Favorable P–T–ƒO2 conditions for abiotic CH4 production in subducted oceanic crusts: A comparison between CH4-bearing ultrahigh- and CO2-bearing high-pressure eclogite

Abiotic Methane Reservoirs in the Western Tianshan HP–UHP Metamorphic Belt, China

“…The protoliths are likely dominated by normal seafloor basalts, including Etype or N-type MORB and OIB (Gao and Klemd, 2003;Klemd et al, 2005;Ai et al, 2006). Peak metamorphic conditions have been estimated as ~550°C and 2.7-3.2 GPa, consistent with the coesite observed in a variety of rocks (e.g., Lü et al, 2008Lü et al, , 2009Lü et al, , 2013Lü et al, , 2014Yang et al, 2013;Tian et al, 2016;Tan et al, 2017;Zhang L F et al, 2018;Wang et al, 2022). Zircon and rutile U-Pb dating indicates that the eclogites underwent peak metamorphism at c. 320 Ma (Su et al, 2010;Li et al, 2011;Yang et al, 2013;Tan et al, 2017;Zhang L F et al, 2018;Zhang L J et al, 2018).…”

Deciphering the Origin of Abiotic Organic Compounds on Earth: Review and Future Prospects

“…6e). Based on garnet (Grt)-clinopyroxene (Cpx) oxybarometry, the ƒO 2 was sufficiently low (FMQ−3) such that the graphite- saturated C-H-O fluid contained CH 4 as the dominant volatile carbon species (Wang et al, 2022). To date, CH 4 appears to be the most common hydrocarbon species hosted in HP-UHP rocks.…”

“…The same approach developed by Connolly and Galvez (2018) has been applied to simulate dissolved carbon species released during the metamorphism of real rocks. For example, Wang et al (2022) used this method to investigate the factors stabilizing CH 4 during metamorphism of UHP mafic eclogite. In coesite-bearing eclogites, garnet cores and mantles and omphacite cores have been observed to contain primary CH 4 + H 2 O (± H 2 ) fluid inclusions, as well as core TFeO and Fe 3+ /ΣFe ratios that were elevated relative to garnet and omphacite rims (Figs.…”

“…) resulted in higher ƒO 2 conditions of FMQ−0.8. Thermodynamic models show that this change in ƒO 2 is primarily driven by the slight change in temperature between peak and early retrograde conditions, where low ƒO 2 is predicted at lower T, whereas ƒO 2 is less sensitive to the pressure drop, with lower ƒO 2 predicted at higher P at fixed T. Furthermore, a comparison was made with an HP eclogite from the same terrane in which CO 2 -rich inclusions were observed; Wang et al (2022) found that the higher bulk CO 2 content and elevated Fe 3+ /ΣFe ratio inherited from pre-subduction alteration, prohibited sufficiently low ƒO 2 to produce CH 4 . However, ƒO 2 conditions between FMQ and FMQ+1 were low enough to stabilize graphite via auto-redox adjustments (O 2 + (Fe 2+ O) mineral ↔ (Fe 3+ 2 O 3 ) mineral , O 2 + C 0 ↔ CO 2 ) at 2.6 GPa, 500°C.…”

“…Organic compounds are usually reducing substances, e.g., CH 4 , and can be produced in a variety of settings, including interstellar environments in solar nebulae to the deep Earth. Indeed, such molecules may even play a role in regulating Earth's redox evolution under hightemperature (HT) and high-pressure (HP) conditions in subduction zones and the deep mantle (e.g., Sverjensky et al, 2014;Brovarone et al, 2017;Walters et al, 2020;Debret et al, 2022;Wang et al, 2022). Some studies have even proposed that deep abiotic hydrocarbons represent potentially untapped energy resources that have not yet been targeted by the oil and gas industry (Kutcherov and Krayushkin, 2010;Truche et al, 2020).…”