Keanu Loiseau

Keanu Loiseau^1,2, Charles Aubourg¹, Pierre Camps³, Christophe Rigollet²

¹ Université de Pau et des Pays de l’Adour, Laboratoire des Fluides Complexes et leurs Réservoirs, 64000 Pau, France

² CVA-Engineering, 64000 PAU, France

³ Géosciences Montpellier Université de Montpellier et CNRS, 34090 Montpellier, France

The production of hydrogen during oxidation of olivine (serpentinization) of mantle rocks is a known process in the temperature range of about 300°C. Serpentine and magnetite, both carriers of Fe2+ (and Fe3+), are by-products of serpentinization. The magnetite can oxidize and produce H2 at temperatures as low as 100°C (Geymond et al., 2023, Frontiers).  In this work we show that further oxidation of the serpentinite produces also magnetite, this reaction is fast whatever the temperature from 100 to 300°C and is speed up by the magnetite content itself. We studied it through the evolution in laboratory of various samples within a hydrated atmosphere.

In the Pyrenees, direct access to these mantle rocks, here lherzolites, is made possible by numerous exhumed mantle bodies, some of which are almost kilometers in size providing access to a unique and ideal collection of fresh samples. In addition, an active H₂ system is functioning and the generating rocks are supposed to be the same mantle rocks present from about 8-10 km depth.

Coupled with the density, the magnetic signal allows to quantify the level of serpentinization of these rocks (Loiseau et al., 2024, Geoenergy). The 7 studied lherzolite samples showed a serpentinization ranging from a few % to 100% (density of 2.4 to 3.4), corresponding to magnetite concentrations from a few hundred ppmv up to around 10%. Some of these lherzolites show low temperature alteration processes with carbonation without excluding the presence of stoichiometric magnetite.

This variety of properties has allowed us to monitor magnetite production in the laboratory on solid natural samples. Our approach involves heating samples with different degrees of serpentinization (0 to 100%) at different temperatures (100°C, 200°C and 300°C) in a hydrated atmosphere while applying a magnetic field of 100 µT. The experimental design makes it possible to qualify the magnetic minerals newly formed during 24 hours of heating. With this technique, concentrations as low as a few ppbv can be identified. Longer trials were originally carried out, but data showed that the full response is achieved in less than 1 day.

Our results demonstrated the formation of magnetite in all samples and at all temperatures. Neoformed magnetite has all the characteristics of stoichiometric magnetite, with sizes in excess of several tens of nanometres. The most remarkable feature of our study is that it shows:

1)     Fully serpentinized samples still generate magnetite in presence of hot water and air.

2)     The kinetics of this magnetite generation are temperature dependent (one order of magnitude more between 100-200°C and 300°C).

3)     A strong correlation between magnetite production and the quantity of initial magnetite. This means that presence of magnetite speeds up production of magnetite, in other words, that magnetite stimulates the processes of Fe2+ oxidation to Fe3+.

Our study indicates a catalytic role for the magnetite and suggests that fully serpentinized zones could still play a significant role in hydrogen production at low temperature context such as mantle bodies at shallow depth (from about 3km).