U–Pb isotopic ages and Hf isotopic composition of single zircons: The search for juvenile Precambrian continental crust

Abstract

U–Pb isotopic ages and Hf isotope compositions by laser microprobe ICPMS and MC-ICPMS can be useful in identifying detrital zircons derived from juvenile continental sources. Hf isotopes in detrital zircons from modern river deposits in Brazil, Australia, India and the Ukraine show evidence for production of juvenile crust at about 2.5 Ga. However, if detrital zircon populations in the 2.4–2.2 Ga time window are representative of the proportion of juvenile crust in their primary sources, they yield little evidence for significant volumes of juvenile crust of this age. Hf isotopic compositions of detrital zircons from the Ukraine and eastern Australia also record production of juvenile continental crust between 1.65 and 1.40 Ga. Zircons from granitoids in south-central Laurentia and in western Brazil have E Hf( T) values that fall near the depleted mantle growth curve recording production of juvenile continental crust in these regions between 1.5 and 1.3 Ga. Although Hf isotope compositions of detrital zircons can be useful in identifying detrital zircons derived from juvenile continental sources, the results do not necessarily equate to the volume of juvenile continental crust produced during specific time intervals.