HR: 11:05h
AN: OS32A-04    [Abstracts]
TI: Osmium Isotope investigation of hydrothermal fluids from Mid-Atlantic Ridge
AU: * Gannoun, A
EM: mouhcine.gannoun@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences Parks Road, Oxford, OX1 3PR, United Kingdom
AU: * Gannoun, A
EM: mouhcine.gannoun@earth.ox.ac.uk
AF: The Open University, Department of Earth Sciences Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
AU: James, R H
EM: r.h.james@open.ac.uk
AF: The Open University, Department of Earth Sciences Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
AU: Burton, K w
EM: k.w.burton@open.ac.uk
AF: The Open University, Department of Earth Sciences Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
AU: Burton, K w
EM: k.w.burton@open.ac.uk
AF: University of Toulouse, LMTG, CNRS, IRD, OMP 14 Av. Belin, Toulouse, F-31400, France
AU: Halliday, A N
EM: Alex.Halliday@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences Parks Road, Oxford, OX1 3PR, United Kingdom
AB: It is generally believed that the variations in the osmium isotope composition of seawater over time, principally reflects a change in the balance of inputs from continental weathering and from hydrothermal exchange with oceanic crust (through submarine alteration and hydrothermal fluids). Some 20 percent of osmium in present-day seawater is thought to be of mantle/cosmic origin with 187Os/188Os composition of 0.13. However, existing Os data from high temperature hydrothermal fluids suggests that hydrothermal alteration does not provide significant unradiogenic Os to the oceans. This study present new Os isotope data for axial high temperature hydrothermal solutions from three localities along the Mid Atlantic Ridge (Snake Pit, TAG, Broken Spur). Os concentrations range from 0.5 to 5.2 picomol/kg and show large enrichment in osmium concentration over seawater by factor of 10 to 100. These concentrations are also one to two orders of magnitude higher than those reported previously. The Os isotope composition varies from 0.112 to 0.220. Most remarkably, the Os rich solutions are extremely unradiogenic relative to MORB or PUM (~0.13). All the data lie on a single mixing line using Os isotope composition versus inverse concentration, between radiogenic seawater (~1.06) and an unradiogenic component with 187Os/188Os composition of 0.11. This unradiogenic component yields a model age of 2.5 Ga and is the oldest yet observed for the sub-oceanic mantle. This observation indicates the existence of ancient depleted mantle accessed by the fluids at the studied localities. All the fluids analysed here show a linear covariation in 187Os/188Os versus Mg/Os or Sr/Os indicating that mixing between hydrothermal fluids and seawater is conservative. These results serve to aid the interpretation of variations in seawater Os over time, and highlight the similarity of sources of Os and Sr to the oceans
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1040 Radiogenic isotope geochemistry
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 1635 Oceans (1616, 3305, 4215, 4513)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting