36 research outputs found

    Noble gases in tracking volcanic processess

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    The expressions able to evaluate the contributions of the three natural sources of helium (e.g., atmosphere,in magma, following the solubility model proposed by Nuccio and Paonita (2001). In particular, variations of He/Ne and He/CO2 ratios have been used to compute the initial and final pressure of an ascending magma (Fig. 2) (Caracausi et alii 2003b, Rizzo et alii 2006). This model was subsequently implemented with S, Cl and F (Aiuppa et alii 2004), providing an useful geochemical tool aimed at giving an early warning for forecasting volcanic eruptions. The high helium flux measured at Mefite d’Ansanto in the Irpinian Apennines, displaying a He isotope composition analogous to that of Mt. Vesuvius (which is located only 40 km away), together with the extremely high heat flow, up to 215 mW m-2 (Fig. 3) and the low rock magneto-telluric resistivity values (down to 10 hom m) at depth of 25 km below the axial zone of Apennines,strongly suggests the presence of a crustal magma intrusion, possibly connected to the mantle wedge formed by the sub-ducting Adria slab below the Apennines (Italiano et alii 2000). The heat source of thermal aquifers in western-Sicily were investigated by means of noble gases (Caracausi et alii 2005).In particular it was possible to recognize: - A mantle source, capable to transfer a heat flow of about 36 mW m-2 by conduction through the crust; - a crustal source, where heat is generated by 238U,235U, 232Th and 40K decay, supporting a heat flow of only 6 mW m-2, because the thick sedimentary carbonates containing an average of 1.9 p.p.m. of uranium, 1.2 p.p.m. of thorium and almost no potassium. The measured heat flows in the region give values up to 38 mW m-2 in excess to the computed contributions of these two sources, implying that an additional source most be involved in the generation of the measured heat flow. On the basis of the helium isotope ratios measured in gas dissolved in the thermal waters, we observed a significant mantle helium component, able to shift the typical crustal isotope helium signature from 0.02 Ra (being 1 Ra the in the atmosphere, where the 3He/4He ratio = 1.39×10-6) up to 2.8 Ra. That He isotope ratio crust and mantle) are described, taking into account both the He isotope compositions and abundances of helium, neon and argon. These evaluations are relevant in many geochemical applications, as in volcano monitoring, tectonics and geodynamics. Data acquired during geochemical monitoring of Mt.Etna clearly show synchronous variations of helium isotope composition (Fig. 1) measured in various peripheral gas manifestations (mofettes, mud volcanoes and bubbling gases), located several kilometers apart to each other, indicating an almost pure magmatic helium source and an Etnean plumbing system much more extensive than previously reported. Furthermore, those variations cannot be related to any change of mixing process between magmatic and crustal helium, while they are related to pre-eruptive pulses of magma ascent towards the surface (Caracausi et alii 2003a). Changes of abundance ratios in magmatic gases are interpreted in terms of different solubility of volatiles causi et alii 2005). In particular it was possible to recognize: - A mantle source, capable to transfer a heat flow of about 36 mW m-2 by conduction through the crust; - a crustal source, where heat is generated by 238U,235U, 232Th and 40K decay, supporting a heat flow of only 6 mW m-2, because the thick sedimentary carbonates containing an average of 1.9 p.p.m. of uranium, 1.2 p.p.m. of thorium and almost no potassium. The measured heat flows in the region give values up to 38 mW m-2 in excess to the computed contributions of these two sources, implying that an additional source most be involved in the generation of the measured heat flow. On the basis of the helium isotope ratios measured in gas dissolved in the thermal waters, we observed a significant mantle helium component, able to shift the typical crustal isotope helium signature from 0.02 Ra (being 1 Ra the in the atmosphere, where the 3He/4He ratio = 1.39×10-6) up to 2.8 Ra. That He isotope ratio can be converted in a mantle helium flux, following O’Nions and Oxburg (1981), by solving an appropriate isotope balance equation involving both crustal and mantle helium. The computed mantle helium flux is 2-3 order of magnitudes above the normal flux in a stable continental crust, implying an advetive transport of helium through the crust and consequently the presence of an outgassing melt intrusion into the crust. It is worth of note that a melt intrusion also explains the calculated excess of heat flow. In turn, this require active lithospheric faults having a extensional component (direct faults or at least trans-tensive faults), through which mantle-derived melts could intrude the continental crust, in a region still characterized by a continental collision geodynamics. The explosion crater lake of Monticchio Piccolo was investigated. That two maar was formed about 130,000 years ago, during the last volcanic activity of Mt. Vulture (Italy). The waters of the lake are stratified, having some analogies with the Lake Nyos (Cameroon). Again the estimated heat flow of about 75.4 mW m-2 is slightly in excess, the helium isotope ratios up to 6.1 Ra is indistinguishable from those measured in fluid inclusions of olivines of the maars ejecta, indicating a clear sub-crustal origin. The total He flux is in the order of 1.8×1014 atoms m-2 sec-1, while the 3He flux is of about 1.52×10-9 atoms m-2 sec-1, indicating a relevant mantle helium contribution. A comparison with other crater lakes clearly shows that the 3He/heat ratio, calculated for Monticchio Piccolo lake, is relatively high considering its formation age of 130,000 years, supporting the possibility that, in spite of the long lasting non-activity period, an outgassing melt is present below the crust

