Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
9533656 | Precambrian Research | 2005 | 25 Pages |
Abstract
Hf isotopic compositions of zircon from juvenile Archean sources are remarkably consistent and define an average ÉHf value of +3.5 ± 0.2 for samples with an average age of 2724 Ma and a best estimate of +2.7 ± 0.4 at 3000 Ma. Thus, the Neaoarchean depleted mantle reservoir beneath the Superior province appears to have been isotopically well mixed. ÉHf values were calculated using a value of 1.865 Ã 10â5 Maâ1 [Scherer, E., Munker, C., Mezger, K., 2001. Calibration of the Lutetium-Hafnium clock. Science 293, 683-687] for the 176Lu decay constant, which is thus far the best reproduced estimate and the one most consistent with depleted mantle evolution results based on Nd isotopes and Nb/Th ratios. A linear Hf mantle growth curve defined by these values and recent MORB intersects the chondritic Hf growth curve during the early Archean (3.4-4.0 Ga). This could indicate that the earliest formation of significant amounts of enriched crust coincides with ages of the oldest preserved rocks, but such a conclusion is contradicted by evidence from 142Nd and 143Nd in early Archean rocks for significant mantle depletion during the Hadean eon (>4.0 Ga). Both lines of evidence might be reconciled if Hadean enriched crust were largely remixed with its depleted mantle source near the beginning of the Archean, leaving only fragmentary evidence of its existence in the oldest rocks.
Related Topics
Physical Sciences and Engineering
Earth and Planetary Sciences
Geochemistry and Petrology
Authors
D.W. Davis, Y. Amelin, G.M. Nowell, R.R. Parrish,