1,721,055 research outputs found
A new hydrothermal moissanite cell apparatus for optical in-situ observations at high pressure and high temperature, with applications to bubble nucleation in silicate melts
No full textSolidification fronts in thermally zoned magma chambers: the case of Sabatini Volcanic District (central Italy)
No full textThe 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)
Anhydrite solubility in differentiated arc magmas
Full text linkThe solubility of anhydrite in differentiated arc magmas was experimentally studied at 200MPa and 800-1000°C over a range of oxygen fugacities, from 0.5 log units above the Ni-NiO buffer to the hematite-magnetite buffer. Anhydrite is stable only at oxidizing conditions (fO2≥Re-ReO2), whereas sulfides only form under reducing conditions. The solubility of anhydrite in the melt ultimately regulates the amount of sulfur available to partition between melt and fluid phase during the eruption. At oxidizing conditions, the solubility product of anhydrite increases with temperature, nbo/t and melt water content. We provide a new calibration of the anhydrite solubility product (KSP=XCaO*XSO3), which reproduces all available experimental data with greatly improved accuracy:lnKSPAnhydrite=8.95-146.5nbot-2.696104T(K)+19.72nbot104T(K)+0.409·H2O(wt.%)In this equation, the molar fractions XCaO and XSO3 in the melt as well as the number of non-bridging oxygen atoms per tetrahedron (nbo/t) are calculated on an anhydrous basis (H2O refers to the melt water content, T is temperature in Kelvin). We apply our model to estimate the sulfur yield of some recent volcanic eruptions and we show that the sulfur yield of the 1991 Mt. Pinatubo dacite eruption was unusually large, because only a small fraction of the sulfur was locked up in anhydrite. In general, high sulfur yields are expected when anhydrite solubility in the melt is high, i.e. for somewhat depolymerized melts. For rhyolitic systems, most of the available sulfur will be locked up in anhydrite, so that even very large eruptions may only have a small effect on global surface temperatures. Our model therefore allows improved predictions of the environmental impact of explosive volcanic eruptions
Magma differentiation in shallow, thermally zoned magma chambers: the example of Sabatini Volcanic District (central Italy)
Full text linkThe 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
Experimental insights into the origin of crystal-poor phonolitic magmas
No full textCrystal-poor phonolitic magmas have been commonly erupted during the quaternary explosive volcanism of Central Italy. The origin of crystal-poor magmas represents a complex issue of igneous petrology and since the first studies on crystal fractionation by settling, many alternative mechanisms of crystal-melt separation have been proposed [see Bachmann and Bergantz, 2004 and references therein]: i) convective fractionation in a crystallizing boundary layer; ii) gas-driven filter press; iii) thermal gradient responsible for mass transport (thermal migration) resulting in segregation of melt from the mushy, boundary zone of magma chambers; iv) melt migration induced by crystal compaction; v) instability of “solidification front”. In the frame of the highly explosive volcanism of Central Italy, that was fed by differentiated and thermally zoned pre-eruptive systems, not all of the abovementioned mechanisms would operate efficiently. Moreover, these mechanisms are mainly based on theoretical models and natural evidences, but poorly constrained by experiments.
In this work, in order to shed light on the origin of large volumes of highly differentiated, crystal-poor magmas in thermally zoned systems, we have experimentally investigated crystallization, differentiation, and crystal-melt separation in the presence of a thermal gradient. Melt differentiation has been investigated under isothermal conditions (i.e. phase equilibria experiments) as well. As a case study, we have used the Sabatini Volcanic District, one of the main volcanic districts of the Roman Province characterized by several explosive eruptions producing large volumes of crystal-poor phonolitic magma [Masotta et al. 2010]. Phase equilibria experiments constrained the liquid line of descent leading to phonolitic magmas but gave no insights into the origin of crystal-poor textures. On the contrary, thermal gradient experiments produced structures clearly showing crystal-rich textures overlaid by crystal-poor batches of differentiated melt. The structures pictured at the cool top of the charges remind the so-called solidification front [Marsh, 1996].
