The discovery of hydrothermal fields at ocean floor opens a new chapter for marine sciences. Fluids in hydrothermal fields are hot and acidic, where at least 400 different biological organisms have been detected, including shrimp, crab and bacteria. Such biological organisms are resistant to high temperature, acidic fluids, and high pressure, and they are dependent on energy and materials (hydrogen gas, methane, ethane and propane, and organic acids) provided by the interaction between basement rocks and seawater (i.e. serpentinization). Hydrothermal fields resemble the early history of Earth. Therefore, serpentinization potentially contributes to the origin and evolution of life.
Numerous experimental studies have been performed on the formation of hydrogen gas and hydrocarbons during serpentinization. However, olivine was usually taken as the starting material to represent peridotite. Although olivine is the most abundant mineral in peridotite, it may not completely represent peridotite during alteration, possibly caused by the effect of pyroxene and spinel. A recent study performed by groups of Guangzhou Institute of Geochemistry and South China Sea Institute of Oceanology, Chinese Academy of Sciences, compared the production of hydrogen gas and hydrocarbons during olivine and peridotite serpentinization. The results show that peridotite serpentinization produces much larger amounts of hydrogen gas and methane than those formed during olivine alteration, while the quantities of ethane and propane are identical. It indicates that pyroxene and spinel promote the formation of hydrogen gas and methane. This study may be significant for the interaction between the atmosphere and lithosphere at the early history of Earth, which is probably crucial for the formation of organism and the origin of life.
The production of hydrocarbons is proposed to depend mainly on catalysts. However, this work shows that initial grain sizes of peridotite and olivine dramatically affect the formation of hydrogen gas and hydrocarbons. At the early stage, serpentinization of peridotite with large grain sizes produces much less hydrogen gas and hydrocarbons than those formed during alteration of peridotite with smaller grain sizes; with progressive reaction, their hydrogen gas is identical, while the former still has much less hydrocarbons than the latter. It indicates the formation of hydrocarbons depends on the kinetics and reaction progresses of serpentinization.
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