Archean depleted mantle: Evidence from Nd and Sr initial isotopic ratios of carbonatites
1987; Elsevier BV; Volume: 51; Issue: 2 Linguagem: Inglês
10.1016/0016-7037(87)90241-9
ISSN1872-9533
Autores Tópico(s)earthquake and tectonic studies
ResumoInitial 87Sr86Sr and 143Nd144Nd ratios of some carbonatites and related alkalic rocks intruded into the Canadian Shield between 2700 Ma and 110 Ma ago define development lines on a plot of initial ratio versus age. The isotopic data show that these complexes have been derived from a mantle source with a RbSr ratio of 0.020 ± 0.002 and a SmNd ratio of 0.358 ± 0.008, which are lower and higher respectively than the values for the silicate portion of the Earth. An estimate of the mean age of formation of this LIL-depleted reservoir, based on the intersection of the development lines with those of bulk Earth, is about 2900 Ma, a figure similar to the age of most Superior Province rocks. A model is proposed that involves differentiation, ca. 2900 Ma ago, of mantle similar in chemical composition to bulk Earth, into granitoid rocks (the Superior Province), and a depleted, residual upper mantle. The source for the Nd and Sr contained in the carbonatites is thought to lie within this depleted sub-continental lithosphere. Measured SmNd ratios of about 0.15 for the carbonatites are much lower than the calculated SmNd ratio of their source, and support enrichment of the light REE either slightly before or at the same time as carbonatite magma generation.
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