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Nd isotopic evolution of the Precambrian mantle

Earth and Planetary Science Letters
Publication Date
DOI: 10.1016/0012-821x(89)90180-5


Abstract The most convincing examples of Proterozoic oceanic crust assemblages occur in the Circum-Superior fold belt of Québec. Nd isotope analyses of the Chukotat and Hellancourt basalts from this belt constrain the ε Nd of the Early Proterozoic depleted mantle reservoir at +4 to +6. Along with data for Phanerozoic ophiolites and modern MORB, the Proterozoic basalts infer a constant rate of evolution of the depleted mantle at a rate of 2.2 ε Nd units per Ga. Recalculation of Sm Nd isotopic data for Archaean greenstone belts to their most reasonable Pb Pb or U Pb ages suggests significantly greater isotopic heterogeneity in the Archaean. Presumed uncontaminated tholeiites and Al-undepleted komatiites fall within the extrapolated growth limits for the Proterozoic-Phanerozoic depleted mantle, while Al-depleted komatiites (ADK) in Early Archaean greenstone belts have significantly higher ε Nd . The latter require derivation from a source whose isotopic composition evolved at 2–4 times the rate of the Al-undepleted komatiite and tholeiite source. The isotopic heterogeneities noted for the Archaean indicate a compositionally layered mantle. By the late Archaean the ADK source was either isolated by declining geothermal gradients, or had become remixed with the overlying mantle giving rise to the present depleted mantle reservoir.

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