Diverse serpentinization and associated abiotic methanogenesis within multiple types of olivine-hosted fluid inclusions in orogenic peridotite from northern Tibet
Diverse serpentinization and associated abiotic methanogenesis within multiple types of olivine-hosted fluid inclusions in orogenic peridotite from northern Tibet
复制标题
藏北造山橄榄岩中多种类型的橄榄石流体包裹体中的多样化蛇纹石化作用和相关的非生物产甲烷作用
DOI：
10.1016/j.gca.2020.12.016
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发表时间：
2021
影响因子：
5
通讯作者：
Wan-Cai Li
中科院分区：
文献类型：
--
作者：
Long Zhang;Qiang Wang;Xing Ding;Wan-Cai Li
通过橄榄岩渗透的热液富含非生物CH4，可以为化学合成微生物活动提供燃料，并可能促进早期生命。与支持非生物甲烷生成和流体循环耦合的模式相反，最近的研究表明，橄榄岩中ch4的浸出可以解释蛇纹岩化流体中ch4水平的升高。因此，造山带橄榄岩中陆相气体渗漏处的ch4排放应主要来源于寄主橄榄岩内的ch4。然而，造山带橄榄岩中包含的ch4的来源仍然难以捉摸，因为据报道，包含的ch4要么是在流体包裹体的蛇纹岩化过程中原地形成的，要么是来自减少的外部流体。此外，造山带橄榄岩含ch4流体包裹体中继子矿物的不同组合表明，流体包裹体中流体-矿物相互作用的多样性。新发现了3种含ch4流体包裹体。ⅰ型流体包裹体含CH4(g)+ 反长辉石 + 水辉石 + 磁铁矿 + 菱镁矿，表明橄榄石直接水化成反长辉石过程中发生了非生物CH4合成。这与之前提出的在反长岩稳定场中高温蛇纹化过程中抑制h2h和ch4生成的观点相矛盾。II型流体包裹体由甲烷(g) + N2 (g) + 利蛇纹石 + 水镁石 ± 磁铁矿或CH4 (g) + N2 (g) + 利蛇纹石 + 磁铁矿与brucite-bearing夹杂物产生更少的CH4compared brucite-free夹杂物。II型流体包裹体中水镁石的结晶可能受捕获流体中溶解硅的浓度控制。类型III流体包裹体是由甲烷(g) + 叶蛇纹石 + 镁 + 石墨 ± 磁铁矿 ± 白云石。III型流体包裹体中高浓度的氧化无机碳可能促进了碳酸盐的沉淀和反长花岗岩的结晶。此外，少量III型流体包裹体中残留的蜥蜴石的存在表明，伴随碳酸盐饱和的反长花岗岩生长可能是一个两步过程，即橄榄石初始水化成蜥蜴石，蜥蜴石转化为反长花岗岩。综上所述，造山带橄榄岩中含ch4流体包裹体非常丰富，可能是蛇纹岩热液体系中非生物ch4的重要储层。此外，本研究发现的造山带橄榄岩中多种类型的含ch4流体包裹体表明，含橄榄石的含ch4流体包裹体有可能成为研究不同条件下蛇纹岩化过程中非生物ch4合成的新窗口。
Hydrothermal fluids percolating through peridotite are highly enriched in abiotic CH4, which can fuel chemosynthetic microbial activity and potentially early life. In contrast to the paradigm favoring coupled abiotic methanogenesis and fluid circulation, recent studies have suggested that leaching of CH4included in peridotite can account for elevated levels of CH4in serpentinization fluids. As such, CH4venting at continental gas seepage hosted in orogenic peridotite should be derived mostly from CH4that originated within the host peridotite. However, the origin of CH4included in orogenic peridotite remains elusive, as the included CH4is reported to form either in situ during serpentinization within fluid inclusions, or to originate from reduced external fluids. Moreover, varying associations of step-daughter minerals documented in CH4-bearing fluid inclusions in orogenic peridotite demonstrate the diversity of fluid–mineral interactions within fluid inclusions. Here we present a detailed petrological investigation into olivine-hosted CH4-bearing fluid inclusions in ophiolitic harzburgite from the North Qilian orogen in northern Tibet, which reveals the occurrence of abiotic CH4synthesis during diverse serpentinization within multiple types of olivine-hosted fluid inclusions. Three types of CH4-bearing fluid inclusions are newly identified in the harzburgite. Type I fluid inclusions contain CH4(g)+ antigorite + brucite + magnetite + magnesite, which imply abiotic CH4synthesis during hydration of olivine directly into antigorite. This contradicts the previous proposal that suggests the inhibition of H2and CH4production during high-temperature serpentinization in the stability field of antigorite. Type II fluid inclusions consist of CH4(g)+ N2(g)+ lizardite + brucite ± magnetite or CH4(g)+ N2(g)+ lizardite + magnetite, with brucite-bearing inclusions yielding less CH4compared with brucite-free inclusions. The crystallization of brucite in type II fluid inclusions was probably controlled by the concentration of dissolved Si in the trapped fluids. Type III fluid inclusions are composed of CH4(g)+ antigorite + magnesite + graphite ± magnetite ± dolomite. High concentrations of oxidized inorganic carbon in type III fluid inclusions likely promoted the precipitation of carbonate and crystallization of antigorite. Moreover, the presence of relict lizardite in minor type III fluid inclusions suggests that antigorite growth accompanying carbonate saturation is probably a two-step process, with initial hydration of olivine into lizardite being followed by transformation of lizardite into antigorite. Above all, this study demonstrates that CH4-bearing fluid inclusions in orogenic peridotite can be very abundant, and may be a significant reservoir of abiotic CH4in serpentinite-hosted hydrothermal systems. Moreover, the multiple types of CH4-bearing fluid inclusions in orogenic peridotite presented in this study indicate that olivine-hosted CH4-bearing fluid inclusions can potentially be a novel window for studying abiotic CH4synthesis during serpentinization under different conditions.