Nd and Sr isotope systematics of clastic metasediments from Isua, West Greenland: Identification of pre‐3.8 Ga Differentiated Crustal Components

Abstract

Recent field work on the Isua metasediments has established that there are two lithological sequences (A and B), which are divided into a number of distinct formations. Each clastic metasedimentary unit has a relatively unique trace element signature. The depleted mantle Sm‐Nd model ages of samples from all major units range from 3.59 to 3.98 Ga, whereas Rb‐Sr model ages cover a very wide range (1.9–4.6 Ga). The samples from the sequence A felsic gneiss formation (A6) yield a relatively good Rb‐Sr isochron age of T = 3.71±0.07 Ga. The sequence B samples do not yield a Rb‐Sr isochron; however, some of the samples plot close to the A6 Rb‐Sr isochron, and sequences A and B appear to have essentially the same deposition age. The Sm‐Nd data for both sequences yield initial εNd values in the range −1 to +3 for a deposition age TSTRAT = 3.64 Ga (if TSTRAT = 3.81 Ga, the range is 0–4). This range appears to reflect real variability of initial εNd values corresponding to the time of deposition. Data trends in εNd; versus 1/Nd diagrams suggest at least three distinct components in each of sequences A and B. Model age calculations suggest that some of these components must be derived from continental crustal sources that predate the time of deposition of these sediments by as much as ≈0.4 b.y. Other components appear to be derived from volcanic sources that are roughly coeval with sediment deposition. The very high initial εNd values implied for early depleted mantle sources (+2 to +4), suggest a very large pre‐3.8 Ga continental crust. Alternatively, these high values could reflect an early terrestrial mantle with highly fractionated cumulate layers that formed in a ≈4.5 Ga magma ocean.