The DEEP ISOTOPIC CYCLE of HYDROGEN – H-DEEPISO

Hydrogen is the most abundant element of the Solar system. On Earth, it has formed H2O, building up the oceans by degassing of the deep Earth interior. Hydrogen is also a ubiquitous proton impurity in nominally anhydrous minerals (NAMs) composing the Earth upper mantle. Furthermore, it plays an essential role on geodynamics by reducing the melting temperature and the viscosity of the Earth’s mantle rock, hence boosting convection. The presence of H in the primordial rocks that build up our planet permit to estimates that three to seven times the current water mass of our oceans is stored inside the deep Earth. To be able to understand and predict the fate of the oceans, the evolution of their hydrogen isotopic composition (D/H ratio, i.e. the ratio of the concentration of the heavy isotope, deuterium, to that of the light isotope, protium) is a fundamental tool. The evolution of the oceanic reservoir volume and isotopic composition results from various processes implying hydrogen escape to space (which leads to preferential loss of the lighter isotope), volcanic degassing (which fills up the hydrosphere) and burial of water into the mantle (depleting the hydrosphere). The amount of water potentially stored in the deep Earth inside NAMs exceeds the mass of the oceans, thus the mantle is controlling the mass and composition of the oceanic hydrogen. In order to model the H-deep cycle, knowing the distribution and fractionation of H between the different reservoirs is crucial. And such data, representative of what occurs at great depth within the mantle, can only be obtained by performing high-pressure high-temperature experiments, combined with very fine analyses to quantify the speciation and isotopic composition of hydrogen. The originality of this project lies in the search for new speciation of hydrogen within deep mantle minerals, and revisiting the fundamental fractionation mechanism of hydrogen isotopes into the deep Earth.
The project H-DEEP-ISO gathers well-known French mineralogists and geochemists from three different laboratories, bringing their complementary expertise to constrain the storage, speciation, isotopic fractionation and mobility of H2 in mantle minerals. The results will allow to constrain the Earth deep cycle of hydrogen and the fate of the oceans. The H-DEEP-ISO team members will share their tools (multi-anvil device, Paterson press, Raman facility) to perform state-of-the art experiments reproducing the deep Earth and develop new spectroscopic methods to detect and constrain the speciation of H in minerals of the mantle with a high spatial resolution. The success of the project also lies on the large sample collection that will allow a survey of H in natural samples. The project will develop and combine new tools at national scale such as spectroscopic techniques and mass spectrometry techniques to improve D/H isotopic measurements on small, synthetic high-pressure samples and natural samples. The expected results will lay the foundation for a better understanding of the H deep Cycle with implications for the fate of the oceans.
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