TEMPORAL SR-, ND- AND PB-ISOTOPIC VARIATIONS IN THE SIBERIAN FLOOD BASALTS: IMPLICATIONS FOR THE PLUME-SOURCE CHARACTERISTICS

Abstract

The Sr-, Nd- and Pb-isotopic compositions of lavas from the 248-Ma-old Siberian Flood Basalt Province (SFBP) become increasingly uniform with height as the main phase of volcanic activity is approached. The late-stage lava sequences from the Putorana subprovince, which constitute approximately 90 vol% of the SFBP, require a homogeneous mantle as their source. Volcanostratigraphic and volume arguments imply that extreme isotopic compositions, displayed by the early lavas from Norilsk subprovince, are not the end-member compositions for the bulk of the SFBP. In ¹⁴⁷/Sm¹⁴⁴Nd-initial εNd space, most of the Putorana basalts define a horizontal array with a majority clustering at (0.17, +1.8). In contrast, the Norilsk lavas show a positively correlated trend indicative of continental lithospheric contributions to the primary plume. Among the three isotopic systems considered, Nd shows the least effect of crustal contamination. We estimate an initial εNd value of +1.8 ± 0.7 (1 σ) for the uncontaminated Siberian plume, which points to a near-chondritic, slightly depleted lower mantle source.