Abstract
Abiotic production of diH2 gas (H2) is commonly associated to, serpentinization, the hydrothermal alteration of mantle peridotites. Serpentinization induces the oxidation of Fe2+ contained in primary mantle minerals, olivine and pyroxenes, into Fe3+ in secondary magnetite and serpentine. Iron oxidation is coupled to the reduction of water leading to H2 production. Ocean-continent transitions (OCTs) of non-volcanic passive margins expose km-scale sections of exhumed serpentinized mantle and are therefore of great interest to investigate H2production. Our study focuses on the Iberian paleo-margin (NW Pyrenees) where mantle relics outcrop.This thesis combined field and experimental approach. The first part presents the results of a multi-analytical study of the petrostructure (EBSD), mineralogy (XRD, Raman, bulk-rock and in-situ Fe K-edge XANES) and geochemistry ((µ)-XRF, EPMA, (LA)-ICPMS)) of 32 samples collected from 3 NW Pyrenees peridotite massifs: Urdach, Turon, and Montaut. The second part presents the results obtained from a long-term hydrothermal experiment performed on natural samples from the Turon massif.Each massif is composed mainly of spinel-lherzolites from sub-continental mantle lithosphere, of contrasted serpentinization degrees (serpentine 14-100 wt%). We show that the three peridotite massifs have contrasted magnetite content, independently of their degree of serpentinization and inversely proportional to the iron content of serpentine minerals. Magnetite precipitation is attributed to temperatures of serpentinization >250°C.Selective enrichments in Fluid Mobile Elements, in particular Cs, Sb and Li, indicate interactions with fluids derived from neighboring sediments and continental crust for the Turon and Montaut peridotites. These results are consistent with the structural position of these massifs that remained below a unit of continental crust and pre-rift cover during the mid-Cretaceous extensional event. The Urdach massif, which is directly exposed to the seafloor, records the formation of ophicalcites and serpentinites. Our results show that ophicalcites contain magnetite while serpentinites are magnetite-free. These features are interpreted as representative of different fluid/rock reaction paths with Ca-Sr rich fluids for magnetite-bearing ophicalcites and Si-rich fluid with a crustal origin, for magnetite-free serpentinites. These contrasted conditions of serpentinization from one massif to the other stem from different structural context.We show that, at equivalent degree of serpentinization, hydration of the sub-continental mantle produces as much H2 as the oceanic mantle, but displays a higher contribution of Fe3+-serpentine for H2 production. It results from the dearth of magnetite induced by lower temperatures of serpentinization and/or higher silica activity of fluid(s).Given our results and the large quantities of ultrabasic rock exposed at the OCT of passive margins, these environments likely represent a significant contribution to the global H2 cycle.To better constrain the distribution and valence of iron between serpentine and magnetite in natural samples, we studied the iron redistribution during a 417-day serpentinization experiment on a poorly serpentinized Turon lherzolite. Alteration occurs in highly oxidizing conditions with the precipitation of iddingsite and hematite and a Si-rich dissolution front is observed at the surface of the sample. Precipitation of Fe-serpentine is observed in the veins of the protolith. Linked to strong fO2 conditions, we suggest a remobilization of Fe from unstable magnetite to serpentine. Results of XANES spectroscopy at the Fe K-edge show contrasted Fe oxidation states between surfaces exposed to the fluid (high water/rock ratio –W/R) displaying high oxidation states and the serpentine contained in the rock (low W/R) displaying low oxidation states. We posit that the behavior of H2 (trapping or release) controls the local fO2 conditions.