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Nd ratios of the samples analyzed are plotted on a ɛNd versus time diagram shown in the figure.De Paolo (1981) fitted a quadratic curve to the Idaho Springs and average ɛNd for the modern oceanic island arc data, thus representing the neodymium isotope evolution of a depleted reservoir.This therefore allows crustal formation ages to be calculated, despite any metamorphism the sample has undergone.Despite the good fit of Archean plutons to the CHUR Nd isotope evolution line, De Paolo and Wasserburg (1976) observed that the majority of young oceanic volcanics (Mid Ocean Ridge basalts and Island Arc basalts) lay 7 to 12 ɛ units above the CHUR line (see figure).
Unambiguous interpretation of the observed age differences has been hindered by a lack of adequate information about grain size, diffusion data for Hf in garnet, and in many cases about peak metamorphic conditions and cooling rates, all of which affect the closure temperatures of these decay systems. In addition, epsilon units will normalize the initial ratios to CHUR, thus eliminating any effects caused by various analytical mass fractionation correction methods applied.age to be calculated, fractionation between Nd/Sm would have to have occurred during magma extraction from the mantle to produce a continental rock.The concentration of Sm and Nd in silicate minerals increase with the order in which they crystallise from a magma according to Bowen's reaction series.
Samarium is accommodated more easily into mafic minerals, so a mafic rock which crystallises mafic minerals will concentrate neodymium in the melt phase relative to samarium.
It is sometimes assumed that at the moment when crustal material is formed from the mantle the neodymium isotope ratio depends only on the time when this event occurred, but thereafter it evolves in a way that depends on the new ratio of samarium to neodymium in the crustal material, which will be different from the ratio in the mantle material.