Fingerprinting the Cretaceous-Paleogene boundary impact with Zn isotopes.

Numerous geochemical anomalies exist at the K-Pg boundary that indicate the addition of extraterrestrial materials; however, none fingerprint volatilization, a key process that occurs during large bolide impacts. Stable Zn isotopes are an exceptional indicator of volatility-related processes, where partial vaporization of Zn leaves the residuum enriched in its heavy isotopes. Here, we present Zn isotope data for sedimentary rock layers of the K-Pg boundary, which display heavier Zn isotope compositions and lower Zn concentrations relative to surrounding sedimentary rocks, the carbonate platform at the impact site, and most carbonaceous chondrites. Neither volcanic events nor secondary alteration during weathering and diagenesis can explain the Zn concentration and isotope signatures present. The systematically higher Zn isotope values within the boundary layer sediments provide an isotopic fingerprint of partially evaporated material within the K-Pg boundary layer, thus earmarking Zn volatilization during impact and subsequent ejecta transport associated with an impact at the K-Pg.

Grain size partitioning of platinum-group elements in road-deposited sediments: implications for anthropogenic flux estimates from autocatalysts.

Twelve road-deposited sediment samples were analyzed for platinum-group elements (PGEs) and Pb in the <63 microm fraction of an urban watershed in Hawaii. Three samples were further fractionated into five size classes, from 63-125 microm to 1000-2000 microm, and these were analyzed for PGEs and Pb. Concentrations in the <63 microm fraction reached 174 microg/kg (Pt), 101 microg/kg (Pd), 16 microg/kg (Rh), and 1.3 microg/kg (Ir). Enrichment ratios followed the sequence Rh>Pt=Pd>Ir. Iridium was geogenic in origin, while the remaining PGEs indicated significant anthropogenic contamination. Palladium, Pt and Rh concentrations and enrichment signals were consistent with PGE bivariate ratios and PGE partitioning in three-way catalysts. Size partitioning indicated that the <63 microm fraction had the lowest PGE concentrations and mass loading percentages. These data suggest that autocatalyst PGE flux estimates into the environment will be significantly underestimated if only a fine grain size fraction is analyzed.

Enriched Pt-Re-Os isotope systematics in plume lavas explained by metasomatic sulfides.

To explain the elevated osmium isotope (186Os-187Os) signatures in oceanic basalts, the possibility of material flux from the metallic core into the crust has been invoked. This hypothesis conflicts with theoretical constraints on Earth's thermal and dynamic history. To test the veracity and uniqueness of elevated 186Os-187Os in tracing core-mantle exchange, we present highly siderophile element analyses of pyroxenites, eclogites plus their sulfides, and new 186Os/188Os measurements on pyroxenites and platinum-rich alloys. Modeling shows that involvement in the mantle source of either bulk pyroxenite or, more likely, metasomatic sulfides derived from either pyroxenite or peridotite melts can explain the 186Os-187Os signatures of oceanic basalts. This removes the requirement for core-mantle exchange and provides an effective mechanism for generating Os isotope diversity in basalt source regions.