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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, B08204, doi:10.1029/2004JB003542, 2005
Carbon dioxide diffuse degassing and estimation of heat release from volcanic and hydrothermal systems G. Chiodini, D. Granieri, R. Avino, S. Caliro, and A. Costa Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, Naples, Italy
C. Werner Institute of Geological and Nuclear Sciences, Taupo, New Zealand Received 16 November 2004; revised 19 April 2005; accepted 3 May 2005; published 16 August 2005.
 We present a reliable methodology to estimate the energy associated with the subaerial
diffuse degassing of volcanic-hydrothermal fluids. The fumaroles of 15 diffuse degassing structures (DDSs) located in eight volcanic systems in the world were sampled and analyzed. Furthermore, each area was measured for soil temperature gradients and for soil CO2 fluxes. The results show that each hydrothermal or volcanic system is characterized by a typical source fluid which feeds both the fumaroles and diffuse degassing through the soil. Experimental data and the results of physical numerical modeling of the process demonstrate that the heat released by condensation of steam at depth is almost totally transferred by conduction in the uppermost part of the soil. A linear relationship is observed between the log of the steam/gas ratio measured in the fumaroles and the log of the ratio between soil thermal gradient and soil-gas flux. The main parameter controlling this relation is the thermal conductivity of the soil (Kc). For each area, we computed the values of Kc which range from 0.4 to 2.3 W m1 C1. Using the CO2 soil fluxes as a tracer of the deep fluids, we estimated that the total heat released by steam condensation in the systems considered varies from 1 to 100 MW. Citation: Chiodini, G., D. Granieri, R. Avino, S. Caliro, A. Costa, and C. Werner (2005), Carbon dioxide diffuse degassing and estimation of heat release from volcanic and hydrothermal systems, J. Geophys. Res., 110, B08204, doi:10.1029/2004JB003542.
1. Introduction  The energy released through hydrothermal and volcanic gaseous emissions is an important component of the energy balance of quiescent volcanoes. Typically energy balances of volcanoes are derived from mass balance calculations at crater lakes. In previous studies at 14 crater lakes, energy outputs ranged from 0.54 108 to 3.85 108 W [Brown et al., 1989; Sheperd and Sigurdsson, 1978; Hurst et al., 1991; Pasternack and Varekamp, 1997]. In four volcanic systems without crater lakes (Vulcano, Solfatara di Pozzuoli, Nisyros, Ischia), the thermal energy released was estimated to range from 0.23 108 to 1.39 108 W [Caliro et al., 2004; Chiodini et al., 2004, 2001a, 1996] based on measurements of volcanic-hydrothermal CO2 released through soil diffusion emission. For instance, the energy dissipated daily by the diffuse degassing structure of Solfatara di Pozzuoli (0.5 km2) is the main source of energy in the entire Campi Flegrei caldera in the current period of quiescence. According to Chiodini et al. [2001a], the thermal energy released daily is one order of magnitude greater than the elastic energy released during 1983 – 1984 seismic crisis, greater than the energy associated with ground deformation, and is about 10 times grater than the conductive heat flux over the Copyright 2005 by the American Geophysical Union. 0148-0227/05/2004JB003542$09.00
caldera (90 km2). These data suggest that the monitoring of energy fluxes associated with diffuse degassing may be an important tool in the surveillance of volcanic activity, and continued development of quick and reliable methods to measure the thermal energy released by the hydrothermal activity will encourage the systematic use of this technique.  In this work we focus our attention on the energy released by subaerial diffuse degassing, i.e., where the gas is not directly emitted by fumaroles. A recent study has shown that diffuse degassing does not occur across entire volcanoes, but rather in restricted areas named diffuse degassing structures (DDSs [Chiodini et al., 2001a]) commonly associated with regions of high permeability (faults, fractures).  Thermal Infrared images of DDSs present an intriguing visualization of the heat flux associated with these structures (Figure 1). It is worth noting both the large extent of the hot soil areas and the generally low magnitude of the thermal anomaly, i.e., during the survey periods, maximum soil temperatures rarely exceeded 20– 30C. At these temperatures steam condenses below the surface and most of the heat should be transmitted by conduction.  In a purely conductive regime, the heat flux is a linear function of the soil thermal gradient with a slope proportional to the thermal conductivity of the medium. On the basis of this consideration, previous studies have estimated the heat released from hot soils measuring soil thermal
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gradients and assuming thermal conductivities of the soils [Chiodini et al., 2001a; Brombach et al., 2001; Lardy and Tabbagh, 1999; Severne and Hochstein, 1994]. Chiodini et al. [2001a] and Brombach et al.  assumed the thermal conductivity to be 1 W m1 C1 following Clauser and Huenges , while Severne and Hochstein [1994, p. 212] used ‘‘appropriate thermal conductivity values’’ to assess the surface heat transfer. Lardy and Tabbagh  directly measured the ground thermal conductivity using active methods in superficial glassy ash, and obtained a mean of 0.51 ± 0.16 W m1 C1 with a large distribution of the data (0.30– 1.06 W m1 C1) depending on the variability of the soil composition. However, the long time required for the direct measurements of the thermal conductivity limits the use of this technique.  An alternative way to estimate the heat flux from DDSs is based on CO2 flux measurements. The degassing of CO2 from a DDS can be computed and mapped with adequate precision applying the sequential Gaussian simulation (sGs) method to the measurement points [Cardellini et al., 2003]. Because CO2 is the second most abundant gas species (after H2O) in volcanic and hydrothermal systems considered here and because it is a noncondensable gas at T-P conditions of the DDSs, it serves as a tracer of the diffuse degassing process. In particular, CO2 fluxes were used to estimate the heat released by DDSs [Brombach et al., 2001; Chiodini et al., 2001a, 2004; Frondini et al., 2004]. These studies calculated the total amount of steam released at depth based on the assumption that the fluids which supply the diffuse degassing process have, at depth, the same composition as those emitted by the fumarolic vents of highest temperature and flow rate. The thermal energy was then computed by multiplying the steam flux by the enthalpy of the steam minus the enthalpy of the liquid at ambient temperature.  In this paper, we discuss the interrelation of CO2 fluxes, thermal gradients, and fumarolic compositions at 15 DDSs from eight different volcanic systems. The aim is to test the reliability of the above method for heat flux estimation by using the measured soil thermal gradients, CO2 fluxes and fumarole compositions. We will describe also a new method for the estimation of the thermal conductivity of the DDSs’ soil based on the combination of the different measured parameters.  In addition, we present the results of a long period of continuous monitoring of the soil temperature at different depths in the crater of Solfatara di Pozzuoli. This data set is used to determine the influence of diurnal and seasonal temperature variations on the thermal heat released from the soil.
