Augustin, Nico

Devey, Colin W.

Eisenhauer, Anton

Garbe-Schönberg, C.-Dieter

James, R. H.

Lackschewitz, Klas

English

OceanRep

Geophysical Research Abstracts, Vol. 8, 02195, 2006SRef-ID: 1607-7962/gra/EGU06-A-02195© European Geosciences Union 2006The history of water-rock interaction at Logatchevhydrothermal field as stored in the secondary mineralparagenesisN. Augustin (1), K. S. Lackschewitz (1), C. W. Devey (1), A. Eisenhauer (1),D. Garbe-Schönberg (2), R. H. James (3)(1) IFM-GEOMAR, Leibniz Institut für Meereswissenschaften, Germany, (2) Institut fürGeowissenschaften, Universität Kiel, Germany, (3) Department of Earth Sciences, The OpenUniversity, Milton Keynes, UKOur new data on mineral compositions, clay and bulk rock chemistry of altered rocksand sediments together with isotopic data for clay minerals show complex and multi-stage alteration processes (e.g., seawater-rock interaction, fluid mixing) in the ultra-mafic hosted Logatchev hydrothermal field (MAR, 15˚N). Our preliminary interpre-tation is that an early low-temperature serpentinization (e.g., lizardite) is overprintedlocally by chlorite and/or talc alteration followed by quartz precipitation. Three activesites at Logatchev reveal an alteration type of strongly lizardite-altered rocks. At theLogatchev-I hydrothermal field, a site of active venting, lizardite is also abundant butlocally smectite, chlorite-smectite mixed layer (e.g., corrensite) and chlorite are partof a “chloritic” alteration assemblage. In addition, talc is locally abundant. There isa strong relationship of the chlorite and talc assemblage to high temperature fluidsbelow the surface. Geochemical modelling suggests that the rocks have been alteredat temperatures of about 200 - 300˚C. Significant changes in trace element concentra-tions in the bulk rock and clay separate chemistry represent the diversity of alterationprocesses. The elements Cr, Cu, Zn, Sr, Ba, Pb and U appear to have a general en-richment in the lizardite and chlorite concentrates in comparison to a depleted mantle.However, Cu and Zn are most probably related to sulfides which are finely dispersedin the clay concentrates. Cr, Sr, Ba, Pb and U are obviously mobile, probably as a re-sult of serpentinization and/or seafloor weathering. In addition, the clay concentratesare characterized by the development of U-shaped REE pattern with pronounced pos-itive and negative Eu-anomalies. We suggest that these differences are most probablyrelated to high fluid/rock ratios under different redox conditions. 87Sr/86Sr ratios ofclay fraction lizardite concentrates suggest the formation of clay minerals under nearlyseawater conditions, whereas Sr-isotopes of chlorite samples from sediments beneatha hydrothermal crust suggest a precipitation from a fluid with lower seawater content.Detailed geochemical and isotopic studies of talc samples will provide further con-strains on the complex interplay between hydrothermal fluids and seawater during theprogressive alteration of predominantly ultramafic rocks.

The history of water-rock interaction at Logatchev hydrothermal field as stored in the secondary mineral paragenesis

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