The transport of volatiles, in particular sulfur and halogens (F, Cl, Br and I), from surface reservoirs to the inner Earth and back in volcanic emissions is suggested by geochemical studies of magmas and volatile measurements on volcanoes. Furthermore, the widespread presence of sulfur-rich liquid phases has been proposed in the inner Earth by geophysical methods. However, the physicochemical properties of these liquids are poorly understood. In particular, the coupling between volatiles and metals, which controls geochemical transport processes from the inner Earth to the surface through porous and focused liquid migration, and then volcanic processes is not well known.
Understanding the behaviour of halogens and sulfur through magmatic processes is key for our society since volcanic emissions of these elements can locally or globally trigger environmental issues. This can happen, for instance, through climatic cooling by sulfur-bearing aerosols or metal pollution of agricultural areas, since these chemical compounds are produced during explosive volcanic eruptions triggered by magma degassing processes.
However, highly precious metals also occur among those which can be concentrated by halogens and sulfur in magmas. These chemical elements include ‘base’ metals (e.g. Cu) and ‘noble’ metals, the latter of which encompass Au, Re and platinum group elements (PGE’s) (Os, Ir, Ru, Rh, Pt, Pd). PGE’s are extremely rare in the earth's crust (<10-9 g.g-1 on average) and their use covers a wide field of applications in medicine, electronics and chemistry. These elements also play a key role in sustainable development, whether it involves reducing emissions of atmospheric pollutants or producing and managing clean and/or renewable energy. Due to the ever-increasing demands of our society, PGE’s are notably classified as critical and strategic resources by the European Union.
The overall objectives of the project HAMA were to investigate the behaviours of halogens and sulfur, together with precious metals, in natural samples from intra-oceanic subduction zones. For this to be achieved, several analytical and methodological developments needed to be undertaken. In parallel, a geochemical model (e.g. involving solubility and mineral/liquid partitioning properties) needed to be developed. The aim of this model is to simulate and predict the couplings between volatile compounds and metals in the mantle and the crust of the Earth.
Combining data from natural samples with the results of numerical simulations during the project HAMA aimed at providing new insights into two fundamental processes involved in the coupled mobilizations of volatiles and precious metals in the inner Earth: (i) partial melting in the deep Earth’s mantle to produce magmas; and (ii), physicochemical evolution of magmas during their emplacement in the upper part of the Earth’s mantle and in the Earth’s crust.
