The distribution and geochemistry of platinum-group elements at the Cretaceous-Tertiary boundary.
Creators: name: Evans, Noreen J.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1992
Subjects: subject: Geochemistry.

description: The globally-deposited clay layer marking the Cretaceous-Tertiary boundary has been studied to determine the mineralogical carrier phases for the anomalously concentrated platinum-group elements, the affinity of these elements for organic complexes in low temperature environments, the degree of terrestrial input to the boundary clay and the usefulness of platinum-group elemental ratios in projectile identification. Grain size separates from marine and terrestrial Cretaceous-Tertiary boundary sites were analyzed for platinum-group elements (Pt, Pd, Ru, Ir, Rh) and gold using sensitive induced coupled plasma mass spectrometry. Detection limits on a five gram sample are 0.05 ppb (Rh, Ru, Ir), 0.1 ppb (Pt, Pd) and 0.2 ppb (Au). Platinum-group elements are concentrated in clay minerals (smectite and illite-smectite clay) formed by the alteration of the original microtektite host. There is also an ubiquitous organic carrier. Ruthenium and Ir were found to be the least susceptible to be fixed in organics. This fact, combined with the geochemical coherence of Ru and Ir makes them more suitable than the other platinum-group elements for estimating the terrestrial platinum-group element input to the boundary clay and for identifying the projectile. The Ru/Ir ratio of marine sections (1.77 $\pm$ 0.53) is statistically different from that of the terrestrial sites (0.92 $\pm$ 0.28), and each represents a relatively coherent group. The marine Ru/Ir ratios are chondritic (1.48 $\pm$ 0.09), but the terrestrial ratios are not. Fractionation of Ru and Ir during condensation from the ejecta cloud may account for the broad differences between marine and terrestrial sites. Post-sedimentary alteration, remobilization or terrestrial PGE input may be responsible for the Ru/Ir ratio variations within the groups of marine and terrestrial sites studied. Modelling indicates that the marine ratios could also be attained if $\approx$15% of the boundary metals were contributed by Deccan Trap emissions. However, volcanic emissions could not have been the principal source of platinum-group elements in the boundary clay because mantle PGE ratios and abundances are inconsistent with those measured in the clay. The Ru/Ir values for pristine Tertiary mantle xenoliths (2.6 $\pm$ 0.48), picrites (4.1 $\pm$ 1.8) and for the Deccan Trap basalt (3.42 $\pm$ 1.96) are all statistically distinct from those measured in the Cretaceous-Tertiary boundary clay. Several Canadian impact craters, believed to have been formed by the impact of chondritic projectiles, were analyzed for platinum-group elements in order to test if the interelement ratios identify the chondrite (i.e., the nature of the impactor). However, the dearth of literature data for various types of meteorites, the overall similarity in their platinum-group element ratios, the unknown fractionation effect upon meteorite volatilization and condensation, and the post-depositional alteration and remobilization of platinum-group elements all hamper application of the technique. Consequently, platinum-group elements cannot be readily utilized for identifying impactors beyond broad groups of meteorites (e.g., chondrite vs. iron). Nevertheless, they can often be used as supporting geochemical evidence, along with other elements (e.g., with Ni, Cr, Co abundance and ratios). This is the case at the K-T boundary, where Ru/Ir ratios, mineralogical and geochemical evidence all support a chondritic nature for the impactor.