1,721,041 research outputs found
Towards a reconstruction of the magmatic feeding system of the 1944 eruption of Mt Vesuvius
No full textAbstract
Geochemistry and mineralogy of both juvenile clasts and xenoliths ejected during the 1944 eruption of Mt Vesuvius, provide major contraints on the magmatic feeding system. Melt inclusions in phenocrysts of juvenile scoriae highlight that the magmas feeding the eruption underwent differentiation at different pressures. A K-tephritic volatile-rich melt evolved to reach K-phonotephritic composition, at pressures higher than 300 MPa, before being fed into a very shallow reservoir (P< 100MPa) in which it mixed with the low-volatile resident K-phonotephritic magma. The newly arrived magma forced the transition from the effusive to the lava fountain phase of the 1944 eruption. The outer portion of the shallow reservoir is formed by a crystallizing margin (glass-bearing fergusites). From this, the transition to the carbonate country rocks occurs throughout a front of infiltration of magmatic melts in porous decarbonating host rocks. Magmatic melts are contaminated by the addition of Ca and Mg deriving from decarbonation reactions and/or melting of host rocks. These modified melts metasomatize the carbonates inducing skarn reactions and forming an endoskarn shell. Isotope (Sr, O) composition of juvenile products and xenoliths marks the amount of magma-carbonate interaction. Isotopic data pointed out that the crystallizing margin of the chamber may present a very limited contamination by carbonates. This suggests that the main volume of magma hosted in the magma chamber did not suffer any mass exchange with the wall rocks
Basaltic volcanoes as large-scale aerators: the example of Mt. Etna
No full textThe recognition and simulation of the patterns of gas release from active volcanoes in relation to those of magma supply and transfer are a major geochemical goal. At basaltic volcanoes such as Mt. Etna (Sicily, Italy) this knowledge would greatly assist our comprehension of the mechanisms of magma rise and injection at different storage levels, from depth up to the shallow systems feeding lava fountains and flows. In this contribution we investigate the H2O-CO2-SO2-H2S-silicate melt system by integrating theoretical models on volcanic degassing with data from plume chemistry, fumarole sampling, chemistry and volatile contents of melt inclusions (MIs). Given an initial bulk composition, we show that the degassing processes behind this ensemble of data can be quantitatively assessed by carefully evaluating the interplay of 1) crystallization, hence phase proportions, 2) redox variables, 3) gas addition (flushing) occurring at different steps along the magmatic column. Because of pervasive CO2-flushing through the magma, we picture Mt. Etna as a big aerator, by analogy with gas absorption techniques in chemical process engineering. CO2-flushing is particularly efficient at P > 140 MPa, where the volatile influx generates a family of degassing paths that embrace the range of variability displayed by H2O, CO2 and S contents dissolved in MIs. The flushing mechanism can work under two extreme scenarios: in one case the rising volatile phase is completely blocked by the shallower magmas, whereas in the other one at each addition of deep gas the pre-existing gas phase is completely separated from the flushed magma. This study shows that equilibrium thermodynamics provides reasonable physico-chemical constraints to interpret the ensemble of data observed, without invoking diffusive regimes acting far from the equilibrium. This is a strong argument for the joint adoption of MI-based volatile contents and volatile-melt saturation algorithms. The results of this study are expected to shed light on the close association between basaltic volcanism and the release of large amounts of carbon dioxide
Mafic magma batches at Vesuvius: a glass inclusion approach to the modalities of feeding potassic strato-volcanoes
No full textMafic magma batches at Vesuvius. A glass inclusion approach to the modalities of feeding stratovolcanoes
No full textInput of deep-seated volatile-rich magmas and dynamics of violent strombolian eruptions at Vesuvius
No full text
Degassing vs. eruptive styles at Mt. Etna volcano (Sicily, Italy). Part I: Volatile stocking, gas fluxing, and the shift from low-energy to highly explosive basaltic eruptions
No full textBasaltic magmas can transport and release large amounts of volatiles into the atmosphere, especially in subduction zones, where slab-derived fluids enrich the mantle wedge. Depending on magma volatile content, basaltic volcanoes thus display a wide spectrum of eruptive styles, from common Strombolian-type activity to Plinian events. Mt. Etna, in Sicily, is a typical basaltic volcano where the volatile control on such a variable activity can be investigated. Based on a melt inclusion study in products from Strombolian or lava-fountain activity to Plinian eruptions, here we show that for the same initial volatile content, different eruptive styles reflect variable degassing paths throughout the composite Etnean plumbing system. The combined influence of i) crystallization, ii) deep degassing and iii) CO2 gas fluxing can explain the evolution of H2O, CO2, S and Cl in products from such a spectrum of activity. Deep crystallization produces the CO2-rich gas fluxing the upward magma portions, which will become buoyant and easily mobilized in small gas-rich batches stored within the plumbing system. When reaching gas-dominated conditions (i.e., a gas/melt mass ratio of ~ 0.3 and CO2,gas/H2Ogas molar ratio ~ 5), magma batches will erupt effusively or mildly explosively. In case of the 122 BC Plinian eruption, open-system degassing conditions took place within the plumbing system, such that earlier CO2-fluxing determined gas accumulation on top of the magmatic system, likely followed by H2O-fluxing further hydrating the shallow magma. The emission of such a cap in the early eruptive phase triggered the arrival of deep H2O-rich magma whose fast decompression and bubble nucleation led to the highly explosive character, enhanced by abundant microlite crystallization and consequent increase of magma effective viscosity. This could explain why open system basaltic systems like Etna may experience highly explosive or even Plinian episodes during eruptions that start with effusive to mildly explosive phases. The proposed mechanism also determines a depression of chlorine contents in CO2-fluxed (and less explosive) magmas with respect to those feeding Plinian events like 122 BC. The opposite is seen for sulfur: low to mild-explosive fluxed magmas are S-enriched, whereas the 122 BC Plinian products are relatively S-poor, likely because of early sulfide separation accompanying magma crystallization. The proposed mechanism involving CO2 separation and fluxing may suggest a subordinate role for variable mixing of different sources having different degrees of K-enrichment. However, such a mechanism requires further experimental studies about the effects on S and Cl dissolution and does not exclude self-mixing between degassed and undegassed parcels within the Etna plumbing system. Finally, our findings may represent a new interpretative tool for the geochemical and petrologic monitoring of plume gas discharges and melt inclusions, and allow tracking the switch from mild-explosive to highly explosive or even Plinian events at Etna
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