Ascent and degassing of basaltic magmas : time-scales and implications for eruptive dynamics and the geochemical cycles of volatile elements – DEGAZMAG

DEGAZMAG is based on a forward approach involving the experimental simulation of magma ascent and degassing in the laboratory and the use of state-of-the-art analytical techniques to characterize the run products. In addition to task no. 1 (Coordination), it comprises three scientific tasks.

Task no. 2 aims at conducting series of decompression experiments that will simulate the ascent and vesiculation of basaltic melts in the laboratory, the objective being to quantify the degassing of major volatile components (CO2, H2O, SO2): depth of bubble nucleation, influence of ascent rate on degassing kinetics, etc. The work in task no. 2 is shared between the two groups of experimental magmatology at LMV and ISTO.

In task no. 3, we shall address the issue of volcanic degassing using isotopic tracers able to record processes occurring over very short time scales (a matter of days): short-lived disequilibria of the U-series and lithium stable isotopes. The effort will first be focussed on determining experimentally the behavior of the studied chemical species during magma degassing. Then, these cutting-edge tools will be applied to the recent volcanic activity of Piton de la Fournaise and Eyjafjallajökull. Task no. 3 is based on the collaborative work of specialists in experimental magmatology and in geochemistry at LMV.

Task no. 4 is devoted to noble gas geochemistry as a tool to investigate the mechanisms and time scales of magma degassing. To improve our understanding of the behaviour of noble gases during magma ascent, two studies will be made: a study of noble gas fractionation in experimentally-degassed samples using bubble-by-bubble laser analysis, and a modeling of kinetic fractionation between a fast- and a slow-diffusing volatile species during diffusive bubble growth. Task no. 4 is led by CRPG and conducted in collaboration with LMV and ISTO.

A renseigner plus tard

A renseigner plus tard

A renseigner plus tard

Magma degassing — the central theme of DEGAZMAG — is a fundamental issue of modern volcanology, with far-reaching implications for eruption dynamics, the environmental impact of volcanism, and the global cycle of volatile elements. Despite significant advances over the past decade, some major aspects of magma degassing remain poorly understood, including the time scales involved, the role of gas transfers in triggering major eruptions, and the importance of kinetic factors in controlling the fractionation of volatile elements and the composition of volcanic gases. Compared to many studies on magmatic volatiles, DEGAZMAG is innovative because it is based on a forward approach involving the experimental simulation of magma ascent and vesiculation in the laboratory and the use of state-of-the-art analytical techniques to characterize the run products.

The first goal of DEGAZMAG is to quantify the behaviour of major volatile components (CO2, H2O) during the ascent of basaltic magmas, and to determine the effect of ascent rate on the kinetics of vesiculation (rate of bubble nucleation and growth). The purpose is to elaborate models of basalt degassing from their sources to the surface (depth of degassing, open vs. closed system degassing, flux of volcanic gases into the atmosphere), and to develop tools to estimate magma ascent rate and degassing mechanisms from studies of eruption products (textures, volatile compositions in melt inclusions) and volcanic gases.

The second goal of DEGAZMAG is to characterize the behaviour of key volatile trace elements that are potential markers of magma degassing: light elements Li and B, short-lived radioactive isotopes (222Rn, 210Po), and noble gases (He, Ar). Due to differences in volatility (e.g., 210Po vs. 210Pb) and/or diffusivity (e.g., 6Li vs. 7Li or He vs. Ar), magma degassing may lead to complex schemes of chemical or isotopic fractionation depending on whether it proceeds at equilibrium or far from equilibrium (diffusion-controlled fractionation) and whether the system is closed or open to gas transfers. Degassing-induced fractionation of volatile trace elements is now quite well documented in eruption products, but the interpretation of these data is severely handicapped by the lack of experimental constraints. Our objective is to provide the experimental database and the theoretical tools necessary to interpret volatile fractionation data in volcanic products in terms of time scales, gas loss or accumulation, and eruption dynamics. A first group of experiments will be carried out on light elements and short-lived radioactive isotopes, and the experimental results will be applied to two recent major eruptions (Piton de la Fournaise, Réunion Island; Eyjafjallajökull, Iceland), which were supposedly triggered by gas transfer processes. A second group of experiments will be focussed on diffusive fractionation of noble gases during ascent and vesiculation of basalts, and the implications for noble gas data in natural basalts.

DEGAZMAG is a 4-year project based on strongly collaborative work between three partner laboratories (LMV, Laboratoire Magmas et Volcans, Clermont-Ferrand; ISTO, Institut des Sciences de la Terre, Orléans; CRPG, Centre de Recherches Pétrographiques et Géochimiques, Nancy), which provide a top-level expertise in experimental magmatology, light element geochemistry, radioactive disequilibria, noble gas geochemistry, and physico-chemical modeling of magmatic processes. As a whole, it involves 12 permanent researchers and 4 non permanent researchers: 2 PhD students at LMV, which are financially supported by the Région Auvergne (2010-2014); and 2 PhD students at ISTO and CRPG, for which we request a financial support from the Agence Nationale de la Recherche.