2. Study Sites  The DDSs investigated in this study are located in 8 different volcanic systems in the world: Stefanos, Kaminakia, Polybotes Micros, and SU area in the Nisyros volcanic system (Greece), Solfatara in the Campi Flegrei district (Italy), Donna Rachele in Ischia Island (Italy), NE rim and Bottom Crater in the Vesuvio system (Italy), Comalito cinder cone in the Masaya Caldera Complex (Nicaragua), Cerro Negro in the Cerro Negro volcanic system (Nicaragua), Vulcano Porto di Levante Beach and
Vulcano Crater in Vulcano island (Italy), Favara Grande and Favara Piccola in Pantelleria island (Italy). Typically, the DDSs are not exclusively contained within crater areas but rather are often related to structural features such as faults/ fracture zones.  Nisyros volcanic island, built up during the last 100 ka, lies at the eastern end of the Aegean active volcanic arc. Hydrothermal eruptions were frequent in historical times. The oldest hydrothermal craters are located at Kaminakia, while the last eruption, which occurred in September 1887, originated in Polybotes Micros crater. At present, Stefanos hydrothermal crater is the most active fumarolic site. According to the geochemical model by Chiodini et al. [1993a], the hydrothermal system of Nisyros is made up of a deep reservoir (>1000 m) with a temperature of 300C, and of a shallower system, with temperatures of 150 – 250C.  Ischia is the westernmost volcanic complex of the Campania area and the last eruption took place in 1301 A.D. [Vezzoli, 1988]. Our study is focused on the so-called Donna Rachele fumarolic area, which is located on the western flank of Mount Epomeo. It is characterized by hydrothermally altered terrain, steaming ground and focused vents. Chiodini et al.  proposed two distinct hydrothermal reservoirs feeding the fumarolic area: a shallower reservoir characterized by a temperature of 250C and a pressure of 40 bars and a deeper reservoir characterized by a temperature of 300C and a pressure of 90 bars.  Solfatara is a tuff cone affected by intense diffuse degassing. It is located inside Campi Flegrei caldera complex. A conceptual geochemical model of the hydrothermal system suggests one or multiple aquifers are located over the magma chamber [Cioni et al., 1984; Chiodini and Marini, 1998]. Gas equilibrates in a ‘‘superheated’’ vapor zone at temperature of 210– 220C in the shallower part of the hydrothermal system [Chiodini et al., 2001a].  Somma-Vesuvio is a central composite volcano located in the southern sector of the Campanian Plain. It was formed by an ancient stratovolcano, Mount Somma, and by a more recent cone, Vesuvio [Santacroce, 1987]. At present, Vesuvio volcano is characterized by widespread fumarolic activity on the inner slopes and bottom of the crater, as well as by diffuse soil CO2 degassing. Geochemical studies of crater fumarolic fluids reveal the presence of a hydrothermal system located inside the Somma caldera [Chiodini et al., 2001b]. On the basis of C-H-O gas equilibria of crater bottom fumarolic fluids, the temperature of this system was estimated to be 400 – 500C. Hydrostatic models suggest a hydrothermal reservoir exists at depths of 2– 4 km within the carbonate sequence, which is present at depths >2.5 km underneath the volcano [Chiodini et al., 2001b].  Masaya is an active basaltic volcano on the Central American volcanic front, situated about 25 km southeast of Managua, Nicaragua. It is an unusual subduction zone volcano because of its shield-like form, consistent tholeiitic basaltic composition (Masaya is one of the few volcanoes known to have hosted basaltic Plinian activity [Williams, 1983]), frequent eruptive activity, and a 25-year cycle of major noneruptive degassing crises [Stoiber et al., 1986]. The flux of SO2 and HCl from Masaya represents the largest reported noneruptive volcanic release of these species in the world [Stoiber et al., 1986]. Comalito cinder cone is located approximately 3.5 km NE of the Masaya crater, along a
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northeast trending lineament. The site investigated was located adjacent to the Comalito cinder cone, and was characterized by steam and diffuse CO2 emissions, minimal vegetation, and soil temperatures up to 80C [Lewicki et al., 2003].  Cerro Negro has been interpreted to be an old cinder cone [Mooser et al., 1956; Walker and Carr, 1986; Simkin and Siebert, 1994] or a young composite volcano [Wood, 1978; McKnight and Williams, 1997] and is located in northwestern Nicaragua on the flank of the El Hoyo – Las Pilas volcanic complex. It erupted 22 times since its formation in 1850, with the last eruption in May– August 1995. Thermal features are limited to superheated fumaroles in the crater (350C), and low-temperature (