Publication Date:
2023-10-03
Description:
Developing appropriate monitoring strategies in long-quiescent volcanic provinces is challenging
due to the rarity of recordable geochemical and geophysical signals and the lack of experienced
eruptive phenomenology in living memory. This is the case in the Massif Central (France) where
the last eruptive sequence formed the Pavin’s Group of Volcanoes, about 7 ka ago. There, current
evidence of a mantle activity reminiscence is suggested by the presence of mineral springwaters,
mofettes, and soil degassing. It appears fundamental as a prerequisite to decipher the evolution of
the gas phase in the magmatic system at the time of the eruptive activity to understand the meaning
of current local gas emissions. In this study, we develop an innovative approach coupling CO2
densimetry and geochemistry of fluid inclusions from products erupted by the Pavin’s Group of
Volcanoes. 3D imagery by Raman spectroscopy revealed that carbonate forming in fluid inclusions
may lead to underestimation of CO2 density in fluid inclusions by up to 50 % and thus to unreliable
barometric estimates. Fortunately, we found that this effect may be limited by focusing on fluid
inclusions with a small diameter (〈4 m) and where no solid phase is detected on Raman spectra.
The time evolution of the eruptions of the Pavin’s Group of Volcanoes shows a progressive
decrease of the pressure of magma storage (from more than 9 kbar down to 1.5-2 kbar) in parallel to
magma differentiation (from basanites at Montcineyre to benmoreites at Pavin). The analysis of the
noble gases entrapped in fluid inclusions yielded two main conclusions: (1) the helium isotope
signature (Rc/Ra = 6.5-6.8) is in the range of values obtained in fluid inclusions from mantle
xenoliths in the Massif Central (Rc/Ra = 5.6±1.1, on average) suggesting partial melting of the
subcontinental lithospheric mantle, and (2) magma degassing (4He/40Ar* from 4.0 to 16.2) mirrors
magma differentiation and the progressive rise of the magma ponding zones of the Pavin’s Group of
Volcanoes. According to our modelling, about 80 % of the initial gas phase would be already
exsolved from these magmas, even if stored at mantle depth. Based on the results obtained from
fluid inclusions, we propose a model of the evolution of the signature of noble gases and carbon
isotopes from mantle depth to crustal levels. In this frame, gas emissions currently emitted in the
area (Rc/Ra = 6.1-6.7 and 4He/40Ar* = 1.7) point to an origin in the lithospheric mantle. This study
strongly encourages the establishment of a regular sampling of local gas emissions to detect
potential geochemical variations that may reflect a change from current steady-state conditions
Description:
Published
Description:
121603
Description:
1V. Storia eruttiva
Description:
2V. Struttura e sistema di alimentazione dei vulcani
Description:
5V. Processi eruttivi e post-eruttivi
Description:
JCR Journal
Keywords:
Fluid inclusions
;
Barometry
;
Noble gases
;
Magma degassing
;
Monitoring
;
04.08. Volcanology
;
04.01. Earth Interior
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
Permalink
