbonate interaction

We conducted quantitative textural analysis of vesicles in high temperature and pressure carbonate assimilation experiments (1200 °C, 0.5 GPa) to investigate CO2 generation and subsequent bubble migration from carbonate into magma. We employed Mt. Merapi (Indonesia) and Mt. Vesuvius (Italy) compositions as magmatic starting materials and present three experimental series using (1) a dry basaltic-andesite, (2) a hydrous basaltic-andesite (2 wt% H2O), and (3) a hydrous shoshonite (2 wt% H2O). The duration of the experiments was varied from 0 to 300 s, and carbonate assimilation produced a CO2-rich fluid and CaO-enriched melts in all cases. The rate of carbonate assimilation, however, changed as a function of melt viscosity, which affected the 2D vesicle number, vesicle volume, and vesicle size distribution within each experiment. Relatively low-viscosity melts (i.e. Vesuvius experiments) facilitated efficient removal of bubbles from the reaction site. This allowed carbonate assimilation to continue unhindered and large volumes of CO2 to be liberated, a scenario thought to fuel sustained CO2-driven eruptions at the surface. Conversely, at higher viscosity (i.e. Merapi experiments), bubble migration became progressively inhibited and bubble concentration at the reaction site caused localised volatile over-pressure that can eventually trigger short-lived explosive outbursts. Melt viscosity therefore exerts a fundamental control on carbonate assimilation rates and, by consequence, the style of CO2-fuelled eruptions

The Viscosity of Carbonate‐Silicate Transitional Melts at Earth's Upper Mantle Pressures and Temperatures, Determined by the In Situ Falling‐Sphere Technique

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Magma differentiation in shallow, thermally zoned magma chambers: the example of Sabatini Volcanic District (central Italy)

The complexity of volcanism in central Italy animated the scientific debate during the last decades. The peculiar potassium-rich magmatism of the Roman Province (Peccerillo, 2005) aimed numerous scientific contributions focused on the petrology of the various volcanic districts. Among these, the Sabatini Volcanic District (hereafter SVD) is one of the largest, being characterized by a number of explosive eruptions emplaced during the last 800 kyr that produced pyroclastic deposits cropping out in a widespread area at north of Rome (~1800 km2, Sottili et al., 2010). During these explosive eruptions (mostly known as yellow tuffs and red tuffs), large volumes of phonolitic magmas were emplaced (an average of 10 km3 dense rock equivalent of magma). The interest for these pyroclastic deposits (quarried since the ancient Roman age) promoted detailed studies, mainly on stratigraphy and geochronology (Karner et al., 2001; Sottili et al., 2010), whereas petrological studies are scarce. The petrological studies are, indeed, limited to lava flows and scoria cones with primitive chemical composition (i.e., Cundari, 1979; Conticelli & Peccerillo, 1992; Conticelli et al., 1997) that represent only a small fraction (less than 10%) of the total volume of the emplaced products. One of the major features of SVD pyroclastic deposits is the textural variability of juvenile clasts. These deposits are commonly characterized by a transition from crystal-poor juvenile clasts at their bottom, toward crystal-rich ones at the top. In general, phonolitic volcanism offers numerous examples of pyroclastic successions showing analogue textural variations of the juvenile component, often accompanied by chemical variation of the juvenile clasts. These variations are commonly interpreted in the light of compositional variation of the erupted magma, resulting from the compositional and/or thermal layering of shallow magma chambers. However, zoning models based on compositional variations invoked for these magmatic systems, do not strictly apply in the case of SVD eruptions, given that no significant chemical variation is observed between the crystal-poor and the crystal-rich juvenile fraction. In addition to the problem of textural variations of the juvenile component, it comes up the paradox on the genesis of crystal-poor, differentiated magmas. Crystallization is intrinsic in the differentiation, but crystal fractionation may be not so obvious in a differentiated magma (i.e., low contrast of densities between melt and crystal, high viscosity of the melt). Hence, mechanisms alternative to crystal settling need to be found to explain the formation of crystal-poor, differentiated magmas. In this study, the products from large explosive eruptions of SVD were collected and investigated in detail. Phase equilibria experiments, coupled with MELTS simulations (Ghiorso & Sack, 1995), were used to constrain both differentiation and pre-eruptive conditions of the phonolitic system. Moreover, temperature gradient experiments were performed with the aim to mimic conditions occurring in a thermally zoned magma chamber and explain the observed textural variation in the deposits. Through the coupling of natural and experimental data, magmatic processes occurring in the shallow, thermally zoned magma system of SVD were modelled, addressing the problems of melt differentiation, crystal-melt separation and achievement of pre-eruptive conditions of phonolitic magmas feeding large explosive eruptions

Solidification fronts in thermally zoned magma chambers: the case of Sabatini Volcanic District (central Italy)

The thermal evolution of magma chambers and its effect on magmatic processes is of major interest for igneous petrology. Theoretical and experimental studies have demonstrated that magma crystallization occurs mostly at the peripheral, cooler regions of magma chambers, where the development of solidification fronts determine the physical evolution of the magmatic system and magma differentiation as well. Solidification fronts, indeed, represent the connection between subvolcanic and volcanic realms, linking the ipoabyssal crystallization of the magma body with the formation of shallow reservoirs, responsible for the eruption of large volumes of crystal-poor, silicic magmas. To explain the formation of these magmas, a mere process of crystal fractionation (thought as a settling of crystals) may result inadequate. Conversely, the extensive crystallization in the solidification front and consequent extraction of differentiated interstitial melt is more likely. When erupted, crystal-poor silicic magmas carry with them fragments of the solidification fronts where they originated, in the form of crystal-rich lithic enclaves. The textural and chemical variability of these rocks provide useful information on the crystallizing conditions in the solidification front, which can be used to constrain the mechanisms of differentiation and pre-eruptive conditions. In this work, the phonolitic volcanism of Sabatini Volcanic District (central Italy) is considered as study case to investigate the formation and evolution of a solidification front. Lithic enclaves emplaced in the course of major explosive eruptions have been analyzed, and natural evidences have been combined with numerical models of the cooling process and experimental simulations of crystallization along temperature gradients. The textural heterogeneity of lithic enclaves reveals the variable crystallization conditions in the solidification front and indicates the coexistence of two magmas at different degree of evolution, produced during the in situ differentiation of the solidification front. This heterogeneity also accounts for efficient crystal-melt separation processes, responsible of the extraction and accumulation of crystal-poor, silicic melt either into isolated batches (i.e., silicic lenses) or voluminous reservoirs (i.e., silicic cap at the roof of the magma chamber)