Thomas Rigaudier

Abiotic solid organic compounds constitute a ubiquitous byproduct of oceanic lithosphere serpentinization but their evolution and fate during subduction remains largely unexplored. Here, we assess the role of prograde metamorphism in modifying the chemical structure and isotopic signature of both biological and abiotic organic carbon hosted in sediments and serpentinites, respectively. Our findings demonstrate that these two carbon types undergo distinct maturation pathways under increasing pressure and temperature, with abiotic solid organic compounds retaining H-, O- and N-bearing organic functional groups along subduction. Notably, abiotic solid organic compounds are the sole carriers of isotopically light carbon in eclogitic terrains thanks to silicate armoring. Their recycling into the deep mantle therefore provides a plausible source for the extremely light δ ¹³ C signatures observed in some mantle reservoirs, including sub-lithospheric and eclogitic diamonds, and more broadly represents a key factor in generating mantle carbon isotope variability.

Analyse isotopique des inclusions fluides des matériaux de la croûte terrestre : caractérisation des sources des fluides et reconstitutions paléoclimatiques: Un des enjeux majeurs en Science de la Terre ces dernières décennies est de comprendre les interactions entre les différentes enveloppes terrestres. Le but poursuivi par ce travail est d’aborder quelques aspects de ces interactions du point de vue des cycles géochimiques des éléments volatils H2O et CO2 en se basant sur l’étude des inclusions fluides piégées dans des roches variées de la croûte terrestre, depuis sa surface jusqu’à l’interface croûte–manteau. La première partie de mon travail présente des travaux expérimentaux effectués sur de la halite. L’étude du fractionnement isotopique de l’oxygène et des cinétiques d’équilibration entre H2O et CO2 lors de l’évaporation de saumures a montré l’importance des corrections à effectuer sur les mesures isotopiques de l’oxygène pour des solutions de haute salinité. L’élaboration d’un protocole expérimental de précipitation de halite en milieu contrôlé a permis de mettre en évidence le potentiel des inclusions fluides pour les reconstitutions paléoclimatiques. La combinaison des données microthermométriques et des compositions isotopiques de l’hydrogène et de l’oxygène des inclusions aqueuses permet d’estimer les sources et la température de l’eau de formation de la halite ainsi que la vitesse du vent à l’interface eau-air pendant la croissance du cristal. Cette méthode a été ensuite appliquée aux dépôts de halite de Sicile datés du Messinien. La deuxième partie de mon travail est consacrée à l’identification des compositions chimiques et des sources des fluides circulant dans la croûte et à l’interface croûte-manteau à travers l’analyse des compositions isotopiques des inclusions fluides des granites et granulites.

Background and aims: Nickel (Ni) hyperaccumulators are a group of plants able to store elevated amounts of this element in their leaves. Some studies indicate that hyperaccumulation may be associated with traits favouring fast resource capture or with traits favouring nutrient and water conservation, but there is no evidence for the role of nickel hyperaccumulation in the leaf economics spectrum. Our study aims to assess the differences in the leaf economics spectrum between Ni hyperaccumulators and non-hyperaccumulators. Methods: We have conducted a field study involving five hyperaccumulators and ten non-hyperaccumulators growing on the same ultramafic community in Sabah (Malaysia). We measured two structural and seven chemical leaf traits and computed a Principal Component Analysis, which was complemented by a test of the phylogenetic signal of each trait and linear mixed models to assess the influence of each leaf trait on nickel accumulation. Results: Our analyses inferred three principal components that reflected the main environmental constraints that shape the resource acquisition strategies of the studied ultramafic plant community: leaf economics spectrum, hyperaccumulation and water-use efficiency, and calcium to magnesium balance. Moreover, the linear mixed models indicated that carbon isotope discrimination and potassium concentrations had a significant effect on Ni accumulation, suggesting that nickel might replace partially potassium in its role in plant water balance. Conclusion: Overall, the data suggest that in the community studied, Ni hyperaccumulation is independent of the leaf economics spectrum and related to plant water economy. More studies with other hyperaccumulator plants are needed to confirm these findings.

Data report: major and trace element composition of silicates and carbonates from Bengal Fan sediments, IODP Expedition 354

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 CO ₂ -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 CO ₂ -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.

Serpentinites are an important sink for both inorganic and organic carbon, and their behavior during subduction is thought to play a fundamental role in the global cycling of carbon. Here we show that fluid-derived veins are preserved within the Zermatt-Saas ultra-high pressure serpentinites providing key evidence for carbonate mobility during serpentinite devolatilisation. We show through the O, C, and Sr isotope analyses of vein minerals and the host serpentinites that about 90% of the meta-serpentinite inorganic carbon is remobilized during slab devolatilisation. In contrast, graphite-like carbonaceous compounds remain trapped within the host rock as inclusions within metamorphic olivine while the bulk elemental and isotope composition of organic carbon remains relatively unchanged during the subduction process. This shows a decoupling behavior of carbon during serpentinite dehydration in subduction zones. This process will therefore facilitate the transfer of inorganic carbon to the mantle wedge and the preferential slab sequestration of organic carbon en route to the deep mantle.