Sm-Nd age data on rock suites from several early Archaean provinces clearly demonstrate that metamorphic and/or
metasomatic events that affected the rocks long after their formation produced open-system behaviour for Sm and/or Nd, leading to effective resetting of the Sm-Nd system accompanied by Nd-isotope homogenisation. This means that extreme caution is required in interpreting highly variable, apparent initial eNd values of ancient rocks in terms of long standing regional mantle heterogeneity. In particular, calculated initial eNd values based on individual high-precision zircon U-Pb dates may be of little or no significance in terms of the geochemical evolution of early mantle or crust source regions.
A striking example is provided by the Acasta gneisses of Northwest Territories, Canada, currently regarded as the oldest known rocks on earth, with SHRIMP U-Pb zircon dates in the range 3.6 - 4.0 Ga and apparent, initial eNd(t) values in the range -4.8 to +3.6 (Bowring & Housh, 1995). A combination of 21 published and new (Oxford) Sm-Nd whole rock data for a wide range of Acasta gneiss rock types yields a regression age of 3397 ± 77 Ma (2 sigma error; M.S.W.D. = 9.4), with an initial eNd of -5.3 ± 0.9. Whilst the highly negative initial eNd provides strong, independent support for the extreme age of the Acasta gneiss protolith, the clear demonstration of resetting of the Sm-Nd system at ~ 3.4 Ga renders calculation of initial eNd based on the U-Pb zircon dates geologically meaningless.
Bowring, S. A. & Housh, T. Science 269, 1535-1540 (1995).