A DUAL ORIGIN FOR THE HYDROTHERMAL COMPONENT IN A METALLIFEROUS SEDIMENT CORE FROM THE MID-ATLANTIC RIDGE

A highly metalliferous sediment core collected from TAG, 26-degrees-N Mid-Atlantic Ridge has been analyzed for uranium series isotopes, rare earth elements (REE), and radiogenic isotopes. Two types of deposition of hydrothermal material have been identified and characterized. The first is sulfide material which has been rapidly deposited and is sometimes clearly turbiditic. This material exhibits high U/Fe and Eu/Sm ratios and low Th-230/Fe and REE/Fe ratios. It is proposed that U is incorporated into the sulfides by diffusion through the oxidized coating with reductive fixation near the sulfide/oxide boundary. The second hydrothermal component is particulate fallout from the neutrally buoyant plume which is diluted by the background pelagic sedimentation. This component exhibits U/Fe ratios around 2 x 10(-5), an order of magnitude greater than reported plume particulate levels, but the Th-230/Fe, REE/Fe and Eu/Sm ratios are similar to plume values. Nd and Pb isotopes have been used to differentiate the hydrothermal and pelagic end-members within this sediment core.