HR: 0800h
AN: PP31B-1539    [Abstracts]
TI: Germanium Isotope Measurements Using Double-Spike HG MC-ICP-MS: Results From Geothermal Fluids and Implications for the use of Ge/Si as a Paleoproxy
AU: * Siebert, C
EM: csiebert@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences Oregon State University, 104 Ocean Admin Bldg, Corvallis, OR 97330 United States
AU: McManus, J
EM: mcmanus@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences Oregon State University, 104 Ocean Admin Bldg, Corvallis, OR 97330 United States
AU: Hammond, D
EM: hammond@usc.edu
AF: Department of Earth Sciences University of Southern California, 3651 Trousdale Ave., Los Angeles, CA 90089 United States
AB: Numerous reports have documented the close coupling between dissolved germanic and silicic acids throughout the global ocean. Despite the apparent similarity in their oceanic behaviors, Ge and Si exhibit marked differences in their oceanic sources and sinks. Rivers supply most of the silica to the modern ocean whereas opaline marine sediments are the primary sink. In contrast to silica, germanium has two primary sources: rivers and hydrothermal inputs, and two sink terms: opaline sediments and reducing continental margin sediments. The differences in the relative importance of the source terms for Ge and Si, combined with the fact that siliceous microfossils may record water column Ge/Si, led to the proposal that the Ge:Si ratio recorded in diatoms could serve as a monitor for the relative importance of these two sources through time. However, an additional large non-opal sink for Ge complicates the interpretation of variations in the oceanic Ge:Si ratio. Distinctive Ge isotope signatures could provide additional constraints for the Ge cycle that would permit the relative rates of weathering and hydrothermal inputs to be deduced provided that Ge isotopes consistently fractionate during at least one of these processes. For our work on Ge isotopes, we use a hydride generation double spike (HG-DS) technique to determine the natural mass dependent isotope fractionation of germanium. The HG-DS technique has multiple advantages, which include correction of analytical mass bias and high sensitivity. We measured high-T geothermal fluids from a number of locations in the Oregon Cascades for their Ge isotope composition and Ge:Si ratios. All samples show light Ge isotope compositions ranging from -1 to -2 permil relative to a JMC laboratory standard. Corresponding Ge:Si ratios range from 6 to 74 micromol/mol. These ratios are higher than those from typical low-temperature weathered fluids (ca. 1) and those of bedrock (3 or lower), and are more elevated than Ge:Si ratios from marine hydrothermal systems (ca. 10 micromol/mol). Although our early results suggest that Germanium isotopes may offer an additional constraint for Ge cycling in the ocean, it is not tenable to extrapolate these results to marine hydrothermal systems at this particular time. Rather, the data here point to significant isotope fractionation during high temperature water rock interaction, which implies that further exploration of Ge isotope variations is warranted.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1051 Sedimentary geochemistry
DE: 1065 Major and trace element geochemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005