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/Sm 144Nd-initial ε Nd 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 ε Nd value of +1.8 ± 0.7 (1 σ) for the uncontaminated Siberian plume, which points to a near-chondritic, slightly depleted lower mantle source. The Siberian basalts, which show an ε Nd of ⩾ 0, display a remarkably uniform Sr-isotopic composition with an average 87/Sr 86Sr of 0.7050 ± 3 (1 σ). However, in a ( 87/Sr 86Sr) i-ε Nd diagram these lavas are significantly displaced to the right-hand side of a mixing line between bulk earth and depleted mantle. We suggest that these basalts suffered selective contamination of Sr during ascent through the continental crust. This element-specific contamination, which may also be concluded from Nd—Pb and Sr—Pb isotopic variations in the basalts, is at a maximum for Pb-isotopes and at a minimum for Nd. The Pb-isotopic ratios of the contaminated basalts indicate significant contributions from the lower crust with little or no input from the upper crust. The early Norilsk lavas define the extreme compositions and help constrain the initial Pb-isotopic composition of the plume source as 206/Pb 204Pb= 18.3, 207/Pb 204Pb= 15.5 and 208/Pb 204Pb= 38.0, which falls slightly to the right of the geochron at 248 Ma B.P.. We propose a two-stage growth history of Pb for the plume source: An early core-mantle differentiation stage, with a μ value of 0.21 for the first 100–150 m.y. after the Earth formed, was followed in the second stage by an increase in the μ value to about 9.2 for the rest of Earth's evolution.