Hydrogen and magnetite are produced within the oceanic lithosphere during interaction between mantle peridotite and seawater (serpentinization). Some ecosystems at deep sea are thought to be sustained by such hydrogen and related hydrocarbons as an energy source [1]. Recently, it has been suggested that the serpentinization proceeds by seawater penetrating to the deep fracture zones at temperatures exceeding 350℃ [2]. However, due to limited experimental studies at high temperatures, it has not been clarified whether hydrogen is produced at the deep parts of the oceanic lithosphere and transported to the microbial life in shallower zone. Hydrogen production is commonly accompanied with magnetite formation [3]. In this study, we conducted the batch type and flow-through type experiments in the systems of olivine (Ol)-H₂O and Ol-orthopyroxene (Opx)-H₂O at temperatures from 200 to 400℃. Based on the thermogravimetric analyses, the chemical composition and magnetic susceptibility, we discussed the effects of temperature and silica on magnetite formation during serpentinization.

In the batch type experiments, the extent of reaction changes systematically with temperature and is greater in the Ol-Opx-H₂O system. The reaction rate in the flow-through type was about 10 times larger. These results indicate that the reaction rate is enhanced by silica derived from orthopyroxene and/or advective mass transfer. The electron microprobe analyses suggest that serpentine minerals contain Fe(III). The magnetic susceptibility of the products related to magnetite amount increases with the extent of reaction in Ol-H₂O system increasing whereas does not increase in Ol-Opx-H₂O system. These results and thermodynamic calculations suggest that in Ol-Opx-H₂O system, magnetite is not produced due to high silica concentration but Fe(III)-serpentine is the dominant host of Fe(III) and the hydrogen production can be higher than that of Ol-H₂O system.

The present study suggests that hydrogen production accompanying serpentinization proceed to orthopyroxene-bearing peridotites (i.e., harzburgite) when seawater circulates in the deep part of the oceanic plate.

Reference

[1] Boetius, 2005, SCIENCE, 307, 1420-1422

[2] Prigent et al, 2020, Earth and Planetary Science Letter, 532, 115988

[3] Klein et al, 2013, Lithos 178, 55-69