台湾东部绿岛热液区烃类气体的成因研究

The occurrence and formation mechanism of abiogenic hydrocarbons within Earth’s interior is a research focus in gas geochemistry. The hydrothermal abiogenic hydrocarbons mainly occur in ultramafic-hosted hydrothermal systems. The Lutao andesite -hosted hydrothermal system in Eastern Taiwan discharges methane with heavy δ13C values, which may be originated from abiogenic pathways. Therefore, in-depth study is needed to distinguish the origin of Lutao hydrocarbons and to unveil their evolution during hydrothermal processes. In this project, we will choose the short-chain hydrocarbons (C1-C5) from Lutao hydrothermal system as a research object and analyze the chemical components, stable carbon and hydrogen isotopic compositions, helium isotopic characteristics, and 14C contents of the related samples. We will investigate the variations of hydrocarbons during hydrothermal circulation and study the factors that controlled these variations. We will distinguish the origin of Lutao hydrocarbons and discuss the relationships between the andesite host rock and the formation and evolution of hydrocarbons. The scientific question is: (1) the effects of secondary processes such as oxidation and CO2-CH4 conversion on the geochemical characteristics of hydrothermal hydrocarbons, and (2) can abiogenic hydrocarbons be produced in andesite-hosted hydrothermal systems and the related formation mechanism. This project will help better understanding the geochemical behavior of gaseous hydrocarbons discharge from hydrothermal systems, and will also complement the occurrence of hydrocarbons in hydrothermal systems with different geological backgrounds.

地球内部的无机成因烃类气体及其形成机制是气体地球化学研究的热点问题。热液无机成因烃类气体主要发现于超镁铁质岩系热液系统中。台湾东部绿岛以安山岩为基岩的热泉中甲烷具有较重的δ13C值，可能来自于无机成因作用，亟需深入研究以揭示绿岛烃类气体的演化过程和成因类型。本项目拟选取绿岛热液短链烷烃烃类气体（C1-C5）为主要研究对象，开展烃类气体的组分特征、稳定碳氢同位素组成、He同位素示踪和14C定年研究，分析在热液活动过程中绿岛烃类气体的变化规律，确定烃类气体的成因类型，并揭示安山岩基岩与绿岛热液烃类气体的形成与演化之间的关系。拟解决的关键问题是1）氧化作用、CO2-CH4转化等次生作用对热液烃类气体地球化学特征的影响；2）以安山岩为基岩的热液系统是否能够生成无机成因烃类气体及其形成机制。本项目研究对认识热液系统中烃类气体的地球化学行为、完善不同地质背景下热液烃类气体的成因机制具有重要意义。

俯冲带热液体系中烃类气体的来源与形成机制对认识俯冲带物质循环、探讨热液循环过程中含碳气体的地球化学行为具有重要意义。本课题以台湾东部绿岛热液体系为研究对象，开展了气体组分关系研究、稳定同位素和团簇同位素分析、微生物培养实验和热液流体地球化学等研究。本项目1）深入剖析了绿岛热液循环体系，在全球范围内首次报道了绿岛热液同时受到台风和潮汐的影响；2）通过静置实验，揭示了微生物氧化作用对绿岛热液烃类气体稳定碳氢同位素特征的影响，表明热解成因或微生物成因的烃类气体通过细菌有氧氧化作用可能呈现类似无机成因烃类气体的地球化学特征；3）绿岛气体的组分关系特征、稳定碳氢同位素和甲烷团簇同位素特征均表明绿岛热液烃类气体主要为无机成因，结合绿岛的幔源氦同位素特征，绿岛烃类气体主要来源于绿岛基岩中的富CO2-H2O的流体包裹体。本研究表明无机成因烃类气体可能广泛存在于火山-热液体系中，而非传统上认为局限于超镁铁质岩或蛇纹岩化的热液体系中。此外，本项目还发现绿岛热液流体具有全球最高的Ca富集程度，Ca富集与循环碳酸盐有关，这为俯冲带的碳循环研究提供了重要的数据和理论支撑。