Genesis of ferromanganese crusts from the TAG hydrothermal field

The rare earth element (REE) geochemistry and Nd isotopic composition of ma rine ferromanganese crusts allow insight into their environment of depositi on. We present REE along with major and trace element data from a suite of samples collected near an active hydrothermal site from the Mid-Atlantic Ri dge and demonstrate extreme REE variability over short spatial scales. Both positive and negative Ce anomalies are observed, along with varied fractio nation of the REEs. Some of the Fe-Mn crusts are formed directly from low-t emperature hydrothermal fluids, channelled over 3 km from any high-temperat ure vent site. A significant hydrothermal component is observed in one Fe-M n crust sampled. Other samples, collected from the same area, are predomina ntly hydrogenous and form at a wide range of accumulation rates depending o n the hydrothermal plume input. The identification of extreme heterogeneity in source and depositional environment within one hydrothermal field sugge sts that (1) this variability can be resolved both spatially and temporally within a fossil hydrothermal deposit and (3) the integrated record provide d by bulk analysis should be interpreted with caution. (C) 2001 Elsevier Sc ience B.V. All rights reserved.