The PROJECT / Context
DEGAZMAG is a research project on magma degassing funded for four years (2012-2015) by the french Agence Nationale de la Recherche (ANR). Magma degassing — the exsolution and subsequent evolution of volatile species initially dissolved into the magmatic liquid — 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. The most abundant volatile components in silicate melts are water and carbon dioxide, followed by sulfur, chlorine, and fluorine. Many other volatile elements can dissolve into silicate melts, but at much lower concentration levels: light elements (Li, B), noble gases, etc. Water and CO2 have a major influence on magma dynamics and are the fuel of explosive eruptions. Along with CO2, sulfur, fluorine and other halogens may have a severe impact on the environment by causing global warming, cooling, acid rains or ecosystem poisoning. Due to their low concentrations, light elements and noble gases have no direct impact on magma dynamics and environment, but are of great interest for understanding geochemical recycling and Earth degassing.
- (1) Magmatic volatiles and eruption dynamics. Volatile components play a fundamental role in the ascent of magmas to the Earth's surface and in the generation of explosive eruptions. The solubility of a volatile component into a silicate melt — that is, the maximum amount that can be dissolved into the melt — increases with increasing pressure. Thus at the high pressures prevailing into magma sources or deep magma chambers beneath volcanoes, silicate melts contain large amounts of volatiles. Pressure decrease during magma ascent to the Earth's surface causes volatile exsolution and the formation of gas bubbles: volcanic eruptions are powered by the tremendous expansion that results from volatile exsolution, and the eruptive style is controlled by the kinetics of magma vesiculation (bubble nucleation and growth).
- (2) Environmental impact of magmatic volatiles. Gases released into the atmosphere by volcanoes affect the environment in different ways (climatic cooling from sulphate aerosols and ashes; greenhouse warming from CO2; ecosystem damage from acid rains) and operate over different time and length scales (Robock, 2000, Rev. Geophys. 38: 191-219). According to the historical records, a single explosive eruption, even of the largest intensity, can only produce a short-lived episode of global cooling. In the past, however, millions of km3 of basalts were emitted over short periods (≈ 1 Ma; Wignall, 2001, Earth-Science Rev. 53: 1–33) in Large Igneous Provinces. In these cases, the huge amounts of CO2 and SO2 liberated in the atmosphere could have a global environmental impact. For instance, the Siberian traps presumably triggered the end-Permian faunal extinction at ≈ 250 Ma. Less dramatically, the 2010 eruptions of Eyjafjallajökull, which caused enormous disruption to air traffic across western and northern Europe, illustrate the impact that volcanic ash and SO2 plumes may have on environment and society.
- (3) Volatiles and chemical geodynamics. Volatiles (H2O, CO2) are first rank actors of the geodynamics of the Earth's interior and of the formation and long-term evolution of the atmosphere and the oceans. For instance, water has a drastic effect on the rheology and melting depth of the mantle, and thus on the rate of mantle convection and differentiation. Volatiles are also fundamental tracers of the structure and evolution of the Earth. In particular, noble gas elemental and isotopic compositions are a primary source of information on Earth degassing (Gonnermann and Mukhopadhyay, 2009, Nature 459: 560-563). For instance, the presence of 3He, an isotope of helium that is not created by any process inside the Earth, in ocean island basalts suggests that ancient volatile components have remained locked inside the lower mantle for about 4.5 billion years. It is, however, very difficult to estimate the volatile signature of mantle source regions because magmas lose almost all their volatiles during the ascent to the Earth's surface. Establishing a precise connection between volatile abundances in erupted lavas and the volatile signature of their sources is a key subject for current and future research effort.