Nd and PB isotope ratios of the Abitibi greenstone belt: New evidence for very early differentiation of the Earth

Details Nd and PB isotope ratios of the Abitibi greenstone belt: New evidence for very early differentiation of the Earth Nd and PB isotope ratios of the Abitibi greenstone belt: New evidence for very early differentiation of the Earth

Dec 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994e%26psl.128..215v&link_type=abstract

Earth and Planetary Science Letters (ISSN 0012-821X), vol. 128, no. 3-4, p. 215-229

Computer Science

11

Differentiation, Earth (Planet), Felsite, Geochronology, Lead Isotopes, Massifs, Neodymium Isotopes, Sedimentary Rocks, Earth Mantle, Mass Spectrometers, Regression Analysis, Volcanoes

Scientific paper

Nd and Pb isotopic compositions were determined for 39 samples from mafic and felsic volcanic and subvolcanic units that underlie volcanogenic massive sulfide deposits in the Noranda and Matagami districts of the Abitibi greenstone belt. At Noranda, Pb and Nd isotopic data plot along a single linear array which implies derivation from an isotopically homogeneous source (average epsilonNd = +2.5). At Matagami, in contrast, the Pb and Nd data define two linear arrays. The more compositionally evolved rhyolites have less radiogenic Pb isotopic compositions and higher average initial (epsilonNd = +3.2) than the more primitive compositions (epsilonNd = +2.4). We interpret these isotopic differences at Matagami, as well as those between Noranda and Matagami, as not due to assimilation of crustal components, but rather primary mantle heterogeneity in the Abitibi mantle at 2.7 Ga. This implies that, although the Abitibi mantle was broadly uniform in terms of initial epsilonNd values (+2.5 +/- 1), it was characterized by small heterogeneities on both local and regional scales in the Archean. On the basic of Pb and Nd isotopic data, we conclude that the Pb isotopic compositions of galena in Noranda and Matagami ores represent initial Pb isotopic compositions of the underlying volcanic rocks. Pb isotopic compositions of such massive sulfide ores throughout the Abitibi greenstone belt plot along a coherent linear Pb-Pb array and Noranda and Matagami galenas are the most and least radiogenic examples, respectively. This array is nearly coincident with the geochron at 2.7 Ga, using an age for the Earth of 4.53 Ga, and indicates not only that the Abitibi mantle was isotopically heterogeneous at 2.7 Ga but also that these heterogeneities were developed within the first few hundred million years of Earth's history. Available data on initial epsilonNd and initial Pb-207/Pb-204 for the Superior Province are negatively correlated, implying that this early U-Pb fractionation was accompanied by Sm-Nd fractionation and involved incompatible element depletion of the mantle.

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