Nd and Sr isotope systematics of clastic metasediments from Isua, West Greenland: identification of pre-3.8Ga differentiated crustal components.

Recent field work on the Isua metasediments has established that there are 2 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. Data trends in epsilon_Nd versus 1/Nd diagrams suggest at least 3 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 approx 0.4b.y. Other components appear to be derived from volcanic sources that are roughly coeval with sediment deposition. The very high initial epsilon_Nd values implied for early depleted mantle sources (+2 to +4), suggest a very large pre-3.8GA continental crust. Alternatively, these high values could reflect an early terrestrial mantle with highly fractionated cumulate layers that formed in a alpha 4.5Ga magma ocean.-from Authors