Abstract:
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 (super 187) Os/ (super 188) Os 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 (super 187) Os/ (super 188) Os 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 (super 187) Os/ (super 188) Os 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.