Diverse serpentinization and associated abiotic methanogenesis within multiple types of olivine-hosted fluid inclusions in orogenic peridotite from northern Tibet

Abstract

Abstract 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 CH4 included in peridotite can account for elevated levels of CH4 in serpentinization fluids. As such, CH4 venting at continental gas seepage hosted in orogenic peridotite should be derived mostly from CH4 that originated within the host peridotite. However, the origin of CH4 included in orogenic peridotite remains elusive, as the included CH4 is 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 CH4 synthesis 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 CH4 synthesis during hydration of olivine directly into antigorite. This contradicts the previous proposal that suggests the inhibition of H2 and CH4 production 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 CH4 compared 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 CH4 in 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 CH4 synthesis during serpentinization under different conditions.