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School of Ocean and Earth Science, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK
1 Department of Trade and Industry, 151 Buckingham Palace Road, London, SW1W 9SS, UK
Submersible and drilling studies of the active TAG hydrothermal mound (26°N, Mid-Atlantic Ridge) have led to new models of fluid flow and evolution within an active mineral deposit which has implications for Fe-oxide and ochre precipitation. Metalliferous sediments from the top of the hydrothermal mound accumulate from a combination of processes including slumping and oxidation of chimney material and in situ precipitation of low-temperature phases from fluids that percolate through the mound. Geochemical proxies of hydrothermal processes allow identification of the mode of formation of one sediment core from the southeastern periphery of the TAG mound. The Fe rich sediment is capped with a
5 cm thick kaolinite, illite, chlorite, smectite layer which formed from alteration and replacement of basalt and diagenetic reactions within the hydrothermal sediment. Underlying this layer is a 10 cm thick zone of Mn, Cu, Zn and Pb enrichment which is controlled by the sharp redox gradients in the core. The base of the core is characterized by Mn-poor, Fe-rich oxide that is dominated by goethite, haematite and amorphous Fe oxides equivalent to ochreous and gossan material. Rare earth element (REE) patterns from the different layers within the core allow interpretation of the modes of formation of sediment in the light of existing fluid flow models for TAG. The basal layer is dominated by in situ precipitation of Fe oxide phases from evolved fluids that result from significant within-mound anhydrite precipitation. The REE data for the upper part of the core demonstrate mixing with sea water which provides the oxidizing conditions for Mn precipitation along with Cu, Zn and Pb enrichment from the evolved fluid. Sea water ingress results in higher V/Fe and P/Fe ratios in the upper part of the core but no enhanced U/Fe ratio. The uppermost clay-rich layer hosts the majority of the REE inventory for the core and the significant positive Eu anomaly indicates recrystallisation of the phyllosilicate phases from the ochreous material during diagenesis. REE data from land-based ochre and gossan sediments demonstrate that the TAG model may be applicable to a wide variety of sites throughout the geological record.
