Journal of the Virtual Explorer

Active volcanism represents a fundamental process by which deep CO₂ is released to the atmosphere. Although relevant CO₂ emissions occur during volcanic eruptions, non-eruptive diffuse degassing at the vent or through the flanks of active volcanoes has been recognized as the principal CO₂ release mechanism to the atmosphere (Table 2; and references therein).

Active volcanoes (Fig. 1) are located in the southern part of continental Italy; (e.g., Vesuvio; Campi Flegrei; Ischia), in Sicily (e.g., Etna) and Sicily channel (e.g., Pantelleria), and in the Southern Tyrrhenian sea (Fig. 1; e.g., Vulcano; Stromboli; Lipari). Based on direct source-point measurements, the overall volcanic CO₂ emission budget is conservatively (lower bound) estimated at about 30 Mt CO₂/y (see references in Table 2). Measured Etna CO₂ fluxes through time vary from 13 to 43.8 Mt/y (average 25 Mt/y; Allard et al., 1991; Gerlach, 1991b). Further CO₂ (about 1-5 Mt/y) emission occurs from the flanks and the summit area of the volcano. Other active volcanoes (i.e., Stromboli, Vesuvio, Campi Flegrei, Ischia, Vulcano, Pantelleria) emit considerably lower - although quite variable - amounts of CO₂ (in the order of 0.2 – 2 Mt/y; Table 2).

Etna represents one of the strongest CO₂-emitting volcanoes at the global scale (Table 2). Gerlach (1991a and b) indicated Etna as a point-source “singularity”, contributing alone to about one third of the global CO₂ by sub-aerial volcanism (cf., Table 1). More recently, the increase of measurements has shown diffuse CO₂ emission from other active volcanoes in the same order of magnitude than Etna, i.e. Nyariagongo, and Popocaptel (Table 2).

The reasons behind extreme CO₂ enrichments in magmas are debated (Gerlach, 1991a and b; Kerrick, 2001). The CO₂ content of degassing magmas reflects mantle source composition, but can be modified by crustal processes during magma rise and rest. In Italy, interaction between magmas and carbonate wall-rocks at crustal levels (i.e., magma chambers) has been proved to contribute substantial CO₂ at Vesuvio and Colli Albani volcanoes, via magma assimilation processes of carbonates. This is testified as by a wealth of petrological and geochemical data, including high oxygen isotopic compositions of igneous rocks and minerals, and by the effects on paths of magma evolution at these centers, which are dominated by clinopyroxene separation induced by heavy carbonate assimilation (e.g., Dallai et al., 2004; Gaeta et al., 2006; Iacono Marziano et al., 2007a and b; Peccerillo et al., 2010).

The relative contributions of these processes and of direct mantle provenance to the overall emission of volcanic CO₂ is highly debated (Iacono Marziano et al., 2007b, 2009; Gaeta et al., 2009). CO₂ degassing at Etna mostly reflects mantle-derived carbon from aa MORB type source enriched by metasomatic fluids, with only subordinate crustal CO₂ contribution (Allard et al., 1997; Nakai et al., 1997). Since Etna alone contributes to more than 90 % of the overall CO₂ emissions from Italian active volcanoes, the bulk of volcanic CO₂ degassing budget in Italy should represent a deep mantle-related process.

CO₂ degassing occurs in Central and Southern Italy via diffuse soil emission and focused vents, located in wide areas where volcanism is absent or is not anymore active. This is indicated as non-volcanic CO₂ emission, and includes degassing from crustal carbonate rocks, fluxing from the upper mantle, and from geothermal fields (Fig. 1; Table 2; e.g., Chiodini et al., 1998; 1999; 2000; 2004; Etiope, 1999; Italiano et al., 2000; Rogie et al., 2000; Mörner and Etiope, 2002; Gambardella et al., 2004). According to Mörner and Etiope (2002), non-volcanic CO₂ fluxes in Italy are in the same order of magnitude of volcanic degassing (from > 4 to 30 Mt/y). For example, the regional mapping of diffuse CO₂ fluxes from an area of 45,000 km² in Central Italy, including Tuscany, Northern Latium geothermal fields, and Central Apennine chain, indicates about 9.7 - 17 Mt CO₂/y (Fig. 1; Gambardella et al., 2004).

A direct association between CO₂ soil emission and location of fault and fractures, often of deep origin, is observed (Table 2): in the axial zones of the Apennines CO₂ emission totals to 4-13 Mt /y (Chiodini et al., 2004 and references therein). In the Mefite of the Ansanto valley (Irpinia; external parts of the southern Apennines), CO₂ fluxes from 0.1 to 0.3 Mt CO₂/y have been measured from gas vents over the hypocenter area of the 1980 earthquake (Italiano et al., 2000; Rogie et al., 2000). In Sardinia, CO₂ degassing occurs in the northern part of the Campidano Graben, (Minissale et al., 1999). In Sicily, high CO₂ fluxes are measured from mofetes distributed along major fault systems that cut the eastern part of the island (De Gregorio et al., 2002).

Non-volcanic CO₂ source might be located in the middle-lower crust, or at greater depth in the mantle, due to decarbonation reactions induced by increasing temperatures, or by active tectonics. Chiodini et al. (2000) proposed that about 40 % of the inorganic carbon of non-volcanic CO₂ derives from a source characterized by a δ13C of -3 ‰, compatible with mixed crust + mantle source, or with a mantle metasomatized by crustal fluids. Also 3He/4He ratios measured in CO₂-rich gases in non-volcanic soil emissions indicate an important mantle component (R/Ra up to 4.48; Minissale, 2004), and, interestingly, are close to values measured in lavas of recent and active volcanoes(cf., Martelli et al., 2008, and references therein).