The volatile flushing triggers eruptions at open conduit volcanoes: Evidence from Mount Etna volcano (Italy) (original) (raw)
Related papers
Impulsive Supply of Volatile-Rich Magmas in the Shallow Plumbing System of Mt. Etna Volcano
Minerals
Magma dynamics at Mt. Etna volcano are frequently recognized as the result of complex crystallization regimes that, at shallow crustal levels, unexpectedly change from H2O-undersaturated to H2O-saturated conditions, due to the impulsive and irregular arrival of volatile-rich magmas from mantle depths. On this basis, we have performed hydrous crystallization experiments for a quantitative understanding of the role of H2O in the differentiation of deep-seated trachybasaltic magmas at the key pressure of the Moho transition zone. For H2O = 2.1–3.2 wt %, the original trachybasaltic composition shifts towards phonotephritic magmas never erupted during the entire volcanic activity of Mt. Etna. Conversely, for H2O = 3.8–8.2 wt %, the obtained trachybasalts and basaltic trachyandesites reproduce most of the pre-historic and historic eruptions. The comparison with previous low pressure experimental data and natural compositions from Mt. Etna provides explanation for (1) the abundant release ...
Volatile dilution during magma injections and implications for volcano explosivity
Geology, 2016
Magma reservoirs underneath volcanoes grow through episodic emplacement of magma batches. These pulsed magma injections can substantially alter the physical state of the resident magma by changing its temperature, pressure, composition, and volatile content. Here we examine plagioclase phenocrysts in pumice from the 2014 Plinian eruption of Kelud (Indonesia) that record the progressive capture of small melt inclusions within concentric growth zones during crystallization inside a magma reservoir. High-spatial-resolution Raman spectroscopic measurements reveal the concentration of dissolved H 2 O within the melt inclusions, and provide insights into melt-volatile behavior at the single crystal scale. H 2 O contents within melt inclusions range from ~0.45 to 2.27 wt% and do not correlate with melt inclusion size or distance from the crystal rim, suggesting that minimal H 2 O was lost via diffusion. Instead, inclusion H 2 O contents vary systematically with anorthite content of the host plagioclase (R 2 = 0.51), whereby high anorthite content zones are associated with low H 2 O contents and vice versa. This relationship suggests that injections of hot and H 2 O-poor magma can increase the reservoir temperature, leading to the dilution of melt H 2 O contents. In addition to recording hot and H 2 O-poor conditions after these injections, plagioclase crystals also record relatively cold and H 2 O-rich conditions such as prior to the explosive 2014 eruption. In this case, the elevated H 2 O content and increased viscosity may have contributed to the high explosivity of the eruption. The point at which an eruption occurs within such repeating hot and cool cycles may therefore have important implications for explaining alternating eruptive styles.
Lithos, 2010
Textural and chemical variations of juvenile clasts are widely observed in pyroclastic deposits. In particular, the co-existence of whitish, pumiceous, and dark grey, scoriaceous, juvenile clasts has been observed in many eruptive units of well-known volcanoes (i.e., Somma-Vesuvius, Vulsini, Colli Albani, Stromboli). Here we report the example of the Tufo Giallo della Via Tiberina (TGVT) pyroclastic succession, which comprises two eruptive units emplaced at ca. 561 and 548 ka, during the early explosive activity of the Sabatini Volcanic District (SVD; Roman Province, central Italy). TGVT deposits, as well as underlying pyroclastic products (FAD, ca. 582 ka), are characterized by coexisting whitish pumice and black-grey scoria clasts showing common phonolitic composition but different textural features: white pumice is highly vesicular, vitrophyric, and contains scarce, N 50 µm-sized, feldspar and clinopyroxene crystals, while black-grey scoria is poorly vesicular, highly crystallized, and contains diffuse leucite phenocrysts. The latter records crystallization under H 2 O-undersaturated conditions, as opposed to the vitrophyric texture of white pumice indicating higher temperature and H 2 O concentration. On these grounds, a thermally and H 2 O-zoned pre-eruptive system has been modelled for the phonolitic magma chambers feeding the early SVD events, in which white pumice and black-grey scoria represent the inner and peripheral portions of the reservoirs, respectively. Extensive leucite + clinopyroxene crystallization in the H 2 O-undersaturated, peripheral portions of the reservoirs, resulted in water flux toward the inner zones, where the higher temperature and increasing H 2 O content acted to delay crystallization in the white pumice-feeder magma. The withdrawal of white pumice at the eruption onset produced decompression of the peripheral magma, triggering black-grey scoria eruption during the late phases of explosive events.
