9526536 Patchett The primary aim of this project is to evaluate the large variability in initial (Nd values that have been reported for several early Archean gneisses. It is essential for a serious and thorough discussion of models of early Earth evolution to determine whether these initial (Nd values are real or if they are produced by disturbances in the Sm-Nd system. Preliminary work on this project has been completed and has shown that the Hf isotopic compositions of zircons from these early gneisses are consistent with their whole-rock Nd isotopic compositions and as such verify the early incompatible-element depletion of the mantle. The P.I.s have not yet been able to analyze zircons from those gneisses with negative (as low as -4.0) and highly positive (up to +4.0) initial (Nd values. The P.I.'s groundwork on this project has demonstrated the complexity of these rocks, and that a solution to this problem can only be achieved by analysis of carefully selected samples. The P.I.s proposes three studies: (1) investigation of highly positive (Nd values reported for Greenland gneisses; (2) evaluation of disturbances in Lu-Hf and U-Pb in samples with known Sm-Nd disturbances; (3) evaluation of the range of Nd values (-4 to +4) in the Acasta gneisses.
