High-pressure synthesis and storage of solid organic compounds in active subduction zones

Abstract

Recent thermodynamic and experimental studies have suggested that volatile organic compounds (e.g., methane, formate, and acetate) can be produced and stabilized in subduction zones, potentially playing an important role in the deep carbon cycle. However, field evidence for the high-pressure production and storage of solid organic compounds is missing. Here, we examine forearc serpentinite clasts recovered by drilling mud volcanoes above the Mariana subduction zone. Notable correlations between carbon and iron stable-isotope signatures and fluid-mobile element (B, As and Sb) concentrations provide evidence for the percolation of slab-derived CO₂-rich aqueous fluids through the forearc mantle. The presence of carbonaceous matter rich in aliphatic moieties within high-temperature clasts (>350°C) demonstrates that molecular hydrogen production associated with forearc serpentinization is an efficient mechanism for the reduction and conversion of slab-derived CO₂-rich fluids into solid organic compounds. These findings emphasize the need to consider the forearc mantle as an important reservoir of organic carbon on Earth.

Fig. 1.. Bathymetry map, pressure and temperature (P-T) path and conceptual cross section of the Mariana forearc system.

Figure modified after (36, 37). (A) The map shows the locations of the three mud volcanoes, Yinazao, Fantangisña, and Asùt Tesoru, drilled during IODP Expedition 366. Hole locations are indicated in red circles. (B) P-T path presenting the different conditions of serpentinization recorded by the sampled forearc ultramafic clasts (i.e., Liz-, Atg-, and blue-serpentinites, from 1 to 3, respectively). Life limit is after Kashefi and Lovley (46). (C) Conceptual model illustrating serpentinization processes in relation to fluid circulation and mantle flow within the Mariana forearc.

Fig. 2.. Characterization of carbonaceous matter wetting antigorite-bearing Mariana forearc serpentinites.

(A) FTIR map of the aliphatic CH₂-CH₃ C─H stretching band area between 2800 and 3000 cm⁻¹ showing the distribution of organic carbon encountered in an Atg-serpentinite. Areas depleted in organic compounds (blue) spatially correlate with the presence of Fe-poor brucite ±antigorite, whereas the organic signal (from green to red) is systematically associated with antigorite. The organic fraction appears chemically homogeneous throughout the analyzed areas (fig. S6). a.u., arbitrary unit. (B) Examples of raw FTIR spectra of brucite, antigorite, and carbonaceous matter associated with antigorite (in red), extracted from the map shown in (A). The organic-related spectrum is characterized by bands at 1: ~2960 cm⁻¹, CH₃ asymmetric C─H stretching, 2: ~2920 cm⁻¹, CH₂ asymmetric C─H stretching, 3: ~2870 cm⁻¹, CH₃ symmetric C─H stretching, 4: ~2850 cm⁻¹, CH₂ symmetric C─–H stretching, 5: ~1730 cm⁻¹, C═O stretching of aliphatic aldehyde, 6: ~1470 cm⁻¹, CH₃ asymmetric C─H bending, CH₂ scissoring, and 7: ~1380 cm⁻¹, CH₃ symmetric C─H bending. Band assignment are from (71, 72). (C and D) SEM-SE (secondary electron) images of carbonaceous matter (CM) films embedding antigorite. (C) The discontinuous carbonaceous matter film covers the mineral surface outcropping at some places. (D) The film of carbonaceous matter completely embeds antigorite needles, locally connecting two crystals (white arrow with magnified view as inset). (E) SEM-BSE (backscatter electron) image with associated EDS element maps (C, Si, Ca, and Fe) of an Atg-serpentinite showing the relationship between CM and the HP paragenesis, made of antigorite, Fe-poor brucite (Brc), andradite (Adr), and magnetite (Mgt).

Fig. 3.. Carbonaceous matter characterization in blue-serpentinites.

(A) SEM-BSE image and associated EDS element maps for carbon and iron in a mesh core crystallizing in blue-serpentinites. The mesh core is composed of a crystal mush with a high porosity and containing brucite, lizardite, and Fe hydroxides and sulfides. The mush porosity is filled with carbonaceous matter. The red and black stars correspond to the localization of punctual EDS (shown in inset) and FTIR (B) analyses. The black inset locates (C). (B) Raw FTIR spectra of the carbonaceous matter (in red) associated with lizardite (in black). The organic-related spectrum is characterized by bands at 1 to 4: 3105, 3082, 3061, and 3028 cm⁻¹, aromatic C─H stretching, 5 and 6: ~2920 and ~ 2850 cm⁻¹, CH₂ asymmetric and symetric C─H stretching, 7 to 9: 1603, 1493, and 1450 cm⁻¹, C═C stretching ± CH₂ scissoring, 1493 cm⁻¹, and 10: 1370 cm⁻¹ aromatic C─N stretching. (C) Magnified SEM-SE image of carbonaceous matter filling the interstitial porosity of the crystal mush. Black arrows indicate micro–Fe hydroxides and sulfides associated with the crystal mush. (D) SEM-SE image of carbonaceous matter with a tubular texture.

Fig. 4.. Covariations of carbonate isotopic composition in serpentinite clasts with elemental tracers.

(A) δ¹³C_TIC versus carbonate abundance (%TIC). (B) δ¹³C_TIC versus δ⁵⁶Fe. (C) δ¹³C_TIC versus As. (D) δ¹³C_TIC versus (B). Plots (B to D) only show data for Atg-serpentinites. HP-serpentinite data are from (44, 45).