Hydrogen cycle within the Deep Earth – HYDEEP

Rocks and minerals from the mantle show that the Earth's interior remains a potentially significant reservoir of water. Moreover, the most recent seismological studies show that a low velocity zone is located just above the discontinuity at 410-km depth. This zone is interpreted as due to the formation of hydrous melt, which would be produced during the phase transition wadsleyite/olivine during the upwelling of mantle material. Indeed, wadsleyite has a higher water solubility than olivine, and the presence of melt suggests that the transition zone is indeed hydrated, in agreement with the interpretations of electrical conductivity data. If geophysical observations indeed suggest the presence of water in the deep Earth, the quantities are still poorly contrained. Moreover, since the mantle is depleted in volatiles by volcanism, it must be re-hydrated by deep subduction in order to keep a hydrous deep Earth.

The HYDEEP project consists of 3 scientific tasks, all using high-pressure experiments: (1) The determination of the curve of maximum solubility of hydrogen in the co-existing nominally anhydrous minerals of the mantle. (2) The amount of water that can be retained in subduction zones by olivine after hydrous phases breakdown, and the nature and kinetics of fluid migration (either by hydro-fracturing, H grain boundary diffusion or reactive infiltration). (3) The infiltration rates of hydrous liquid generated by the phase transformation of hydrated wadsleyite from less hydrated olivine, just above 410 -km depth. These different experimental approaches will be coordinated and conducted by N. Bolfan-Casanova using the multi-anvil press (CNRS/INSU national facilities) at Laboratoire Magmas & Volcans (LMV, Université Blaise Pascal, Clermont-Ferrand, France).

The originality of HYDEEP lies in overcoming several technological obstacles. We propose here to study the mineralogical peridotite as a bulk, thus integrating the textural and chemical complexities of a mantle rock. We will particularly study the effect of oxygen fugacity on water and ferric iron incorporation. The difficulty here lies in obtaining a grain size sufficient enough for infrared spectroscopy measurement. The infiltration experiments at high pressures will be the first conducted in Europe. Another innovation of the project HYDEEP is the study of water diffusion along grain boundaries, an innovative research subject for the Geosciences community at the international level.

N. Bolfan-Casanova is the principal investigator of the HYDEEP project and will be assisted by specialists in the field of ionic diffusion (Demouchy S., Geosciences Montpellier), XANES spectroscopy (M. Munoz, LGCA), fluid infiltration (T. Hammouda, LMV) and metamorphic petrology of the serpentinized mantle (R. Debret and C. Nicollet, LMV). The results will allow us to obtain a complete and quantitative fate of the Hydrogen cycle in the deep Earth. Combining the expected results from the project HYDEEP will permit to build up-to-date geodynamic models, which are fully dependent on experimental data set.

A significant portion of funding requested by HYDEEP is a PhD contract. Indeed, experiments on fluid flow (part (2) from above focusing on hydro-fracturing, grain boundary diffusion or reactive infiltration) require a full-time researcher. The rest of the funding requested corresponds to the purchase of multi-anvil experimental supplies, in particular tungsten carbide anvils, a few travels to present results at international conferences, and travels to allow S. Demouchy to run experiments at LMV.