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.

Pulsated Global Hydrogen and Methane Flux at Mid‐Ocean Ridges Driven by Pangea Breakup

“…Our estimates oscillate between those of Worman et al (2016) and Cannat et al (2010). Outside of ridge environments, other sources of hydrogen derived from serpentinization include active subduction zones (e.g., Vitale‐Brovarone et al, 2017) and continental rifting that leads to hyperextended passive margins (e.g., Manatschal et al, 2015). Our analysis does not yet include either of these environments.…”

“…°From Precambrian continental crust: SL14, Sherwood Lollar et al (2014). +From serpentinization in subduction zones (no upper limit was given): Vitale‐Brovarone et al (2017).…”

Devolatilization of Subducting Slabs, Part II: Volatile Fluxes and Storage

“…The second limitation stems from the binary H 2 O‐CO 2 molecular fluids assumed for the liquid phase in our model. Variations of pressure, temperature and particularly oxygen fugacity during subduction can give rise to the dominance of methane (CH 4 ) or precipitation of graphite (Brovarone et al, ; Galvez et al, ). Moreover, ionic carbon species (e.g., HCO , CO ) are inferred to be common in subduction zone settings (Frezzotti et al, ; Ague and Nicolescu, ), although detailed thermodynamic modeling suggests that molecular carbon species can dominate over ionic ones in fluids derived from subducted basalts (Galvez et al, ) and sediments (Connolly and Galvez, ).…”

Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex, Spain)

“…VM: Voltri massif (Scambelluri et al, 2016); ZS: Zermatt-Saas; B: Bracco Unit, Internal Ligurides; UV: Ubaye Valley, Western Alps (all from Collins et al, 2015). L, L*: Lanzo massif peak conditions and PT of graphite-formation in ophicarbonate, respectively (Vitale Brovarone et al, 2017); BA: contact-metamorphic ophicarbonates, Bergell aureole (Pozzorini & Früh-Green, 1996;Trommsdorff & Connolly, 1996); BD: Bellinzona-Dascio unit, Central Alps (Stucki, 2001). Dark grey field: maximum P-T conditions for hydrothermal ophicarbonate formation in seafloor serpentinites (Schwarzenbach et al, 2013).…”

“…Many examples are preserved in brucite-or olivine-bearing Atg-serpentinites that underwent metamorphism in intermediate to warm subduction settings (cf. orange and pink dotted paths in Figure 1; Collins et al, 2015;Scambelluri et al, 2016;Vitale Brovarone et al, 2017). Examples of serpentinite-hosted meta-ophicarbonate recording metamorphic grade approaching or exceeding the P-T conditions of antigorite breakdown are very rare (e.g.…”