Serpentinization processes and hydrogen production in the North-Western Pyrenees: from field to laboratory experiments
Marguerite Godard – Géosciences Montpellier
Manuel Muñoz – Géosciences Montpellier
Eric Gaucher – Total, CSTJF, Pau
- Montaut massif shows evidences of HT hydrous metasomatism in sub-continental mantle. Turon massif indicates concomitant refertilization and mylonitization during exhumation.
- Serpentinization began at T>350°C in Montaut and Urdach highly altered peridotites while Turon records only a few rare occurrences of serpentinization at T<200°C.
- Montaut and Urdach have Fe3+/ΣFe > 0.6, with formation of Fe3+-serpentine, indicating higher hydrogen production than Turon peridotites.
Aqueous fluids interaction with ultramafic rocks at low temperatures (<400°C) leads to serpentinization. This hydration reaction is coupled to the oxidation of ferrous iron contained in primary minerals to ferric iron in secondary minerals (magnetite and/or serpentine), and to the release of hydrogen. Serpentinization is a widely studied process in mid-ocean ridges and in subduction contexts but recent studies also report the production of H2 in continental areas via serpentinization reactions. We used an integrated petrological, mineralogical and geochemical approach on 32 variably serpentinized peridotites from three orogenic massifs located in the North-Western Pyrenees, Turon, Montaut and Urdach to characterize their sequence of serpentinization. This study is coupled to a bulk and µ-XANES spectroscopy at the Fe K-edge, to derive the oxidation state of iron and to determine the hydrogen budget for each massif. The three massifs expose subcontinental mantle having contrasted serpentine contents (14, 60, and 100 wt.%, respectively) associated to contrasted bulk rock Fe3+/ΣFe ratios (0.18, 0.73 and 0.63 respectively). It suggests distinct hydrothermal alteration paths. We show that the pre-serpentinized subcontinental mantle was exposed to heterogenous fluid metasomatism (Montaut) and melt refertilization reactions (Turon), leading to minor compositional heterogeneities between the protoliths. We show the massifs have variable magnetite and Fe3+-serpentine contents, decreasing with increasing iron contents in serpentine minerals.
Serpentinization in Montaut is associated to magnetite and Fe3+-serpentine, suggesting temperatures of serpentinization >200°C. Montaut displays a continuous alteration, from amphibole formation (~800°C), down to clay formation (<100°C), enhancing the bulk rock Fe3+/ΣFe ratio. The massif is located along the North Pyrenean Thrust, constituting an efficient pathway for fluids over time. Turon mesh serpentine lacks magnetite, which suggest a serpentinization at temperature lower than that of Montaut (<200°C). The low Fe3+/ΣFe ratio indicates Turon peridotites still have a high redox and H2 production potential. Urdach comprises 1) a magnetite-free/low-temperature pervasive serpentinite domain, and 2) an ophicalcite domain with magnetite-mesh Fe3+-serpentine assemblages. It also shows element transport (Ca and Sr) from serpentinites to ophicarbonates. We posit that the crust-mantle detachment was the main pathway for hydrothermal fluids and that incipient fluid/crust interaction led to an increase in silica activity and the formation of Fe-rich serpentinites. Ophicalcites were formed by serpentinite-derived fluids having higher pH and lower silica activities leading to Fe3+-serpentine and magnetite formation. Fe oxidation state and distribution over secondary minerals, explain the equivalent Fe3+/ΣFe observed for Urdach serpentinites and ophicalcites. The Fe-rich serpentinites have a high redox potential.