Bulletin of Volcanology, 2008
Mount Etna volcano was shaken during the summer 2001 by one of the most singular eruptive episodes of the last centuries. For about 3 weeks, several eruptive fractures developed, emitting lava flows and tephra that significantly modified the landscape of the southern flank of the volcano. This event stimulated the attention of the scientific community especially for the simultaneous emission of petrologically distinct magmas, recognized as coming from different segments of the plumbing system. A stratigraphically controlled sampling of tephra layers was performed at the most active vents of the eruption, in particular at the 2,100 m (CAL) and at the 2,550 m (LAG) scoria cones. Detailed scanning electron microscope and energy dispersive x-ray spectrometer (SEM-EDS) analyses performed on glasses found in tephra and comparison with lava whole rock compositions indicate an anomalous increase in Ti, Fe, P, and particularly of K and Cl in the upper layers of the LAG sequence. Mass balance and thermodynamic calculations have shown that this enrichment cannot be accounted for by “classical” differentiation processes, such as crystal fractionation and magma mixing. The analysis of petrological features of the magmas involved in the event, integrated with the volcanological evolution, has evidenced the role played by volatiles in controlling the magmatic evolution within the crustal portion of the plumbing system. Volatiles, constituted of H2O, CO2, and Cl-complexes, originated from a deeply seated magma body (DBM). Their upward migration occurred through a fracture network possibly developed by the seismic swarms during the period preceding the event. In the upper portion of the plumbing system, a shallower residing magma body (ABT) had chemical and physical conditions to receive migrating volatiles, which hence dissolved the mobilized elements producing the observed selective enrichment. This volatile-induced differentiation involved exclusively the lowest erupted portion of the ABT magma due to the low velocity of volatiles diffusion within a crystallizing magma body and/or to the short time between volatiles migration and the onset of the eruption. Furthermore, the increased amount of volatiles in this level of the chamber strongly affected the eruptive behavior. In fact, the emission of these products at the LAG vent, towards the end of the eruption, modified the eruptive style from classical strombolian to strongly explosive.
Journal of Volcanology and Geothermal Research, 1997
The Villa Senni Eruption Unit (VSEU) belongs to the Tuscolano-Artemisio phase of volcanic activity in the Alban Hills Volcanic District, the closest to Rome of the recent or active volcanoes of central Italy. The most important products of this eruption are represented by pyroclastic flow deposits, named lower and upperflow unit (LFU and UFU, respectively). Three main rock types form VSEU as follows: (1) juvenile K-foiditic scoria clasts of the LFU; (2) juvenile phonotephritic scoria clasts of the UFU; and (3) holocrystalline phonotephritic lithic inclusions (halites) in the UFU. On the basis of the chemistry, mineralogy and petrography of the three studied rock types their phase relations have been discussed. Other petrologic constraints from laboratory melting experiments are presented and used to investigate the role of volatiles on the evolution of the magma chamber system. Some broad implications on withdrawal pattern are also presented. It has been verified the LFU rock type can be obtained from a parental melt of UFU composition by a CO,-controlled crystal-liquid fractionation of a solid assemblage close in composition to that of halites. Because it can be proved that the storage of magma occurred at shallow depth within the Mesozoic carbonate country rocks, it is proposed that CO, diffusion, originating from thermal decomposition of wall-rock carbonates, controlled the evolution trend of the melt at the periphery of the magma chamber, whereas the inner part of the magma body retained the volatile component (essentially H,O) of the original melt. The corresponding eruption model is therefore comprehensive of an early eruptive phase (LFU rock type) involving the more differentiated central magma bulb, whereas the late eruptive phase (UFU rock type) tapped the more ma& peripheral magma. The halite xenoliths are believed to represent the chilled margins of the magma chamber.
The role of magma composition and water content in explosive eruptions
Journal of Volcanology and Geothermal Research, 1998
The role of anhydrous magma composition, water content, and crystal content on the dynamics of explosive eruptions is investigated by modeling the ascent of magma along volcanic conduits and the subsequent pyroclastic dispersion in the w atmosphere, described in a companion paper Neri, A., Papale, P., Macedonio, G., 1998. The role of magma composition x and water content in explosive eruptions: 2. Pyroclastic dispersion dynamics. J. Volcanol. Geotherm. Res., 87, 95-115.. The conduit model used is based on the solution of the fundamental transport equations assuming steady-state and isothermal flow conditions, and includes a composition-based description of magma properties and their variations along the conduit. This study stems from the well-documented vertical compositional variation of many pyroclastic deposits, often associated with reconstructed variations in initial water content. The results of the modeling show complex and sometimes non-intuitive dependence of the distribution of the flow variables on magma composition, crystal and water contents. In general, a water content decrease is expected to produce a decrease in mass flow-rate, decrease in pressure and velocity along the conduit, an increase in the exit gas volume fraction, and a decrease in velocity, pressure, and mixture density at the conduit exit. Reverse variations are expected to occur by decreasing the degree of chemical evolution of the liquid at a constant water content, apart from exit velocities which show more complex variations. The overall effect of increasing crystals is in general similar to that of increasing the degree of chemical evolution of the liquid, or decreasing the water content. The above results are to a large extent interpreted in terms of variations in magma viscosity, which is recognized as the critical magma property besides water content in the dynamics of magma ascent. The common compositional trend of explosive eruptions characterized by chemically evolved, water-richer and crystal-poorer magma erupted first is predicted to be associated with variations in the evolution of the eruption dynamics, depending on the relative magnitude of the changes. However, the exit velocity always decreases in the above trend, and the mass flow-rate increases in most relevant cases, comparing well with the results of chemical and stratigraphic studies of the deposits from explosive eruptions.
