Mineralogical evidence for H2 degassing during serpentinization at 300 °C/300 bar

Hydrogen is produced in large amounts during hydrothermal alteration of peridotite in low-spreading-rate mid-ocean ridges. This production is directly linked to reducing conditions in hydrothermal fluids induced by the oxidation of Fe2+ in primary minerals (olivine and pyroxene) to Fe3+ in secondary minerals (magnetite and serpentine). A better knowledge of iron speciation in serpentine is therefore crucial to the quantification of hydrogen production during the serpentinization process. For the first time, we have determined the amount of ferric iron in altered peridotite as a function of alteration time. We investigated experimentally the alteration of powdered Iherzolite in pure water at 300 degrees C/300 bar. For each experimental run (0, 7, 18,34 and 70 days), H-2 degassing was measured using in-situ gas chromatography and the experimental products were analyzed using XRD, Raman and X-ray absorption spectroscopy at the iron K-edge. In parallel, aqueous solutions were analyzed by ICP-AES. Our results show quasi-complete scrpentinization at 70 days with replacement of primary olivine and pyroxene by secondary lizardite and magnetite. The Fe3+/Fe-total ratio is linearly dependent on the hydrogen production and ranges from 0 to 0.66 at the end of the experiment. Our results reveal strong variations in Fe3+ in serpentine for different alteration times, from 0 to 100% of ferric iron, including up to 12% of tetrahedral iron. Hydrogen was produced in three main stages: (1) a first stage during which the H-2 production rate reaches a maximum at 18 days and is controlled by the crystallization of magnetite, (2) an intermediate stage during which serpentine incorporates ferric iron and thus plays a major role (up to 50%) in the hydrogen formation, and (3) a final stage during which magnetite amount increases from similar to 2 to similar to 5% of the mineral assemblage. The last alteration stage is accompanied by a slight increase of the Fe3+/Fe-total ratio, while the rate of hydrogen production decreases at the end of the experiment. Consequently, variations of the ferric iron contents in natural oceanic peridotites may constitute a good indicator of the hydrogen-potential of various ultramafic hydrothermal fields. (C) 2011 Elsevier B.V. All rights reserved.