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Australia: The Land Where Time Began |
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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.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |