Ion microprobe zircon geochronology of the Uivak Gneisses: Implications for the evolution of early terrestrial crust in the North Atlantic Craton

Ion microprobe U-Pb results for zircons from three Uivak I gneisses and one specimen of Uivak II gneiss, from the Saglek-Hebron area of Northern Labrador are reported. These results are compared with interpretations based on published conventional U-Pb zircon results and with conclusions about crustal evolution in the NAC derived from Rb-Sr, Sm-Nd and Pb-Pb isotopic studies

Strontium and neodymium isotopic variations in early Archean gneisses affected by middle to late Archean high-grade metamorphic processes: West Greenland and Labrador

Relicts of continental crust formed more than 3400 Ma ago are preserved fortuitously in most cratons. The cratons provide the most direct information about crust and mantle evolutionary processes during the first billion years of Earth history. In view of their polymetamorphic character, these terrains are commonly affected by subsequent tectonothermal events. Hence, their isotope systematics may be severely disturbed as a result of bulk chemical change or local isotopic homogenization. This leads to equivocal age and source information for different components within these terrains. The Sr and Nd isotopic data are presented for early Archean gneisses from the North Atlantic Craton in west Greenland and northern Labrador which were affected by younger metamorphic events

Origin of ocean island basalts: A new model based on lead and helium isotope systematics

Free to read at publisher's site. Current models of ocean island basalt (OIB) Pb isotope systematics based on longterm isolation of recycled oceanic crust (with pr without sediment) are not supported by solutions to both terrestrial Pb paradoxes. St follows that the linear arrays of OIB data in Pb isotope diagrams are mixing lines and have no age significance. A new model is presented that takes into account current solutions to both terrestrial Pb paradoxes and that explains combined Pb and He isotope evidence in terms of binary mixing. The key feature of this model is a two-stage evolution: first, long-term separation of depleted mantle from undepleted lowermost lower mantle. Mixing between these two reservoirs results in the wide spread in Pb-207/(204)Pbti` and generally high (but variable) He-3/He-4 ratios that typify enriched mantle 1 (EM1) OIBs. The second stage involves metasomatism of depleted upper mantle by EM1 type, lowermost mantle-derived melts. Evolution in the metasomatized environment is characterized by variable but generally high (Th+U)/(Pb+He) ratio that leads to a rapid increase in Pb-208/Pb-204 and Pb-206/Pb-204 ratios and decrease in He-3/He-4. Mixing between depleted mantle and melts from metasomatized mantle portions reproduces the characteristics of high mu (HIMU) OIBs. The Sr versus Nd isotope array is compatible with binary mixing between depleted mantle and near-chondritic lowermost mantle because of the large variation in Sr/Nd ratios observed in EMI and HIMU OIBs. OIBs contaminated by subcontinental lithospheric mantle (EM2) exhibit more complex isotope systematics that mask their primary geochemical evolution