High-pressure serpentinization and abiotic methane formation in metaperidotite from the Appalachian subduction, northern Vermont

Serpentinization is the process of hydroxylation of olivine-rich ultramafic rocks to produce minerals such as serpentine, brucite and magnetite. This process is commonly accompanied by Fe oxidation and release of H-2, which can be involved in abiotic reaction pathways leading to the genesis of abiotic light hydrocarbons such as methane (CH4). Examples of this phenomenon exist at the seafloor, such as at the serpentinite-hosted Lost City hydrothermal field, and on land in ophiolites at relatively shallow depths. However, the possibility for serpentinization to occur at greater depths, especially in subduction zones, raises new questions on the genesis of abiotic hydrocarbons at convergent margin and its impact on the deep carbon cycle. High-pressure ultramafic bodies exhumed in metamorphic belts can provide insights on the mechanisms of high-pressure serpentinization in subduction zones and on the chemistry of the resulting fluids. This study focuses on the ultramafic Belvidere Mountain complex belonging to the Appalachian belt of northern Vermont, USA. Microstructures show overgrowth of both primary (Mg# 0.91) and metamorphic (Mg# 0.95) olivine by delicate antigorite crystals, pointing to at least one stage of serpentinization at high-temperature conditions and consistent with the high-pressure subduction evolution of the Belvidere Mountain complex. Formation of ubiquitous magnetite and local Fe-Ni alloys testifies to the partial oxidation of Fe2+ into Fe3+ and generation of reduced conditions. Fluid inclusion trails cross-cutting the primary olivine relicts suggest their formation during the antigorite serpentinization event. MicroRaman spectroscopy on the fluid inclusions reveals a CH4-rich gaseous composition, as well as N-2, NH3 and H2S. Moreover, the precipitation of daughter minerals such as lizardite and brucite in the fluid inclusions indicate the initial presence of H2O in the fluid. High-pressure serpentinization driven by the infiltration of metasediment-derived aqueous fluids is proposed at the origin of CH4 and other reduced fluid species preserved in the fluid inclusions. This suggests the Belvidere Mountain complex as an example of deep abiotic hydrocarbon genesis related to high-pressure serpentinization in an early Paleozoic subduction zone. (C) 2021 Elsevier B.V. All rights reserved.