Ruthenium/Iridium Ratios in the Cretaceous-tertiary Boundary Clay: Implications for Global Dispersal and Fractionation Within the Ejecta Cloud

Ruthenium (Ru) and iridium (Ir) are the least mobile platinum group elements (PGE's) within the Cretaceous-Tertiary (K-T) boundary clay (BC). The Ru/Ir ratio is, therefore, the most useful PGE interelement ratio for distinguishing terrestrial and extraterrestrial contributions to the BC. The Ru/Ir ratio of marine K-T sections (1.77 +/- 0.53) is statistically different from that of the continental sections (0.93 +/- 0.28). The marine Ru/Ir ratios are chondritic (C1 = 1.48 +/- 0.09), but the continental ratios are not. We discovered an inverse correlation of shocked quartz size (or distance from the impact site) and Ru/Ir ratio. This correlation may arise from the difference in Ru and Ir vaporization temperature and/or fractionation during condensation from the ejecta cloud. Postsedimentary alteration, remobilization, or terrestrial PGE input may be responsible for the Ru/Ir ratio variations within the groups of marine and continental sites studied. The marine ratios could also be attained if approximately 15 percent of the boundary metals were contributed by Deccan Trap emissions. However, volcanic emissions could not have been the principal source of the PGE's in the BC 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 +/- 0.48), picrites (4.1 +/- 1.8), and Deccan Trap basalt (3.42 +/- 1.96) are all statistically distinct from those measured in the K-T BC

U-Pb geochronology of detrital and igneous zircon grains from the Águilas Arc in the Internal Betics (SE Spain): Implications for Carboniferous-Permian paleogeography of Pangea

New U–Pb detrital zircon and U–Pb zircon ages of metaigneous rocks in the Águilas Arc (Betic Chain, SE Spain) allow us to determine the maximum depositional ages of the rocks. Within the Nevado-Filábride Complex, a Late Carboniferous depositional age for the Lomo de Bas schists and quartzites, and a Permian-Triassic maximum depositional age for the Tahal Fm are determined. Within the Alpujárride Complex, the maximum depositional age of the Micaschists and Quartzite Fm is Late Carboniferous and the Meta-detrital Fm was deposited in the Early Permian. Furthermore, the maximum depositional age of the Saladilla Fm in the Maláguide Complex is also Early Permian. The age distribution patterns for the Carboniferous rocks of the Nevado-Filábride and Alpujárride complexes are similar to those from the Cantabrian Zone of the Iberian Massif, suggesting deposition in Carboniferous foreland basins located eastwards of the Iberian Massif. However, age patterns in Maláguide and samples from the North-eastern Iberian Peninsula and South France show strong similarities suggesting that it can be located near those areas in the Late Carboniferous times. The samples with Early Permian maximum depositional ages from the three complexes contain more Paleozoic zircon grains relative to the older Carboniferous samples, but have similar age distribution patterns, suggesting that they were deposited in the same basin. Samples from unconformable Middle Miocene sediments have Early Permian youngest zircon populations and age distribution patterns corresponding to a mixing of detrital zircon grains from the Alpujárride and Maláguide complexes. Furthermore, there is no record of any major felsic rocks formation and/or exhumation event after the Early Permian in those two complexes

Mixed and recycled detrital zircons in the Paleozoic rocks of the Eastern Moroccan Meseta: Paleogeographic inferences

Ministerio de Economía y Competividad (MINECO) of Spain through the project CGL2015-71692-P and the Pre-Doctoral scholarship BES-2016-078168. Zircon analyses and imaging were carried out on the SHRIMP II, LA-ICPMS and SEM facilities at the John de Laeter Centre, Curtin University, with the financial support of the Australian Research Council (LE150100013) and Auscope NCRIS (AQ44 Australian Education Investment Fund program)The paleogeographic evolution of the Moroccan Variscides has been a matter of discussion for several decades, with current theoriesmostly based on classical geological correlations. In this regard, the scarce number of studies devoted to U-Pb geochronological analyses of detrital zircon populations is particularly limiting when trying to ascribe the different domains to a single continental piece either derived from the West African Craton or to different sources, with some located in the Nubian Shield or the SaharanMetacraton. In thiswork, detrital zircon grains from 10 samples of sandstones from the Paleozoic (Ordovician to Devonian) sequence of the Eastern Meseta andMiddle Atlaswere dated in order to identify possible sediment sources and elucidate the paleogeography of this easternmost portion of the Moroccan Variscides. The main detrital zircon populations have Ediacaran-Cryogenian ages (610–670 Ma, related to the Cadomian and/or Pan-African orogeny) and middle Paleoproterozoic ages (1980–2080 Ma, related to the Eburnean orogeny), which are in agreement with previous data from the Western Meseta, suggesting similarity between both Mesetas, and strong West African Craton affinity. Such an affinity verifies themost accepted paleogeographic interpretation considering that theMoroccan Mesetas remained attached to northern Gondwana during the entire Paleozoic period. The main differences between our samples and those from the Western Meseta concern the minor detrital zircon populations, such as the Cambro-Ordovician and the Tonian-Stenian ones. In particular, Eastern Meseta and Middle Atlas samples lack a Cambro-Ordovician detrital zircon population, usually interpreted as related to the rifting that opened the Rheic Ocean. This population is locally reported in the Western Meseta and widely described in southwestern Europe, where magmatism of this age is well known. Furthermore, the most northeastern samples are also characterized by a Tonian-Stenian detrital zircon population (up to 30% of the data), which might imply northeastern African sources (Saharan Metacraton and/or Arabian-Nubian Shield)Ministerio de Economía y Competividad (MINECO) of Spain CGL2015-71692-P, BES-2016-078168Australian Research Council (LE150100013)Auscope NCRIS (AQ44 Australian Education Investment Fund program

Fractionation of ruthenium from iridium at the Cretaceous-Tertiary boundary

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 Ir, 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