Outgassing in the upper mantle: constraints on the source and evolution of volcanic CO2
By combining barometry and the composition of fluid inclusions trapped within lava crystals and recent enclaves in the Piton de la Fournaise, a new study has shed new light on the evolution of CO2 from its mantle source to its degassing into the atmosphere. One of the most remarkable results is the presence of significant early degassing (90%) of CO2 within the upper mantle. This result raises major questions about the volcanic contribution to the atmospheric carbon budget and its impact on climate dynamics.
Publication date: 14/11/2018
Observatories, Press, Research
Related observatories : Volcanological Observatory of Piton de la Fournaise (OVPF-IPGP)
Related themes : Natural Hazards
Given the role of CO2 in past, present and future climate dynamics, the assessment of the terrestrial carbon budget is a hot topic for the scientific community(Deep Carbon Observatory). In recent years, a number of studies have shown that estimates of carbon concentration in the mantle and of the CO2 budget associated with volcanic outgassing could be significantly underestimated.
Thanks to a Franco-Italian cooperation involving researchers from the Laboratoire Géosciences Réunion (LGSR), the Institut de Physique du Globe de Paris (IPGP) and the Istituto Nazionale di Geofisica e Vulcanologia (INGV), a new study published in June 2018 in the journal Geochimica et Cosmochimica Acta sheds new light on this issue. Conducted at Piton de la Fournaise, in an “OIB” (Ocean Island Basalt) type context that is still not very constrained compared to the “MORB” (Mid-Ocean-Ridge Basalt), this study is based on a new approach coupling (i) barometry of fluid inclusions trapped in enclave crystals and lavas representative of different levels of the associated magmatic plumbing and (ii) geochemistry of the trapped gas phase(δ13C, He-Ar-CO2).
Such coupling has made it possible to trace the evolution of CO2 from the upper mantle to the most superficial magma reservoirs located within the volcanic edifice. The researchers proposed a model of CO2 evolution (concentration, δ13C) at different depths in the magma plumbing of the volcanic edifice. By analysing the temporal evolution of the carbon isotopic signature in theCO2 of gaseous emissions, this model could be used to characterise the depth and/or state of progress of the degassing of the magma involved, thus providing valuable clues for monitoring volcanic edifices.
This new approach has also revealed that almost 90% of the CO2 initially contained in the primary magmas of Piton de la Fournaise was already exsolved within the upper mantle. This result contrasts with observations made on other ‘OIB’ edifices such as Kilauea (Hawaii), but provides a reasonable explanation for the evolved and degassed nature of the lavas emitted at Piton de la Fournaise, as well as the low level of inter-eruptive gaseous emissions in the active part of the edifice. This process is thought to depend on the rate at which magma rises through the multiple magma storage zones in the magma plumbing of the edifice, and could be common to a number of volcanic edifices.
Such early degassing within the upper mantle also raises important questions about the interpretation of the depleted isotopic signatures (δ13C) associated with certain mantle reservoirs and the calculation of carbon concentrations in the Earth’s mantle. The researchers propose a reassessment of the carbon concentration in the mantle source feeding the magmatic activity of Piton de la Fournaise (716±525 ppm) in agreement with that estimated for the bulk mantle. As a result, at Piton de la Fournaise, the real CO2 flux associated with this would be at least an order of magnitude greater than the CO2 flux measured conventionally by direct methods in gaseous emissions. On a larger scale, these results call into question current estimates of volcanic CO2 flows. They argue in favour of a reassessment of the volcanic contribution to the atmospheric carbon budget and its impact on climate dynamics.
Ref: Boudoire, G., Rizzo, A.L., Di Muro, A., Grassa, F., Liuzzo, M. (2018). Extensive degassing in the upper mantle beneath oceanic basaltic volcanoes: First insights from Piton de la Fournaise volcano (La Réunion Island). Geochimica et Cosmochimica Acta 235, 376-401.
