title: Massive production of abiotic methane during subduction evidenced in metamorphosed ophicarbonates from the Italian Alps.
abstract: Alteration of ultramafic rocks plays a major role in the production of hydrocarbons and organic compounds via abiotic processes on Earth and beyond and contributes to the redistribution of C between solid and fluid reservoirs over geological cycles. Abiotic methanogenesis in ultramafic rocks is well documented at shallow conditions, whereas natural evidence at greater depths is scarce. Here we provide evidence for intense high-pressure abiotic methanogenesis by reduction of subducted ophicarbonates. Protracted (≥0.5-1 Ma), probably episodic infiltration of reduced fluids in the ophicarbonates and methanogenesis occurred from at least ∼40 km depth to ∼15-20 km depth. Textural, petrological and isotopic data indicate that methane reached saturation triggering the precipitation of graphitic C accompanied by dissolution of the precursor antigorite. Continuous infiltration of external reducing fluids caused additional methane production by interaction with the newly formed graphite. Alteration of high-pressure carbonate-bearing ultramafic rocks may represent an important source of abiotic methane, with strong implications for the mobility of deep C reservoirs.
authors.affiliation: Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie UMR 7590 CNRS-UPMC-IRD-MNHN, Campus Jussieu, Case courrier 115, 4 Place Jussieu, 75005 Paris, France.
authors.forename: Alberto
authors.initials: A
authors.lastname: Vitale Brovarone
authors.affiliation: Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, F-75005 Paris, France.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Martinez
authors.affiliation: Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie UMR 7590 CNRS-UPMC-IRD-MNHN, Campus Jussieu, Case courrier 115, 4 Place Jussieu, 75005 Paris, France.
authors.forename: Agnès
authors.initials: A
authors.lastname: Elmaleh
authors.affiliation: Dipartimento di Scienze della Terra, Università degli Studi di Torino, via Valperga Caluso 35, 10100 Torino, Italy.
authors.forename: Roberto
authors.initials: R
authors.lastname: Compagnoni
authors.affiliation: Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, F-75005 Paris, France.
authors.forename: Carine
authors.initials: C
authors.lastname: Chaduteau
authors.affiliation: Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie UMR 7590 CNRS-UPMC-IRD-MNHN, Campus Jussieu, Case courrier 115, 4 Place Jussieu, 75005 Paris, France.
authors.forename: Cristiano
authors.initials: C
authors.lastname: Ferraris
authors.affiliation: Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie UMR 7590 CNRS-UPMC-IRD-MNHN, Campus Jussieu, Case courrier 115, 4 Place Jussieu, 75005 Paris, France.
authors.forename: Imène
authors.initials: I
authors.lastname: Esteve