Carbon and sulfur trapping during contact metamorphism: Impact on volatile transfers in sedimentary basins with magmatic activity, example of the Guaymas Basin.

Abstract

The opening of sedimentary basins is often accompanied by magmatic activity, and the emplacement of magmatic sills in the sediments. Contact metamorphism at the edges of the sills releases  large amounts of volatiles (H2O, CO2, CH4, and H2S), disrupting the carbon and sulfur cycle within the basin. The volatiles then migrate toward the seafloor and may be released into the oceans and atmosphere. While the reactions of dehydration, decarbonation, and desulfurization during contact metamorphism are well understood, the interactions between the produced fluids and the rocks remain unclear, especially when the fluids remain trapped within the metamorphic aureoles. This study, based on samples collected during the IODP Expedition 385 in the Guaymas Basin, aims to quantify the fluids produced and trapped in the metasediments during the emplacement of magmatic sills. Geochemical and mineralogical characterization of sediments in contact with the sill indicates metamorphic reactions, primarily affecting silica polymorphs (opal-A and opal-CT), diagenetic sulfides (pyrite), and detrital minerals (mainly quartz, clays, and feldspars), and cracking of the organic matter. The transitions from opal-CT to metamorphic quartz and from pyrite to pyrrhotite are good markers of the metamorphic aureoles. The size and the mineralogical assemblages differ in the upper and the lower aureoles suggesting major fluid rock interactions below the sill.  Indeed in this metamorphic aureole, the precipitation of carbonate-pyroxene-pyrrhotite patches within a quartz-plagioclase matrix is a good indicator of fluid-rock interactions at about 200-400°C. These newly formed carbonate patches indicate the trapping of metamorphic fluids below the sill. Isotopic data (δ18O and δ13C) were used to estimate the peak temperature of metamorphism and precipitation of carbonates. Additionally, thermodynamic modeling was conducted to understand the conditions of these reactional textures in the metasediments below the sill and quantify the amount of carbon and sulfur trapped under the sill. We show that a third of the carbon released to the fluid by organic matter cracking is directly trapped under the sill as carbonates. These results indicate that a significant fraction of the carbon initially released upon emplacement of the sill is sequestered deeply and doesn't rise up to the ocean floor.