In situ study at high pressure and temperature of the environment of water in hydrous Na and Ca aluminosilicate melts and coexisting aqueous fluids

What is it about?

Water is the main gas released during volcanic eruptions. At depth, prior to eruption, it is dissolved in magmas (also called silicate melts) as water molecules or hydrogen ions bonded to the silicate molecules that build the molecular backbone of magmas. This greatly affects the magma properties and, in turn, determines the behaviour of volcanic eruptions at surface or the mobility of magmas in the Earth crust and mantle. In this regard, it is important to know how water reacts with the molecules constituting silicate melts. In this study, we looked at how water dissolves in magmas as a function of temperature, pressure, and melt chemical composition. We show that the hydrogen environment in magmas strongly depends on temperature. On the other hand, when considering a system with magma and aqueous fluids in equilibrium, the global chemical composition of the system drives the difference of hydrogen environment in the two phases (magma and aqueous fluids). This could be determining for driving the exchange of the hydrogen isotopes, a.k.a H and D, that are used to infer about the global cycle of water in the Earth system.

Featured Image

Why is it important?

This publication is amongst the firsts to show in situ spectroscopic data about water in melts relevant to the deep Earth. It brings a unique understanding of the effect of temperature on the hydrogen molecular environment in those melts, at conditions relevant to the Earth crust and upper mantle. It shows that, if fluids and magmas are in equilibrium, as it can be the case in subduction zones for instance (the areas where the oceanic crust sinks into the Earth mantle), the isotopes of hydrogen are likely to fractionate in a manner that will depend on the system chemical composition. Such knowledge is important for any future research that will use hydrogen isotopes to infer about the water exchanges between the Earth interior (crust and mantle) and exterior (oceans, atmosphere, biosphere...) layers.