Mathematics – Logic
Scientific paper
Nov 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984e%26psl..71...46m&link_type=abstract
Earth and Planetary Science Letters, Volume 71, Issue 1, p. 46-58.
Mathematics
Logic
18
Scientific paper
Measurements of 143Nd/144Nd and 147Sm/144Nd are reported for whole rocks and mineral separates from granulites of the Napier Complex at Fyfe Hills. Charnockites, leuconorites and gabbros yield a whole rock Sm-Nd isochron age of 3060 +/- 160 m.y. and an initial 143Nd/144Nd ratio of 0.50776 +/- 10 (ɛNd(3060 m.y.) = -2.0 +/- 1.8). The negative ɛNd value and the presence of geologically induced dispersion in the data suggest that the isochron age does not represent the time of primary crystallization of the complex but instead indicates a time of later redistribution of Sm and Nd and partial re-equilibration of 143Nd/144Nd ratios. This probably occurred during the upper granulite facies metamorphism which has also been dated at ~ 3100 m.y. by Rb-Sr and U-Pb zircon studies [1]. Coexisting clinopyroxene, apatite and total rock fractions in two adjacent samples define an approximately linear array corresponding to an age of 2300 +/- 300 m.y. This array indicates that redistribution of Sm and Nd and re-equilibration of 143Nd/144Nd ratios occurred on an intermineral scale during the upper amphibolite to lower granulite facies metamorphism at ~ 2450 m.y.
Due to the resetting of the Sm-Nd system on both whole rock and mineral scales, the primary crystallization age of the igneous protolith is not well constrained by the present data, although it is clearly> 3100 m.y. If it is assumed that the complex was derived initially from a depleted mantle reservoir (ɛNd(T) ⋍ 2), evolution of the negative ɛNd value of -2.0 with the observed Sm/Nd ratios requires a prehistory of ~ 380 m.y. This implies a primary age of ~ 3480 m.y. However, substantially older primary ages can be inferred if the source reservoirs had ɛNd(T) > 2 and/or substantial reductions in the Sm/Nd ratio occurred in whole rocks during the granulite facies metamorphism at 3100 m.y. Such an inferred reduction in the Sm/Nd ratio may have been the result of preferential loss of Sm relative to Nd, or introduction of a low Sm/Nd fluid with ɛNd >= 0 during granulite facies metamorphism.
Black Lance P.
McCulloch Malcolm T.
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