Extreme Nd-isotope heterogeneity in the early Archaean - Fact or fiction? Case histories from northern Canada and West Greenland

• Published in: Chemical Geology
• Main Authors: Moorbath, S., Whitehouse, M. J., Kamber, B. S.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier 1997
• Subjects: Archaean; mantle; crust; Nd isotopes; geochronology; isua supracrustal belt; early archean crust; sm-nd; amitsoq gneisses; whole-rock; isukasia area; enderby land; rb-sr; age; metamorphism; Geochemistry & Geophysics; Canada; Greenland; Enderby Land
• Online Access: https://eprints.qut.edu.au/126207/

Sm-Nd data on rock suites from early Archaean provinces in northern Canada and West Greenland clearly demonstrate that tectonothermal (i.e. igneous, metamorphic, tectonic) processes which affected the rocks long after their formation produced open-system behaviour leading to effective resetting of the Sm-Nd system accompanied by complete, or near-complete, Nd-isotope homogenisation. This means that extreme caution is required in interpreting highly variable initial epsilon(Nd) values of ancient rocks in terms of long-standing regional mantle heterogeneity or of complex mantle-crust interaction processes. In particular, calculated initial epsilon(Nd), values based on high-precision zircon U-Pb dates may be of little or no significance in terms of geochemical evolution of early mantle and crust source regions. A striking example is provided by the Acasta gneisses of northern Canada, with published SHRIMP U-Pb zircon dates in the range 3.6-4.0 Ga and apparent, initial epsilon(Nd) values in the range -4.8 to +3.6 (Bowring and Housh, 1995). A combination of 34 published and new Sm-Nd whole-rock analyses for a wide range of rock types yields a regression (errorchron) age of 3371 +/- 59 Ma (MSWD = 9.2), with initial epsilon(Nd) = -5.6 +/- 0.7. Whilst the very negative initial epsilon(Nd) provides strong, independent support for the extreme age of the Acasta gneiss protolith, resetting of the Sm-Nd system at similar to 3.4 Ga renders calculation of initial epsilon(Nd) based on the zircon U-Pb dates geologically meaningless. Analogous considerations for early Archaean Akilia enclaves and host Amitsoq gneisses of West Greenland suggest that their published range of initial epsilon(Nd) values of similar to -4.5 to +4.5 at similar to 3.73-3.87 Ca (Bennett et al., 1993) may be unrealistically wide and, therefore, inappropriate for modelling upper-mantle heterogeneity. In an attempt to determine a realistic initial epsilon(Nd) value, we have regressed 58 published and new Sm-Nd data for two major rock units of the Isua ...