In situ thermodynamic monitoring of serpentinization and H2 generation - An experimental study

Abstract:

To quantify the H₂ production associated with serpentinization at slow spreading mid-oceanic ridges, hydrothermal alteration experiments have been performed. Starting materials are a fine-grained harzburgite from Pindos ophiolite and artificial seawater (3.2 wt.% NaCl). Two grain sizes (P1 and P2), both <60μm, are used. The water-rock ratio is ~1.8 (40g of rock and 72g of water). Experiments are carried out at 250, 300 and 350°C, and for a constant pressure of 500 bar. For each temperature, two experiments are conducted in parallel. In the first, the hydrothermal fluid is periodically sampled and its H₂ concentration ([H₂]) determined by gas chromatography. In the second, the H₂ fu/gacity (ƒH₂) inside the reaction cell is continuously monitored with a H₂semi-permeable membrane made of Au₂₀Pd₈₀.

[H₂] is maximum at 300°C with a plateau at ~115 mmol/kg_water against ~50 mmol/kg_water at 250°C (for the same duration, the experiment at 350°C (still in progress) shows [H₂] ~10 times lower). The kinetics of H₂ production is controlled by the particle size. At 300°C, the [H₂] plateau is reached in ~35 days for P2 against ~55 days for P1. At 250 and 300°C, an ƒH₂ increase is observed after the 2^nd day and the rate of fH₂ increase reaches a maximum during the 4^th day. At 300°C, the ƒH₂ stabilizes after ~40 days at 23 bar (fO₂ ≈ NNO - 4.1 to 4.3). At 250°C, it takes ~70days to reach a plateau ƒH₂ of 38 bar (fO₂≈ NNO - 4.6 to 5.1). The solid products recovered and analyzed at the end of experiment are characterized by a serpentinization rate > 60% and they contain variable amounts of magnetite and brucite, in addition to serpentine minerals.

To link ƒH₂ and [H₂], experiments coupling the two types of measurement have been performed in the H₂O-H₂ system. For [H₂] <50 mmol/kg_water, at 250 and 300°C, the first results show ƒH₂/[H₂] ≈ 660 and 380 bar.kg_water/mol_H2respectively.

These results imply that H₂-bearing fluids associated with serpentinization strongly positively deviate from ideal behaviour, consistent with the proximity of the two-phase field of the H₂O-H₂ system. This specific behaviour of H₂O-H₂ fluid mixtures needs to be accounted for in thermodynamic models of serpentinization.