DEGAZMAG

The PROJECT / Scientific goals

Despite significant advances over the past decade, some major issues 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. DEGAZMAG will use modern techniques of experimental magmatology and geochemistry to solve some of these major issues. DEGAZMAG's rationale is to simulate in the laboratory the ascent and degassing of silicate melts, especially basalts, and to characterize the run products using cutting-edge analytical techniques in order to achieve two main goals:

• 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's research is divided in 3 scientific tasks (A, B, C in the table below) and 9 subtasks; task A corresponds to the first goal above, tasks B and C to the second goal.

DEGAZMAG's tasks and subtasks and their leaders: DL, D. Laporte; MP, M. Pichavant; AP, A. Provost; IV, I. Vlastelic; ERK, E. Rose-Koga; OS, O. Sigmarsson; PB, P. Burnard; KK: K. Koga.;