    Mantle CO2 degassing at Mt. Vulture volcano (Italy): relationship between CO2 outgassing of volcanoes and the time of their last eruption

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    Mantle volatiles are mainly lost from the Earth to the atmosphere through subaerial and submarine volcanism. Recent studies have shown that degassing of mantle volatiles also occurs from inactive volcanic areas and in tectonically active areas. A new challenge in Earth science is to quantify the mantle-derived flux of volatiles (e.g., CO2), which is important for understanding such diverse issues as the evolution of the atmosphere, the relationships between magma degassing and volcanic activity, gas pressure and seismogenic processes, and the hazards posed by volcanic lakes. Here we present a detailed study of mantle-derived CO2 budget from Mt. Vulture volcano in the Apennines, Italy, which latest eruption occurred 141±11 kyr ago. The relationship between 13CCO2 and total dissolved carbon at Mt. Vulture volcano indicates that the emitted CO2 is a mixture of a biogenic end-member with an average 13CCO2 of about –17‰ and a mantle-derived CO2 end-member with 13CCO2 values from 3‰ to 2‰. These values of mantle-derived 13CCO2 are in the range of those for gas emitted from active volcanoes in the Mediterranean. We calculated the contribution of individual components (CO2 in groundwater, in lakes and from main pools) to the total CO2 budget in the area. We used new measurements of water flow, combined with literature data, to calculate the CO2 flux associated with groundwater, and measured the gas flux from the main pools on the volcanic edifice. Finally, we calculated the CO2 flow in the lakes based on the gradient concentration and eddy diffusivity. The total mantle-derived CO2 budget in the area is 4.85 108 mol yr-1, which is more than double previous estimates. This is higher than those observed in younger volcanic systems elsewhere, thereby supporting the existence of actively degassing mantle melts below Mt. Vulture volcano. A structural map highlights the tectonic control on CO2 flow across the Mt. Vulture volcanic edifice. Indeed, the tectonic discontinuities that controlled the magma upwelling during the most recent volcanic activity are still the main active degassing structures. The new estimate of CO2 budget in the Mt. Vulture area, together with literature data on CO2 budget from historically active and inactive Italian volcanoes, suggests a power-law functional relationship between the age of the most recent volcanic eruption and both total discharged CO2 (R2 = 0.73) and volcano size-normalized CO2 flux (R2 = 0.66). This relation is also valid by using data from worldwide volcanoes highlighting that deep degassing can occur over very long time too. In turn, the highlighted relation provides also an important tool to better evaluate the state of activity of a volcano, which last activity occurred far in time. Finally, our study highlights that in the southern Apennines, an active degassing of mantle-derived volatiles (i.e., He, CO2) occurs indiscriminately from west to east. This is in contrast to the central-northern Apennine, which is characterized by a crustal radiogenic volatile contribution, which increases eastward, coupled to a decrease in deep CO2 flux. This difference between the two regions is probably due to lithospheric tears which control the upwelling of mantle melts, their degassing and the transport of volatiles through the crust

