橄榄岩蛇纹石化过程中氢气和烷烃的形成

Formation of hydrogen gas and alkane during peridotite serpentinization

蛇纹石化过程中形成氢气、烷烃和有机酸,为海底热液区生命活动提供物质和能量来源,可能对地球和其他行星早期生命起源和演化有重要影响。目前关于蛇纹石化过程中氢气和烷烃形成的研究大多以橄榄石为初始物,且温度和压力较低（≤300℃,500bar）。本研究通过一系列的水热实验,研究300~500℃、1~3kbar时橄榄石、斜方辉石、单斜辉石、橄榄岩、玄武岩以及玄武岩与橄榄岩混合物发生蛇纹石化反应后氢气和烷烃的生成。300℃、3kbar时,橄榄石蛇纹石化后产生的氢气远大于辉石蚀变后产生氢气的量。随着温度的增加,400~500℃、3kbar时,橄榄石蚀变程度极低,产生氢气的量低于斜方辉石。单斜辉石实验后没有发生蚀变,不产生氢气和烷烃。400~500℃、3kbar时,橄榄岩蛇纹石化后产生的氢气和烷烃远高于橄榄石、斜方辉石和单斜辉石。玄武岩蛇纹石化后生成氢气和甲烷的量低于橄榄岩,但与玄武岩和橄榄岩混合物相当。这是因为玄武岩的单斜辉石蚀变后形成富铁的透辉石（~8.1%FeO）,透辉石的Fe以Fe2＋为主,这降低了Fe3＋和氢气的量。以上表明,橄榄岩的蛇纹石化不同于橄榄石和斜方辉石。不仅是海底热液蚀变产生氢气和甲烷,洋壳俯冲过程中地幔楔橄榄岩蛇纹石化也会生成氢气和甲烷,但由于洋壳玄武岩的加入,氢气和甲烷的量会远小于橄榄岩蚀变时的量。

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℃, 500bar）. Here, we performed a series of hydrothermal experiments at 300 ~500℃ and 1 -3kbar to explore the formation of hydrogen gas and alkanes during serpentinization of olivine, orthopyroxene, elinopyroxene, peridotite, basalt, and mixtures of peridotite and basalt. At 300℃ 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 ~ 500℃ 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 ~ 500℃ is identical, while the amounts of alkanes are much less. The quantities of hydrogen gas and alkane during basah aheration 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 FeE ＋ -rich diopside （ - 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.