Italy offers a unique opportunity to investigate localised solid/fluid transfer of C towards the surface. Italy emits 2.5-5x1011 mol. a-1 of CO2. Additional evidence for CO2 fluxing in Italy includes diatremes and maars that contain carbonatite, melilitite and mantle nodules. 1020kg of solid C is stored in the deep Earth, while the flux of C-O-H at the surface may catastrophically change. Mantle/core outgassing is considered to have a δ13C of about -5‰. Italian carbonatites have δ13C‰ between -8 and -4, R/Ra up to 7.33 (plume-type values). Fresher Italian carbonatites have δ13C, δ18O and δ11B values of −6.7‰ (-4.8‰ intrusive), +10.2‰ (+10.3‰ intrusive) and −5.9‰, respectively (mantle values). The change from natrocarbonatite to Ca-carbonatite at Oldoinyo Lengai is accompanied by a change in δ18O from +6.5‰ to +25‰, while δ13C remains unchanged. Nyerereite inclusions from Italian carbonatites suggest that similar processes contributed to δ18O evolution. Radiogenic mantle end-members, detected in Italian carbonatites, are related to deep alkali-carbonatites and metasomatic reactions with pyrope harzburgite. A specific concourse of geological causes is required to produce the above features, which together imply geologically instant mantle decompression, concentration of volatiles in a melt phase and explosive detachment from the source (diatresis). Magmatic convoy ascent needs to be 10-30 m/sec. to prevent mantle xenoliths settling. The only realistic agent of acceleration is deep-seated concentration of volatiles in fluidisated system with a high proportion of carbon dioxide. The nature of the Italian ultra-alkaline rocks implies that the high concentration of juvenile propellant resulted in extremely violent volcanic activities and that the potential volcanic risk implied by carbonatite occurrences and CO2 explosion along the Apennine structural trends must be re-evaluated in terms of this factor. The proposed carbonatite eruption triggering mechanism may generate LP supersonic shocks in the mantle, which would be possibly recognised as a precursor of surface eruption.

CO2 discharge and volcanic risk in Italy

STOPPA, Francesco
2010-01-01

Abstract

Italy offers a unique opportunity to investigate localised solid/fluid transfer of C towards the surface. Italy emits 2.5-5x1011 mol. a-1 of CO2. Additional evidence for CO2 fluxing in Italy includes diatremes and maars that contain carbonatite, melilitite and mantle nodules. 1020kg of solid C is stored in the deep Earth, while the flux of C-O-H at the surface may catastrophically change. Mantle/core outgassing is considered to have a δ13C of about -5‰. Italian carbonatites have δ13C‰ between -8 and -4, R/Ra up to 7.33 (plume-type values). Fresher Italian carbonatites have δ13C, δ18O and δ11B values of −6.7‰ (-4.8‰ intrusive), +10.2‰ (+10.3‰ intrusive) and −5.9‰, respectively (mantle values). The change from natrocarbonatite to Ca-carbonatite at Oldoinyo Lengai is accompanied by a change in δ18O from +6.5‰ to +25‰, while δ13C remains unchanged. Nyerereite inclusions from Italian carbonatites suggest that similar processes contributed to δ18O evolution. Radiogenic mantle end-members, detected in Italian carbonatites, are related to deep alkali-carbonatites and metasomatic reactions with pyrope harzburgite. A specific concourse of geological causes is required to produce the above features, which together imply geologically instant mantle decompression, concentration of volatiles in a melt phase and explosive detachment from the source (diatresis). Magmatic convoy ascent needs to be 10-30 m/sec. to prevent mantle xenoliths settling. The only realistic agent of acceleration is deep-seated concentration of volatiles in fluidisated system with a high proportion of carbon dioxide. The nature of the Italian ultra-alkaline rocks implies that the high concentration of juvenile propellant resulted in extremely violent volcanic activities and that the potential volcanic risk implied by carbonatite occurrences and CO2 explosion along the Apennine structural trends must be re-evaluated in terms of this factor. The proposed carbonatite eruption triggering mechanism may generate LP supersonic shocks in the mantle, which would be possibly recognised as a precursor of surface eruption.
2010
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11564/170694
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