Petrological factors controlling abiotic methane formation in mafic-ultramafic rocks

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Υφαντή, Ελένη
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Abiotic methane occurrence in ultramafic rocks produced via CO2 hydrogenation, is an emerging research topic. It is considered that metals occurring in mineral phases of some ultramafic rocks catalysed the Sabatier reaction. However, it is still unclear which mineralogical features and/or petrological factors control its formation, and which lithotype is the gas source. The occurrence and isotopic composition of gas occluded in rocks, is routinely quantified via whole-rock analysis, after crushing and milling of rock samples. The purpose of this study is to give new insights into the literature gaps mentioned above. The aim was to identify mineral reactions, which indicate the formation of methane and to locate the mineral-catalysts indicating past or ongoing, gas-rock reactions. Deep understanding on the petrological aspects that control the formation of abiotic methane during continental serpentinisation, would allow the extraction of research criteria that could be used in other studies to predict the potential of abiotic methane occurrence in unstudied areas. This study was performed in two parts. The first part of the thesis, focused in determining which lithologies host abiotic methane among samples deriving from different ophiolitic suites and various flood basalt localities in Greece. The results were used to investigate the mineralogical aspects that affect the formation of abiotic methane, and extract research criteria for investigation in unstudied areas. In the second part, the mineralogical and geochemical indicators of abiotic gas synthesis set in the first part, were tested for their suitability to assess the potential of a prospecting ophiolite complex to host abiotic methane. For this purpose, the Ranau peridotites in Sabah, east Malaysia, was chosen. The results of this study showed the chromitites from Moschokarya and Aetorraches areas host the highest amount of methane. Moreover the chromitites from Moschokarya are enriched in IPGE, which occur as laurite and IPGE-Ni sulphides. The secondary IPGE alloys in the Moschokarya chromitites resemble deactivated and spent catalysts, after extensive gas-rock reactions with a fluid phase. Furthermore, four main categories of research criteria were set to assist future investigation of abiotic methane in new areas (field evidence, lithologies to act as reagents, source lithology, catalyst indicators). These criteria set were used to investigate the potential of abiotic methane occurrence in an unstudied area (Ranau peridotites). Preliminary data showed that the Ranau peridotites host methane of potential abiotic origin. In summary, this research showed that massive chromitites are the catalyst-bearing lithologies that host the highest amount of abiotic methane and are considered as its source lithology. The Ru-bearing PGM are the potential minerals catalysts of the Sabatier reaction, while alteration of PGM under reducing conditions resembles a deactivated and spent catalyst, after extensive gas-rock reactions with the fluid phase.
Abiotic methane, Chromitites, Platinum group minerals, Othrys, Peridotites