Tracking volcanic eruptions from the dynamics of magma degassing processes – VOLGASPEC

Better understanding of magma degassing processes is crucial to improved forecasting of volcanic
eruptions and related hazards. The progressive exsolution of volatile components (H2O, CO2, S, Cl, F,
etc.) during magma decompression generates an expanding gas phase (bubbles) that affects the
physical properties of the melt, its crystallisation path and dynamics of ascent, and then constitutes
the driving force of eruptions at the surface. Moreover, the high mobility of this gas phase and its
chemical evolution as a function of pressure, due to different solubility behaviour of volatiles, make it
a very sensitive tracer for subsurface conditions and eruptive potential of magma bodies
(geochemical precursors). Therefore, an integrated study of both dissolved/exsolved volatiles in
magmas and gas emissions from volcanoes unquestionably constitutes a key approach to
improve our capability to forecast volcanic eruptions and associated hazards.
The development of this approach has long been hampered by intrinsic field difficulties, the absence
of adequate methodologies and the lack of systematic investigations. In this VOLGASPEC research
project we propose exhaustive and innovative investigations of magma degassing processes – from
magma chamber via conduits to the atmosphere – through the combined use of powerful new
technologies for the analysis of both dissolved and emitted magmatic volatiles. The dynamics of
magma degassing processes, and their control of eruptive activity, will be studied by careful linking of
state-of-the-art laboratory and field investigations:
1) Study of the initial abundances, exsolution pressures and degassing modes (closed/open) of the
five main volatile species (H2O, CO2, S, Cl, F) dissolved in magma (melt inclusions), complemented
by isotopic analysis of H2O (D/H) and S (electron, ionic and nuclear microprobes; Micro-Raman);
2) Dosing of H2O and halogen residual contents in lavas and tephra, in relationship with their
texture (vesicularity, microcrystallinity) and eruption dynamics (SEM, MET, pyrohydrolyse, ICPMS,
Micro-Raman);
3) Remote sensing of the molecular composition of magmatic gas emissions at the surface (H2O,
CO2, SO2, H2S, HCl, HF, CO, COS) using open-path Fourier transform infrared spectroscopy (OPFTIR),
correlations between chemical ratios in the gas phase and the magma (inclusions, glasses) as
a function of T, P and redox conditions;
4) UV absorption remote sensing of SO2 fluxes, in order to establish the mass budgets for each
volatile species and the amounts of degassing magma (supply rates, dimensions of reservoirs); and
5) Tracing the kinetics of degassing (bubble transfer time and magma renewal rate) from
radioactive disequilibria among the short-lived daughters of radon-222 in the gas phase (210Po, 210Bi,
210Pb) and in lavas (226Ra/210Pb ratio; beta and gamma spectrometry, ICPMS).
These investigations will be closely coordinated and conducted on a few volcanoes that are
archetypes of the diversity in eruptive regimes, magma compositions and tectonic contexts, but also
of special interest for France (Piton de la Fournaise, Antilles, Nyiragongo, Vanuatu arc – links with
projects ANR2005 'Usine subduction Antilles' and ANR2006 'Arc-Vanuatu'). The results obtained,
correlated with other parameters monitored by local Observatories, will be integrated, modelled and
used to improve current models of magma degassing and bubble flow.
VOLGASPEC is a thematic project of fundamental research, with wide implications, that will also have
direct applications to volcanic hazard mitigation. Through its objectives, methodologies and expected
results, it will allow French and other European volcanologists to make significant advances in the
understanding of magma degassing processes and eruptive phenomena. In its structure, it is a
coherent and multidisciplinary project which harnesses most French specialists and institutes in this
field of research (LPS, IPGP, LMV, UPMC, LPMMP, CRPG, Reunion Univ.: Partner 1), as well as
internationally leading colleagues from the UK (Cambridge, York) and Italy (INGV): Partner 2. This
partnership, combining various disciplines and capacities (volcanologists, geochemists, geologists,
specialists in optics, physicists), will guarantee international impact (publications, meetings, shared
expertise). The project requires the funding of an OP-FTIR spectrometer (Bruker OPAG-33), a tool
with revolutionary potential for the remote analysis of magmatic gases, even during eruptions, as
illustrated by the results obtained by the proponents, in particular on Etna (Nature, 2005). French
volcanology needs urgently this instrument, of recent use in 4 countries (Japan, UK, USA and Italy),
but whose funding could not be obtained from INSU in 2004 then 2006 and ANR in 2005. This mobile
instrument has the potential to also be an intervention tool in case of emergency on any volcano in
France and Europe (Mediterranean basin, Azores, Canaries, Iceland). Finally, it will allow us to
evaluate the interest for future implementation of OP-FTIR spectroscopy in French Observatories,
including the training of local scientists, in complement to UV sensors planned in NOVAC EC-project.