2006
, raising the possibility of changing magmatic conditions. Here we decipher the origin and mechanisms of the second eruption from the composition and volatile (H 2 O, CO 2 , S, Cl) content of olivine-hosted melt inclusions in explosive products from its south flank vents. Our results demonstrate that powerful lava fountains and ash columns at the eruption onset were sustained by closed system ascent of a batch of primitive, volatile-rich (!4 wt %) basaltic magma that rose from !10 km depth below sea level (bsl) and suddenly extruded through 2001 fractures maintained opened by eastward flank spreading. This magma, the most primitive for 240 years, probably represents the alkali-rich parental end-member responsible for Etna lavas' evolution since the early 1970s. Few of it was directly extruded at the eruption onset, but its input likely pressurized the shallow plumbing system several weeks before the eruption. This latter was subsequently fed by the extrusion and degassing of larger amounts of the same, but slightly more evolved, magma that were ponding at 6-4 km bsl, in agreement with seismic data and with the lack of preeruptive SO 2 accumulation above the initial depth of sulphur exsolution ($3 km bsl). We find that while ponding, this magma was flushed and dehydrated by a CO 2-rich gas phase of deeper derivation, a process that may commonly affect the plumbing system of Etna and other alkali basaltic volcanoes.
2016
18 Fluids exsolved from mafic melts are thought to be dominantly CO2-H2O ± S fluids. 19 Curiously, although CO2 vapor occurs in bubbles of mafic melt inclusions (MI) at room 20 temperature (T), the expected accompanying vapor and liquid H2O has not been 21 found. We reheated olivine-hosted MI from Mt. Somma-Vesuvius, Italy and quenched 22 Magmatic fluids in melt inclusion-Revision2 2 the MI to a bubble-bearing glassy state. Using Raman spectroscopy, we show that the 23 volatiles exsolved after quenching include liquid H2O at room T and vapor H2O at 150°C. 24 We hypothesize that H2O initially present in the MI bubbles was lost to adjacent glass 25 during local, sub-micron-scale devitrification prior to sample collection. During MI 26 heating experiments, the H2O is redissolved in the vapor in the bubble, where it remains 27 after quenching, at least on the relatively short time scales of our observations. These 28 results indicate that (1) a significant amount of H2O may be stored in...
Petrological investigations of active volcanoes are often supported by mass balance, thermodynamic calculations and/or experiments performed at key conditions. Conversely, the compositions of mineral phases found in natural products are generally used as input data for predictive models calibrated to derive the intensive variables of the magmatic system. In order to evaluate the extent to which mineral chemistry records crystallization conditions, we have compared the compositions of olivine, clinopyroxene, plagioclase and titanomagnetite in 2001-2012 trachybasaltic lavas at Mt. Etna with those obtained through thermodynamic simulations and experiments conducted under anhydrous, water-undersaturated and water-saturated conditions. This systematic comparison allows us to track recent differentiation processes beneath Mt. Etna, as well as the P-T-fO 2 -H 2 O variables controlling the solidification path of magma. Two compositionally distinct populations of olivine and clinopyroxene phenocrysts are found in these lavas: Mg-rich and Mg-poor minerals formed at 600-1100 MPa and 1100-1250°C, and 0.1-500 MPa and 1050-1175°C, respectively. The oxygen fugacity varies by 1-2 log units suggesting water exsolution during magma ascent in the conduit and magma emplacement near the surface. The nucleation and growth of normally zoned plagioclases occur at P b100 MPa, when the amount of H 2 O dissolved in the melt abruptly decreases from about 3.0 to 0.2 wt.% due to magma decompression and degassing. This leads to the conclusion that Etnean magmas fractionate throughout the entire length of the vertically developed plumbing system where magma mixing, volatile exsolution and degassing are the most important processes driving eruptions.
The Role of H 2 O in Subduction Zone Magmatism
Annual Review of Earth and Planetary Sciences, 2012
Water is a key ingredient in the generation of magmas in subduction zones. This review focuses on the role of water in the generation of magmas in the mantle wedge, the factors that allow melting to occur, and the plate tectonic variables controlling the location of arc volcanoes worldwide. Water also influences chemical differentiation that occurs when magmas cool and crystallize in Earth's continental crust. The source of H 2 O for arc magma generation is hydrous minerals that are carried into Earth by the subducting slab. These minerals dehydrate, releasing their bound H 2 O into overlying hotter, shallower mantle where melting begins and continues as buoyant hydrous magmas ascend and encounter increasingly hotter surroundings. This process is controlled by plate tectonic variables that ultimately influence the location of the active volcanic arc above subduction zones. Water also modifies the thermodynamic properties of melts, leading to the unique chemical composition of arc volcanic rocks and Earth's continental crust. 413 Annu. Rev. Earth Planet. Sci. 2012.40:413-439. Downloaded from www.annualreviews.org by Massachusetts Institute of Technology (MIT) on 07/01/12. For personal use only.