Australia: The Land Where Time Began

A biography of the Australian continent 

Hadean Age Confirmed for a Zircon Crystallised Following the Magma Ocean by Atom-Probe Tomography

According to Valley et al. ancient zircons that can be dated by the U-Th-Pb geochronometer are the only physical evidence of the physical conditions in the earliest phases of the evolution of the Earth (1).  The time at which the hydrosphere and habitable conditions for life were established have been inferred by measuring oxygen isotope ratios in such zircons (2,3). Though it has not been possible to date directly the chemical homogenisation of the Earth’s crust and the existence of the magma ocean, they must have occurred earlier (4). Poorly understood processes of intracrystalline Pb mobility can plausibly bias the accuracy of U-Pb ages (5-7). In this paper Valley et al. present results of their study using atomic-probe tomography (8) to identify and map individual atoms in the oldest known concordant grain found on Earth, a zircon from 4.4 Ma in the Hadean with a high-temperature overgrowth that formed about 1 billion years after the formation of the crystal core,  Isolated nanoclusters, which measured about 10 nm and spaced 10-50 nm apart, were fund to be enriched in incompatible elements that included radiogenic Pb that had unusually high ²⁰⁷Pb/²⁰⁶Pb ratios. The length scales of these clusters that make U-Pb biasing impossible, was demonstrated by Valley et al., as well as their formation during the later reheating event. It was confirmed by the tomography data that resulted from the study that any event of mixing of the Earth must have occurred prior to 4.4 Ga, consistent with the formation of the ocean by an early impact that formed the moon about 4.5 Ga.

Sources & Further reading

1. Valley, J. W., A. J. Cavosie, T. Ushikubo, D. A. Reinhard, D. F. Lawrence, D. J. Larson, P. H. Clifton, T. F. Kelly, S. A. Wilde, D. E. Moser and M. J. Spicuzza (2014). "Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography." Nature Geosci 7(3): 219-223.