Hydrothermal sediments associated with a relict back-arc spreading center in the Shikoku Basin, recovered from the Nankai accretionary prism, Japan

The hemipelagic mudrocks of the Nankai accretionary prism, Japan, contain h ydrothermal deposits associated with a relict spreading center in the Shiko ku Basin. Initial work on core samples from Ocean Drilling Program site 808 found several samples with elevated concentrations of calcium, magnesium, iron and manganese, at depths of between 1060 and 1111m below sea floor. Ho wever, the origin of these sediments was uncertain, due to a lack of data. There was no recorded evidence of whether these elevated concentrations wer e present throughout this interval of core, or if they were present as disc rete layers with the background hemipelagic mudrocks in between. In the pre sent study the core was resampled, and the sediments with anomalous chemica l compositions were found to be present in discrete layers. This fact, alon g with a detailed interpretation of their geochemistry, has allowed them to be identified as hydrothermal sediments, associated with the relict spread ing center in the Shikoku Basin. The lower (older) two layers display a che mical composition typical of umbers, while the upper (younger) two layers a re metalliferous mudrocks typical of deposits found further from the spread ing center.