Coupled 186Os-187Os enrichments in the Earth's mantle -: core-mantle interaction or recycling of ferromanganese crusts and nodules?

Os-186 enrichments in volcanic rocks and peridotite-derived iridosmine grains have been attributed to contributions from Earth's outer core to the mantle, and apparently constrain the scale of mantle convection and an early timing for inner-outer core segregation more than 3.5 Gyr ago. Here, we highlight that marine ferromanganese crusts and nodules are characterised by high Pt/Os ratios and Pt-Os contents that develop much larger Os-186 excesses over geological time (greater than or equal to0.2%/Gyr) than those hypothesised for Earth's outer core ( < 0.005-0.01%/Gyr). Os-187/Os-188 ratios in ferromanganese crusts are radiogenic due to sequestering of continental Os from seawater. Similarly, ancient ferromanganese materials may have had Os-186 excesses ( > 0. 1%) as a result of high Pt/Os ratios in continental crust, even prior to in-growth of Os-186 after formation due to their high Pt/Os ratios. Past recycling of small amounts of these materials into the Earth's mantle will produce coupled Os-187-Os-186 excesses and little change in Re and platinum- group-element concentrations, as observed in Hawaiian picrites, and in contrast to the predicted result of outer core addition to the mantle. Os-187 and Os-186 enrichments in the Hawaiian mantle source are potentially consistent with it comprising recycled oceanic lithosphere, pelagic sediments and ferromanganese materials, and questions the notion that Os isotopes can be used to uniquely identify core-mantle interactions and the depth at which mantle sources for volcanism originate. (C) 2004 Elsevier B.V. All rights reserved.