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| Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago Simon A. Wilde, John W. Valley, William H. Peck & Colin M. Graham (2001) Nature, v. 409, pp. 175-178. |
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| No
crustal rocks are known to have survived since the time of the intense meteor
bombardment that affected Earth between its formation about 4,550 Myr ago
and 4,030 Myr, the age of the oldest known components in the Acasta Gneiss
of northwestern Canada. But evidence of an even older crust is provided
by detrital zircons in metamorphosed sediments at Mt Narryer and Jack Hills
in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, where
grains as old as ~4,276 Myr have been found. Here we report, based on a
detailed micro-analytical study of Jack Hills zircons, the discovery of
a detrital zircon with an age as old as 4,404 ± 8 Myr—about 130 million
years older than any previously identifed on Earth. We found that the zircon
is zoned with respect to rare earth elements and oxygen isotope ratios (d18O
values from 7.4 to 5.0‰), indicating that it formed from an evolving
magmatic source. The evolved chemistry, high d18O
value and micro-inclusions of SiO2 are consistent with growth
from a granitic melt with a d18O value
from 8.5 to 9.5‰. Magmatic oxygen isotope ratios in this range point
toward the involvement of supracrustal material that has undergone low-
temperature interaction with a liquid hydrosphere. This zircon thus represents
the earliest evidence for continental crust and oceans on the Earth.
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