Resetting of Sm-Nd systematics during metamorphism of > 3.7-Ga rocks: Implications for isotopic models of early Earth differentiation

New Sm-Nd isotopic results for sixteen samples from the Garbenschiefer Amphibolite Unit of the > 3700-Ma Isua supracrustal belt of West Greenland are reported. The data illustrate the difficulty in identifying primary epsilon(Nd)(T) values in poly-metamorphic and poly-deformed rock sequences. The investigated samples are interpreted as derived from a Mg- and Al-rich igneous protolith which can be argued to have been essentially isotopically homogeneous at least on the outcrop scale at the time of its emplacement. However, the data do not give uniform epsilon(Nd)(T) values even from single outcrops, The wide variation of epsilon(Nd)(T) which we obtain (- 1.0 to + 5.0) demonstrates that the Sm-Nd systematics of the samples were disturbed by one (or more) metamorphic event(s) which took place some considerable time after the original crystallization of the Isua belt. A Sm-Nd isochron age of 2849 +/- 116 Ma has been obtained on plagioclase and amphibole separates which represent the oldest metamorphic assemblage now preserved at Isua. This result indicates that the earliest amphibolite-facies metamorphism of the Isua belt for which pressure and temperature conditions can be obtained (P approximate to 5 kbar; T approximate to 500-600 degrees C) is late Archaean, and not early Archaean as had been assumed earlier by some authors, 2850 Ma is also thought to represent the main time of disturbance of the Sm-Nd system in the Garbenschiefer Amphibolite Unit. A consideration of available isotopic data suggests that many of the rocks from Isua underwent some Nd isotopic re-equilibration and/or fractionation of their Sm-147/Nd-144 ratio during the 2850-Ma metamorphic episode. In these rocks, the computed epsilon(Nd)(T) are apparent values only and need not represent either the primary isotopic composition of their protoliths or early Archaean mantle values. Only Nd-rich rocks (> 20 ppm) such as some felsic schists and the grey gneisses adjacent to the belt are arguably sufficiently robust to retain epsilon(Nd)(T) values which are close to their original composition. These average + 2.0, which may represent a close approximation of the epsilon(Nd) value of the Earth's mantle at similar to 3800 Ma, The results presented in this paper show that great care should be exercised in the use of Sm-Nd data from very early Archaean rocks as constraints for isotopic models of early Earth's evolution, particularly in terranes which have been affected by several metamorphic episodes.