IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A1_2G-O6

Room B2

Date/Time: July 23 10:00-10:15

First measurement of magma degassing processes and co-varying seismic tremor
during an eruption of Piton de la Fournaise hot spot basaltic volcano, Reunion island
Patrick Allard1 , Alessandro La Spina2 , Giancarlo Tamburello3 , Alessandro Aiuppa3 , Andrea Di Muro4 ,Diego Coppola5 , Jean Battaglia6 , Mike Burton7 , Florent Brenguier4 , Thomas Staudacher41Institut de Physique du Globe de Paris (IPGP), UMR7154 CNRS, Paris, France, 2 Istituto Nazionale diGeofisica e Vulcanologia (INGV), Catania, Italy, 3 DISTEM, Universita di Palermo, Palermo, Italy,
4Observatoire Volcanologique du Piton de la Fournaise, IPGP, La Reunion, France, 5 Dipartimento diScienze Mineralogiche e Petrologiche, Universita di Torino, Italy, 6 LMV, CNRS-Univ. Blaise Pascal,Clermont-Ferrand, France, 7 Istituto Nazionale di Geofisica e Vulcanologia (INGV), Pisa, Italy
E-mail: pallard@ipgp.fr

Piton de la Fournaise (PdF), in the western Indian Ocean, is a very active hot spot basaltic volcano whose eruptions(1 to 2 per year on average) are well anticipated by the local geophysical monitoring network but whose magmaticgases had never been measured. Here we report on the first data for magmatic gas composition (OPFTIR
absorption spectroscopy, in situ MultiGas analysis plus filterpack sampling) and budget (DOAS), coupled withrecording of seismic tremor and the lava extrusion rate (space-borne MODIS sensing), during a fissure eruptionof PDF in October 2010. Dyke ascent from about 2 km beneath the summit crater was tracked by a few hoursof precursory seismic signals and volcano deformation. After a strong initial burst coinciding with eruptive fissure
opening, both the tremor amplitude, lava extrusion rate and SO2 flux coherently decreased during the first week oferuption. The emitted magmatic gases, whose composition varied slightly over time, were found to be water-richand to display high S/Cl but low C/S, Cl/F and Cl/Br ratios. These features are consistent with a hydrous hot spot
mantle source and previous volatile fractionation during shallow magma storage. Volatile fluxes referred to themagma extrusion rate and available melt inclusion data point to essentially syn-eruptive (closed system) degassingof sulfur, chlorine and fluorine during the first half of the eruption. In contrast, additional inputs of CO2 and H2O arerequired to explain the gas composition. Differential CO2 bubbling is supported by high-frequency correlations
between the FTIR-sensed CO2/HCl ratio and seismic tremor, while entrainment of hydrothermal steam might
have enhanced the H2O gas content. The second week of the eruption was marked by a spectacular decoupling
between declining lava extrusion and reincreasing seismic tremor amplitude (especially in the low-frequency band),indicating a key control of tremor and degassing activity by differential (open system) gas bubbling across the feeder
dyke. Finally, the end of the eruption was preceded by a new sharp tremor increase, with remarkable anti-correlated
variations of the 1-3 Hz and 3-5 Hz spectral signals, which we attribute to abrupt geometrical changes prior to dykeclosure. Combined geochemical and geophysical survey of Piton de la Fournaise volcano in the future should
permit a better understanding and forecasting of its eruptive behavior.

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