Sm-Nd Isotopic Data and Earth's Evolution.

Samuel A. Bowring and Todd Housh (1) report Sm/Nd isotopic data giving a wide range of initial ENd values, from +3.5 to -4 at 4.0 billion years ago (Ga) and from +4 to -7 at 3.6 Ga. Their samples were from the Acasta gneisses of northern Canada, which represent the oldest known outcrops of continental crust (2). Their modeling of the Sm-Nd isotopic data is taken to suggest that the Acasta granitoid rocks were formed from mixtures between mantle-derived melts and crustal melts derived from extremely ancient (about 4.3 Ga), heterogeneous, depleted (high Sm/Nd) and enriched (low Sm/Nd) reservoirs. We consider it likely that the wide range of initial ENd values has a much simpler explanation. We plot the Acasta gneiss Sm-Nd data of Bowring and Housh (1) and Bowring et al. (3) on a Sm-Nd isochron diagram (Fig.