HR: 10:50h
AN: OS32B-03    [Abstracts]
TI: Search for the proverbial cosmic/mantle osmium sources to the oceans
AU: * Sharma, M
EM: mukul.sharma@dartmouth.edu
AF: Dartmouth College, Department of Earth Sciences, Hanover, NH 03755 United States
AU: Rosenberg, E J
EM: erin.rosenberg@dartmouth.edu
AF: Dartmouth College, Department of Earth Sciences, Hanover, NH 03755 United States
AU: Butterfield, D A
EM: david.a.butterfield@noaa.gov
AF: PMEL, NOAA, Seattle, WA 98115 United States
AB: We report the first measurements of Os isotopes in low-temperature hydrothermal fluid samples from on- and off-axis vent fields associated with the Juan de Fuca Ridge (JdFR). The ¹⁸⁷Os/¹⁸⁸Os ratio of the oceans is thought to be governed by inputs from the continents (rivers, groundwater, aerosols), extraterrestrial matter (cosmic dust/micrometeorites) and from the oceanic crust (hydrothermal fluids and submarine alteration). On average, the ¹⁸⁷Os/¹⁸⁸Os ratios of the mantle and cosmic materials are nearly identical (~0.13) and about ten times lower than that of average continental matter (1.26). The ¹⁸⁷Os/¹⁸⁸Os ratio of present-day deep oceans = 1.06 indicating that about 20 % of Os in seawater is of mantle/cosmic derivation. However, the Os data from high temperature hydrothermal fluids from the JdFR have indicated that while there is a nearly complete Os isotopic exchange between the interacting seawater and MORB, high temperature hydrothermal alteration does not provide significant unradiogenic Os to the oceans (Sharma et al., 2000). Our investigations have also revealed that during atmospheric entry and heating of cosmic dust, most of Os is sequestered in small (1 μg) metal nuggets which have a short residence time in seawater column (Chen, et al., 2005) and may not contribute significant amounts of Os to seawater. These observations would suggest that low temperature hydrothermal alteration of MORB may the dominant source of Os. We collected low temperature hydrothermal fluids using an all Teflon apparatus (Butterfield sampler mounted on Jason-II) linked to hollow steel-spikes that were driven into the seafloor (LEXEN Scientific Party, 2003). We also sampled fluids from ODP borehole 1026B. The samples were processed and analyzed for Os isotopes at Dartmouth. The concentrations of Os in low-temperature fluids range from 7.4 to 207 fmol/kg (Avg. = 27 fmol/kg). The ¹⁸⁷Os/¹⁸⁸Os ratios vary from 0.430 to 1.033 (Avg. = 0.878). These results indicate that during low temperature alteration (1) there is partial Os isotopic exchange between the interacting seawater and MORB, and (2) the resulting average Os contents of the low temperature fluids are less than seawater. If these data are representative they indicate that low temperature alteration of MORB is a sink of seawater Os and that another source of mantle Os (such mantle periodtites) must be investigated.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 4875 Trace elements (0489)
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005