Carbonate metasomatism and CO2 lithosphere–asthenosphere degassing beneath the Western Mediterranean: An integrated model arising from petrological and geophysical data

Abstract

We present an integrated petrological, geochemical, and geophysical model that offers an explanation for the present-day anomalously high non-volcanic deep (mantle derived) CO 2 emission in the Tyrrhenian region. We investigate how decarbonation or melting of carbonate-rich lithologies from a subducted lithosphere may affect the efficiency of carbon release in the lithosphere-asthenosphere system. We propose that melting of sediments and/or continental crust of the subducted Adriatic-Ionian (African) lithosphere at pressure greater than 4 GPa (130 km) may represent an efficient mean for carbon cycling into the upper mantle and into the exosphere in the Western Mediterranean area. Melting of carbonated lithologies, induced by the progressive rise of mantle temperatures behind the eastward retreating Adriatic-Ionian subducting plate, generates low fractions of carbonate-rich (hydrous-silicate) melts. Due to their low density and viscosity, such melts can migrate upward through the mantle, forming a carbonated partially molten CO 2 -rich mantle recorded by tomographic images in the depth range from 130 to 60 km. Upwelling in the mantle of carbonate-rich melts to depths less than 60 -70 km, induces massive outgassing of CO 2 . Buoyancy forces, probably favored by fluid overpressures, are able to allow migration of CO 2 from the mantle to the surface, through deep lithospheric faults, and its accumulation beneath the Moho and within the lower crust. The present model may also explain CO 2 enrichment of the Etna active volcano. Deep CO 2 cycling is tentatively quantified in terms of conservative carbon mantle flux in the investigated area. IntroductionThe role of Earth degassing in present-day global carbon budget and consequent climate effects has been focused chiefly on volcanic emissions (e.g. Gerlach, 1991a;Varekamp and Thomas, 1998, Kerrick, 2001). The non-volcanic 1 escape of CO 2 from the upper mantle, from crustal carbonate rocks, from hydrocarbon accumulations, and from geothermal fields is not considered in the budgets of natural release processes (cf. IPCC reports). However, the impact of these processes on atmospheric CO 2 budget is relevant if considered at the regional scale.Italy constitutes an extraordinary example of massive CO 2 subaerial fluxes in the Western Mediterranean region (e.g. Chiodini et al., 1999Chiodini et al., , 2004Rogie et al., 2000;Chiodini and Frondini 2001;Minissale, 2004). In Italy, CO 2 emissions occur both in those areas of young or active volcanism (e.g. the Tyrrhenian border of Central Italy; Fig. 1), and at zones in which there is no evidence of magmatic activity, such as Central and Southern Apennines (Fig. 1).Interactions between magmas and carbonate rocks at walls of magma chambers locally contribute CO 2 in some recent and active volcanic zones (e.g., Alban Hills, Mt. Ernici, Vesuvius;Federico and Peccerillo, 2002; Iacono Marziano et al., 2007a, b). However, the bulk of present-day soil degassing in Italy is a regional feature affecting both volcanic areas and z...

Are ‘hot spots’ hot spots?

“…There is also abundant geochemical evidence for water and carbonatite contributions to the melt sources, e.g., at Hawaii [Huang et al, 2009;Sisson et al, 2009], and the western Mediterranean [Frezzotti et al, 2009]. Concentrations of CO 2 of up to 6% have been determined for some Hawaiian magmas [Barsanti et al, 2009].…”

Thermal weakening of the carbonate basement under Mt. Etna volcano (Italy): Implications for volcano instability

“…Etna (e.g. see Allard et al, 1991;Frezzotti et al, 2009;Chiodini et al, 2011). In these scenarios, the contribution of decarbonation to the CO 2 budget at Mt.…”

Non‐volcanic CO2 Earth degassing: Case of Mefite d'Ansanto (southern Apennines), Italy

“…This amount of non-volcanic CO 2 is ∼10% of the estimated present-day total CO 2 discharge from sub-aerial volcanoes of the Earth [Kerrick, 2001] but it is more likely an underestimation because the contribution of the numerous CO 2 vents widespread located in the Italian region is not included . The most probable source of this regional Earth degassing process is the mantle wedge, located in the Tyrrhenian sector of Italy, which is anoma-lously rich in fluids originated in the subducted Adria slab [Chiodini et al, 2004;Frezzotti et al, 2009].…”