Geochemistry of Ce and Nd isotopes and REE abundances in the Amîtsoq gneisses, West Greenland

Abstract

Ce and Nd isotope ratios and rare earth element (REE) abundances were determined on the Archean Amîtsoq gneisses, West Greenland; samples used were four grey gneisses, two white gneisses and one banded gneiss from the Isukasia area and one banded grey gneiss from the Godthaab area. All of the Amîtsoq gneisses studied here have negative ε Nd and positive ε Ce values for the present Nd and Ce isotopic compositions. Calculated initial values of ε Nd and ε Ce give the following evolution histories of the REE patterns. The grey gneiss in the Godthaab area is considered to have been derived from a chondritic source with (La/Ce) N and (Nd/Sm) N ratios of 1.1 ± 0.4 and 0.99 ± 0.02 in the chondrite-normalized values. On the other hand, it is concluded that the grey gneisses and the banded gneiss in the Isukasia area were formed from a light-REE-depleted source with (La/Ce) N = 0.6 ± 0.1 and (Nd/Sm) N = 0.97 ± 0.03. Furthermore, it is suggested that the white gneisses in the Isukasia area may have been produced from a source with a convex REE pattern. Our study on Ce and Nd isotopes and REE abundances provides further evidence that fractionated REE patterns were established already in the very early stages of the Archean.