The DePaolo and Wasserburg (1976a,b) model of a two-layered mantle consisting of a depleted upper layer overlying a lower primitive one, described in Section 3, seemed to be supported by mass balance calculations indicating that the incompatible element inventory of the continents could be derived from a third or so of the mantle. The key assumption in these calculations, that the Earth had a chondritic Sm/Nd ratio and therefore chondritic 143Nd/144Nd, seemed beyond question. The chondritic assumption was extended to all refractory lithophile elements (RLE’s) – lithophile elements with high nebular condensation temperatures (Section...

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