Re-evaluation of the origin and evolution of > 4.2 Ga zircons from the Jack Hills metasedimentary rocks

Abstract

New data are presented on internal structures, U–Pb systematics and oxygen isotope compositions of eight detrital zircons with ages greater than 4.2 Ga, from the Jack Hills metasedimentary belt, Australia. Cathodoluminescence imaging, ion-microprobe U–Pb and oxygen isotope results show evidence for an extensive period of complex zircon growth, secondary reaction and U–Pb isotopic disturbance from 4.36 to 3.90 Ga. In addition many of the zircons have discordant U–Pb systems and excess common Pb indicating a superimposed, relatively recent, reaction between radiation damaged zircon and low temperature fluids. The significance of oxygen isotope compositions for zircons with complex internal structures and U–Pb systems is complicated by uncertainty in the origin of the grains and the unknown effect of later reactions. However, a minority of grains with sharp oscillatory zoning, uniform and concordant U–Pb systems, igneous Th–U ratios and low common Pb contents, are interpreted as undisturbed primary magmatic zircons. The oldest identified, oscillatory zoned, magmatic grain, with an age 4363 ± 20 Ma, is one of a few reported magmatic grains with this age, which is interpreted as the oldest reliable age for Hadean magmatic zircons. Mantle δ 18O values are reported for these zircons. Younger oscillatory zoned zircon, including oscillatory zoned cores in complex grains, have δ 18O values lower than 6.5‰, which are within the range of ion microprobe analysed δ 18O values for zircons in high temperature equilibrium with the normal mantle rocks of 5.3 ± 0.6‰ (2 standard deviations). These values are also within the range of δ 18O values found in lunar zircons. The absence of heavy oxygen in the grains that can be interpreted as primary magmatic zircons and the complex history over the period from 4.36 to 3.9 Ga, seen in all other Jack Hills zircons and reflected in the internal structures and U–Pb isotopic systems, questions the model for the early Earth involving long intervals of relatively temperate conditions from 4.4 to 4.0 Ga that were conducive to oceans and possibly life.