Quantification and kinetics of H2 generation during hydrothermal serpentinisation experiments
Résumé
H2-rich hydrothermal fluids generated by serpentinisation of mantle rocks at slow-spreading ridges have been revealed by recent studies [1, 2]. Fluxes and the future of the H2 produced by this process are poorly constrained [1, 3]. In this study, we aim to quantitatively evaluate the H2 production fluxes associated with these hydrothermal systems and to document the kinetics of the hydrogen-producing reaction. For this matter, hydrothermal serpentinisation experiments are being undertaken on mixtures composed of a natural peridotite from the Pindus ophiolite and olivine crystals from San Carlos. The experiments are conducted at a temperature of ~ 300° C and a pressure of 450-500 bars in large-volume Dickson-Seyfried bombs for periods of × 1 month. Starting materials are powders between 1 - 100 μm for the peridotites and individual grains ranging from 1 - 2 mm for the San Carlos olivine. They are reacted with a homemade artificial seawater in such proportion that water-rock ratio = 1.8. The reactants are loaded in a modified Ti cell fitted with a semi-permeable Au-Pd membrane simultaneously allowing direct sampling of the hydrothermal fluid and in situ monitoring of the pH2 during the advancement of the reaction. The gas fraction of the fluid sampled is then analyzed by gas chromatography (GC). The pH2 readings show traces of H2 to be present from the second day of experiment. The increase of the pH2 reaches a maximum after ~ 6 days and the pH2 finally stabilizes after ~ 16 days at ~ 12.5 bars, which corresponds to a local fO2 of about NNO-4. The GC measurements, performed after 30, 43, 51 and 65 days, yield respectively, H2 concentrations of 82.4, 89.7, 90.3 and 101 mmol.kg-1 of water, in reasonable agreement with results from previous studies [4-6]. Further experiments are being undertaken in order to: duplicate observations, especially the pH2 readings, more closely link the GC measurements and the in situ pH2 readings, especially during the first 15 days of experiment, and relate H2 production with the mineralogical composition of products of the serpentinisation reaction. The possible influence of the oxidation of the Ti cell on the H2 production will be also checked by using a Au bag instead of a Ti cell. However, from our results, it appears that H2 generation via serpentinisation is surprisingly rapid. [1] ] J.-L. Charlou et al., Chem. Geol., 191, 2002. [2] C. Mével, C.R. Geosc., 335, 2003. [3] M. Cannat et al., Geophys. Mono. Series, 188, 2010. [4] D.G. Allen, and W.E. Jr Seyfried, Geochim. Cosmochim. Acta 67 (8), 2003. [5] M.E. Berndt, et al., Geology 24 (4), 1996. [6] W.E. Seyfried,et al., Geochim. Cosmochim. Acta 71, 2007.