Noble gases in tracking volcanic processess

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.