Formation of hydrogen gas and alkane during peridotite serpentinization

Serpentinization potentially contributes to the origin and evolution of life during early history of Earth and other planets, indicated by biological activities in hydrothermal fields those are supplied with hydrogen gas, alkanes and organic acids produced during serpentinization. Previous studies have been investigated on olivine serpentinization, and temperatures and pressures were very limited (<= 300 degrees C, 500bar). Here, we performed a series of hydrothermal experiments at 300 similar to 500 degrees C and 1 similar to 3kbar to explore the formation of hydrogen gas and alkanes during serpentinization of olivine, orthopyroxene, clinopyroxene, peridotite, basalt, and mixtures of peridotite and basalt. At 300 degrees C and 3kbar, the quantity of hydrogen gas after olivine serpentinization is much higher than that after pyroxene alteration. As temperatures increase, e. g., at 400 similar to 500 degrees C and 3kbar, the amount of hydrogen gas after olivine alteration is much lower due to its sluggish alteration rate, which is slightly less than that after orthopyroxene serpentinization. Clinopyroxene is completely unaltered after experiments, and therefore no hydrogen gas and alkanes are produced. Compared to peridotite, the quantity of hydrogen gas after olivine and orthopyroxene at 400 similar to 500 degrees C is identical, while the amounts of alkanes are much less. The quantities of hydrogen gas and alkane during basalt alteration are comparable with those after hydration of mixtures of basalt and peridotite, but they are much lower than those after peridotite alteration. This is mainly due to production of Fe2+ -rich diopside (similar to 8. 1% FeO) after basalt alteration, which dramatically reduces the amount of Fe3+ and consequently hydrogen gas. This study suggests that olivine and pyroxene cannot completely represent peridotite for the formation of hydrogen gas and alkanes during serpentinization. Moreover, hydrogen gas and alkane are produced not only after serpentinization of abyssal peridotite, which could be also formed during hydration of oceanic crust in subduction zones. However, the amounts of hydrogen and alkane during hydration of oceanic crust should be very low due to incorporation of basalt.