Sources of iron-formations in the archean isua and malene supracrustals, West Greenland: Evidence from La-Ce and sm-nd isotopic data and REE abundances
Isotopic ratios of Ce and Nd and abundances of rare earth elements (REEs) and Ba are reported for the iron-formations of the Archean Isua and Malene supracrustals, West Greenland. It is suggested that the Sm-Nd system of the Isua iron-formations was disturbed by metamorphisms at 1680 Ma (oxide facies iron-formation) and 2740 Ma (carbonate facies iron-formation). However, the Malene iron-formation samples show similar Nd isotopic ratios and REE patterns indicating that they suffered little disturbance after deposition at 2800 Ma. Initial ∊ Ce and ∊ Nd values estimated at 2800 Ma for the Malene iron-formation are close to CHUR (chondritic uniform reservoir); of 3 samples, two show negative ∊ Ce and positive ∊ Nd, and the other gives chondritic ∊ Ce and negative ∊ Nd These initial values suggest that the sources for the Malene iron-formation had a chiefly chondritic or rather mantle-like depleted nature without a notable long-term negative Ce anomaly. Similar REE features are indicated for the sources of the Isua iron-formation by the estimated initial ∊ Ce and ∊ Nd values at 3760 Ma of these samples.