Gas Hydrates-bearing Sediments from Southern Hydrate Ridge (ODP Leg 204): Geochemical and Mineralogical Approach

Mineralogical and geochemical analyses of more than 500 samples were carried out in order to characterize the southern Hydrate Ridge sediments and to evaluate the role of sediment composition in fluid migration and gas hydrate (GH) distribution. Bulk and clay mineralogy were obtained by XRD and trace element contents by ICP-MS. Bulk mineral composition of southern Hydrate Ridge sediments does not change significantly from the summit to the flanks and slope basin. This composition is dominated by clays (30-60% ), quartz (25- 40% ), feldspars (10-25% ) and calcite (<5% ). Regarding the clay mineralogy, the most abundant mineral is detrital mica (50% ), whereas smectites, kaolinite and chlorite are less abundant (<30% ). Some noticeable trends are recognized in smectite content, increasing with depth at both flanks, from low values near the seafloor to high values (>40% ) downcore. Such increase is accompanied by a decrease in detrital mica content, suggesting the presence of deep fluid flows from below the accretionary complex. Sediments from all sites show high TOC contents from seafloor down to 50 mbsf. Within this interval, high CaCO3 contents and carbonate nodules are observed. High values of REE, Hf and Zr concentrations correlate with increasing terrigenous input, which resulted in coarse grain-size layers, identified as turbidites and formed by silty-clay sediments with different amounts of sand content (up to 10% ). The comparison between turbidite and gas hydrate distributions (inferred from IR imaging, disturbance fabrics and gas hydrate identification) suggests that GH predominantly form within the terrigenous coarser grain-size layers. At the summit, near the seafloor, where the highest GH content was observed (40% of pore volume), this trend is not recognizable, suggesting that gas-methane availability is high enough to generate GH even within finer grain-size layers.