Os isotopes and platinum group elements in K-T boundary clays; impact signatures versus post-impact processes

Author(s): Pearson, D. G.; Sigurdsson, H.; Woodland, S. J.; Shirey, S. B.; Murray, R. W.; Lyons, T. L.; Schmitz, B.
Author Affiliation(s): Primary:
Durham University, Department of Geological Sciences, Durham, United Kingdom
Other:
University of Rhode Island, United States
Carnegie Institute of Washington, United States
Boston University, United States
University of Missouri-Columbia, United States
University of Goteborg, Sweden
Volume Title: Geological Society of America, 1999 annual meeting
Source: Abstracts with Programs - Geological Society of America, 31(7), p.123-124; Geological Society of America, 1999 annual meeting, Denver, CO, Oct. 25-28, 1999. Publisher: Geological Society of America (GSA), Boulder, CO, United States. ISSN: 0016-7592 CODEN: GAAPBC
Note: In English
Summary: The enrichment of Platinum Group Elements (PGEs), together with non-crustal Os isotope signatures in K-T boundary clays (KT-BC), are key geochemical indicators of the K-T impact event. Inter-element PGE fractionations in K-T boundary clays have been suggested to result from fractionation processes associated with the impact. We have undertaken a detailed PGE and Re-Os isotope study across the ODP Leg 165 site 1001 K-T boundary section plus analyses of other KT-BC around the globe to better constrain the processes that produce the observed "impact signatures". Our chemistry technique gives isotope dilution analyses of Re, Pt, Pd, Ru, Ir & Os together with Os isotopes for a single digest, eliminating nugget effects. Although KT-BC are characterised by unradiogenic Os compared with crust, their range in 187Os/188Os is large (0.137 to 0.212) and significantly above the range for chondrites. If interpreted simply as bolide-crust mixtures these ratios require very large crustal inputs, considerably greater than that estimated from mass balance calculations based solely on elemental abundances. Globally, there is no correlation between 187Os/188Os and Os concentration. Decoupling of isotopes from elemental concentrations may suggest intimate mixing of the bolide with crustal material during impact, followed by relatively little mixing with crust during settling of ejecta/condensates. The Leg 165 transect shows good correlation between 187Os/188Os and 1/Os that is interpreted as diagenetic re-mobilisation of Os in the sediment pile. This is supported by PGE elemental fractionations. Os/Ir in KT-BC is fractionated to significantly sub-chondritic values (0.1-0.25) in abyssal samples compared to shallow marine deposits. Although some evidence exists for volatile PGE loss during impact (Re, Os, Ru?) most PGE inter-elemental fractionation appears to be the result of processes occurring during deposition and subsequent diagenesis.
Year of Publication: 1999
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Atlantic Ocean; Caribbean Sea; Cenozoic; Chemical fractionation; Clastic sediments; Clay; Condensates; Cretaceous; Deposition; Diagenesis; Ejecta; Geochemistry; Isotope ratios; Isotopes; K-T boundary; Leg 165; Lower Paleocene; Marine environment; Mass balance; Mesozoic; Metals; Mixing; North Atlantic; Ocean Drilling Program; Os-188/Os-187; Osmium; Paleocene; Paleogene; Platinum group; Processes; Rhenium; Ruthenium; Sediments; Shallow-water environment; Stable isotopes; Stratigraphic boundary; Tertiary; Upper Cretaceous
Coordinates: N103000 N193000 W0650000 W0830000
Record ID: 2000031026
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