Major element and isotope geochemistry (Sr Nd and Hf) of mantle derived peridotites carbonatites and kimberlites from Canada and Greeland insights into mantle dynamics

Abstract

Illhérité de la differentiation initiale de la Terre. Notre étude confirme donc qu'une telle hétérogénéité était toujours présente aux environ de 3 Ga, mais plus elusive dans les roches juvéniles d'âge <2.7 Ga. Les nouvelles données isotopiques de l'Hf ici présentées favorisent une source et origine communes pour les carbonatites, kimberlites et certain basaltes océaniques. IV ABSTRACTWe report on the major element composition of a suite of kimberlite-hosted peridotite xenoliths from the northern margin of the western Archean craton of Greenland, as well as the isotopic composition (Nd, Sr and Hf) of carbonatites and kimberlites from eastern Canada and southwestern Greenland.Our results indicate that the lithospheric mantle underneath the Sarfartoq area of southwestern Greenland extends to approximately 225 km, and consists of refractory (olivine Mg # >0.9) harzburgites and lherzolites. Moreover, the Sarfartoq peridotites are not characterized by the OPX enrichment observed in peridotites from the Kaapvaàl craton of South Africa, and show more affinity with peridotites from east Greenland (Wiedemann Fjord), Somerset Island (Canadian arctic) and Tanzania. Detailed analyses of constituent minerals reveal that the lithospheric mantle underneath the Sarfartoq area is compositionally layered as follows: (1) an internally stratified upper layer (70 to 180 km) consisting of coarse, un-deformed, refractory garnet-bearing and garnet-free peridotites and, (2) a lower layer (180 to 225 km) characterized by fertile, CPX-bearing, porphyroclastic garnet lherzolites. The stratification observed in the upper refractory harzburgite layer (70-180 km) is reflected by an increase in fertility (e.g. lower olivine abundance and fosterite content) with depth. The sharp nature of the stratification may indicate a two-step process for the formation of this lithosphère, i.e. multistage growth of the lithospheric root.In Sr values suggests that apatite and carbonate precipitated predominantly under non-equilibrium conditions. The isotopic variations observed within individual hand specimens may therefore reflect larger-(regional) scale open-system processes, possibly involving mixing of carbonatitic melts derived from distinct mantle sources, or from a common isotopically heterogeneous mantle. This analytical technique provides a rapid and effective approach to determine the extent of isotopic heterogeneity within a single sample or individual grains, with analytical precision approaching that obtained by conventional TIMS.High-precision Hf isotopic analyses and U-Pb ages of carbonatites and kimberlites from Greenland and eastern North America, including Earth's oldest known carbonatite (3 Ga), indicate derivation from an enriched mantle source. This previously unidentified mantle reservoir of unradiogenic Hf isotopic composition, preserved in the deep mantle for at least 3 billion years, may account for the mass imbalance in Earth's Hf-Nd budget. Our results show that the Early to mid-Archean terrestrial mantle was isotopically much more...