Thermal gradient experiments, being performed under controlled conditions, allowed us to determine parameters necessary to model the processes responsible for the formation of these batches of differentiated, crystal-poor melt. We advocate that these experiments represent an important tool to model crystal-melt separation in thermally zoned natural systems as well
Fluid-melt partitioning of sulfur in differentiated arc magmas and the sulfur yield of explosive volcanic eruptions
Full text linkThe 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
Ancient to modern metallurgical slags: evolving smelting techniques and their interaction with the environment
No full textThe discovery of metals and how to extract and use them was a turning point in human history, because it changed the economy and socio-cultural structure of ancient civilisations and started to severely affect the impact of human activities on the environment. In fact, a lot of societies developed near extraction sites and founded their economy on the use and trade of metals.
In Tuscany (Italy) there has been a long history of mining and metal extraction. From archaeological studies it has been reconstructed that the earliest records of these activities date back to the Etruscan period (VII century B.C.). Exploitation continued intermittently until a few decades ago. This extended period of mining exploitation left a wealth of both iron and copper metallurgical slags that can usually be found as abandoned and unsupervised heaps.
These slags, apparently just a waste from the metallurgical process, actually carry information about the evolution of the metallurgical process through which they were generated. Information about the charge, flux and fuel can be inferred from chemical and mineralogical composition of the slags.
Slags from three different smelting districts, ranging from ancient Etruscan-Roman period to modern age (1900 A.D.) were studied macroscopically, identifying distinctive features related to the smelting process in different time periods. Then, thin sections obtained from representative samples were examined, using optical microscopy and electron microscopy. Chemical analyses were performed for major and trace elements by X-ray fluorescence spectroscopy and by inductively coupled plasma mass spectrometry, respectively.
Leaching experiments on some carefully selected samples were also completed, to investigate the release of potentially toxic elements during the interaction of the slags with the surrounding environment.
This kind of investigation allows to reconstruct part of the history of metal utilisation as well as to predict the impact that these remains will have on the environment
Sulfur budget in differentiated arc magmas
No full textSulfur compounds (H2S and SO2) constitute a major fraction of volcanic emissions, with abundance next only to H2O and CO2. Explosive eruptions can inject large amounts of sulfur into the stratosphere, inducing atmospheric perturbations that may eventually result in changes of the Earth’s average temperature [1]. Before being released during volcanic eruptions, sulfur can be stored in the silicate melt or in a separate fluid phase [2]. Sulfur contained in sulfur-bearing minerals is usually not released during eruptions because the decomposition reactions are not efficient at the eruptive timescale. The solubility of these minerals in the silicate melt is, however, important because it directly controls the concentration of sulfur in the melt and, indirectly, the amount of sulfur that partitions in the fluid phase. Sulfur partition coefficients for most of the differentiated arc magmas at oxidizing conditions are poorly constrained and large uncertainties exist because of the experimental difficulty to get reliable results.
Here we present new experimental data on anhydrite solubility [3] and fluid-melt partition coefficients of sulfur in differentiated arc magma compositions. Melt polymerization, temperature and water content strongly influence the distribution of sulfur among minerals, fluid and melt, by controlling both the anhydrite solubility and the fluid-melt partitioning. Sulfur in less polymerized melts is preferentially accumulated in the melt, because of the high anhydrite solubility and low partion coefficient. At increasing melt polymerization, the solubility of anhydrite decreases and sulfur is preferentially partitioned into the fluid phase or locked up in anhydrite. The interplay between anhydrite solubility and sulfur partitioning yields a maximum distribution of sulfur in the fluid phase at intermediate nbo/t (between 0.10 and 0.15)
Crustal Magmatic System Evolution: Anatomy, Architecture, and Physico‐Chemical Processes
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