The distribution of volatile elements in the Earth's mantle

Hydrogen (H) and carbon (C) are the two most abundant volatile elements in the Earth’s mantle and, despite being present at trace concentrations, strongly influence many geochemical and geophysical properties of rocks, and in turn, control the geodynamical evolution of the planet. While many studies have investigated the effects of H and C on the properties of minerals and rocks, less data are available regarding the effects of these volatiles in systems where they are both present. The aim of this thesis was to shed new light on the behavior of H and C in the mantle, this was done following two main approaches: (i) studying of a large set of nominally anhydrous mineral inclusions hosted in lithospheric diamonds from different kimberlites and cratons. The H2O content of these inclusions was determined, and H diffusion modelling demonstrated that the H2O contents recorded from inclusions reflects fast re-equilibration with the diamond-forming agent. Using the measured H2O contents together with H2O partition coefficients allowed the H2O content of the diamond-forming agents to be constrained, which was estimated to be on average ≤ 3 wt%. The trace element composition of some inclusions that were extracted from their diamond hosts was also determined. Comparisons of H2O contents and trace elements patterns revealed two different metasomatic trends for eclogitic minerals, one characterized by high H2O contents associated with low LREE concentrations and the other by low to moderate H2O contents and a large range of LREE concentrations. The H2O-rich metasomatism is likely due to alteration operated by fluids produced by deserpentinization reactions taking place during subduction of the oceanic slab, while the H2O-poor trend is probably due to decoupling between H2O and LREE during metasomatism. (ii) performing a series of high-pressure and high-temperature experiments at 1200°C and 2 GPa using hydrous and hydrous-carbonated basaltic systems, producing large clinopyroxene crystals in equilibrium with pools of basaltic-andesitic melt (comprising > 60 wt% of the system). Measurement of the H2O contents of clinopyroxenes and melts in equilibrium at different CO2 contents allowed quantification of the effect of CO2 on the clinopyroxene/melt partition coefficient of H2O, which was summarized in an empirical equation. This equation was then used to: (i) model melting of a hydrous and hydrous-carbonated eclogite in the mantle and (ii) to calculate the expected H2O content of mantle domains responsible for the production of oceanic island basalts with the HIMU isotopic signature.