Platinum-group element abundances in background samples and the K-T boundary interval from DSDP Hole 91-596 (Table 2)

New data on Ru/Ir abundance ratios are presented for nonmarine (Hell Creek, Montana; Frenchman River, Saskatchewan) and marine Cretaceous-Tertiary boundary sites (Brazos River, Texas; Beloc, Haiti; DSDP 577 and DSDP 596). The Ru/Ir ratio varies from 0.5 to 1 within 4000 km of Chicxulub and increases to 2-3 at paleodistances (65 Ma) of up to 12,000 km from the impact site. For CI chondrites, Ru/Ir = 1.5. A ballistic model of ejecta cloud cooling and expansion, which employs the available vapor-pressure versus temperature data for Ru and It, predicts qualitatively similar global variation in the Ru/Ir ratio but by only a factor of 1.5. We infer that several other factors, such as remobilization of PGE during diagenesis, preferential oxidation of Ru, condensation kinetics and atmospheric chemical and circulation processes, may account for the observed larger Ru/Ir variation.

(Table 1) Platinum group element abundanves in ODP Leg 135 samples

Ocean Drilling Program Leg 135 backarc basin lavas are characterized by anomalously high Au contents (1.0-11.4 ppb) and strongly fractionated relative platinum group element (PGE) abundances (Pd/Ir ratio, approximately 100). The Rh and Ir contents are very low, ranging from below detection (approximately 0.02 ppb) to 0.08 ppb. The Pd and Pt contents range from <0.3 to 4 ppb. Rh, Pd, and Pt values are consistently and significantly higher in Site 836 and 839 samples relative to those from Sites 834 and 835. Major, trace, and rare earth element (REE) data suggest Sites 836 and 839 have a more pronounced arc signature than Sites 834 and 835. No correlation exists between noble metal abundance and indices of alteration or fractionation (e.g., loss on ignition (LOI), Mg#, and Cr or Ni contents), suggesting that measured values and ratios are primary and reflect characteristics of the mantle source. The evaluation of Leg 135 noble metal data with respect to potential mantle-source components is hindered by the lack of data on magmas derived from such sources. However, analyses of the limited available data for the different magma types suggest that the characteristic enrichment of Leg 135 lavas in Au, relative to Pd and Cu, cannot be derived solely from simple MORB-type or ocean-island-type mantle, or mantle depleted by a previous melt extraction event. The Au-enriched signature of the Lau basin lavas could, however, be produced through the addition of a sedimentary component from the downgoing slab. Separation of Au from the PGE occurs within oceanic hydrothermal systems and gold values of the resultant precipitates are 2-3 orders of magnitude higher than other oceanic crustal components. Even small additions of this component from the downgoing oceanic crust to a supra-subduction zone mantle melt could account for the high mean Au/Pd ratios of the Leg 135 samples (Sites 834 and 835, Au/Pd = 5.04; Sites 836 and 839, Au/Pd = 2.26).