    Elemental and isotope covariation of noble gases in mineral phases from Etnean volcanics erupted during 2001-2005, and genetic relation with peripheral gas discharges

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    During 2001–2005, Mount Etna was characterized by intense eruptive activity involving the emission of petrologically different products from several vents, which involved at least two types of magma with different degrees of evolution. We investigated the ratios and abundances for noble-gas isotopes in fluid inclusions trapped in olivines and pyroxenes in the erupted products. We confirm that olivine has the most efficient crystalline structure for preserving the pristine composition of entrapped gases, while pyroxene can suffer diffusive He loss. Both the minerals also experience noble gas air contamination after eruption. Helium isotopes of the products genetically linked to the two different magmas fall in the isotopic range typical of the Etnean volcanism. This result is compatible with the metasomatic process that the Etnean mantle is undergoing by fluids from the Ionian slab during the last ten kyr, as previously inferred by isotope and trace element geochemistry. Significant differences were also observed among olivines of the same parental magma that erupted throughout 2001–2005, with 3He/4He ratios moving from about 7.0 Ra in 2001 volcanites, to 6.6 Ra in 2004–2005 products. Changes in He abundances and isotope ratios were attributed to variations in protracted degassing of the same magma bodies from the 2001 to the 2004–2005 events, with the latter lacking any contribution of undegassed magma. The decrease in 3He/4He is similar to that found from measurements carried out every fifteen days during the same period in gases discharged at the periphery of the volcano. To our knowledge this is the first time that such a comparison has been performed so in detail, and provides strong evidence of the real-time feeding of peripheral emissions by magmatic degassing

    A helium isotope cross-section study through the Vulture line, southern Apennines

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    We report the results of a geochemical study of gas emissions and spring waters collected along a NE–SW transect through the southern Apennines in order to quantify the contribution of mantle-derived helium in crustal fluids and consequently to evaluate the existence of a structural discontinuity (the “Vulture line”). This tectonic discontinuity is interpreted as N40°–50° trending deep fault, cutting the entire chain-foreland system in southern Apennines (Schiattarella et al., 2005). The lithospheric discontinuity was generated by variation in the velocity of subduction rollback along the length of the subducting plate and has generated a vertical slab window (i.e., Doglioni et al., 1994; Govers and Wortel, 2005; D'Orazio et al., 2007), that is responsible for the origin of Mt. Vulture volcano. Mount Vulture is the eastern-most occurrence of the Quaternary Italian volcanism, and is the only volcano to the east of the Apennine mountain belt. Its volcanic activity started at 742±11 kyr and continued until 142±11 kyr, interrupted by several long inter-eruptive periods (Buettner et al., 2006, and references therein). The volcanism is strongly silica undersaturated, from alkaline potassic to ultrapotassic affinities. We investigated lavas from the Mt. Vulture displaying 3He/4He (up to ~6.0 Ra) and Sr isotopes that are consistent with an origin in mantle that has had minimal pollution from subducted Adriatic slab. This value is rather constant along the history of the volcano, and represent the highest helium isotope signature of the Italian peninsular magmatism even if it is slightly lower than that of the most uncontaminated Sicilian terms. Similar 3He/4He in fluids from around Mt. Vulture indicate that the deep volcanic system is still degassing. The 3He/4He of the investigated fluids along the NE–SW transect of the Vulture line highlights that degassing of mantle-derived helium occur from the Apulian foreland to the Tyrrhenian sea. The highest contribution of mantle-derived fluids is present at Mt. Vulture volcano and the surrounding area, while it decreases toward the Tyrrhenian sea. This may be due to different causes: a) volatiles degassing from near-surface melts beneath Mt. Vulture are quantitatively dominant with respect to crustal gases, in contrast to gas emissions located close to the peri-Thyrrenian area and/or b) the 3He/4He of the peri-Tyrrhenian magmas is expected to be lower than 6 Ra. Our data suggest the active role of Vulture line (lithospheric faults) to transfer towards the surface mantle-derived fluids from magmatic bodies or from asthenospheric upwelling of hot, possible molten material (Ökeler et al., 2009) accumulated to the base of the crust
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