The oldest known fragment of continental crust on Earth is 4.4 billion years old, reports a study published online this week in Nature Geoscience. Although past studies infer such an age for Earth’s oldest minerals, the rigour of the isotopic techniques used, and thus the robustness of the age, had been vigorously debated.
John Valley and colleagues map the distribution of lead isotopes within a single grain of the mineral zircon, taken from the Jack Hills region of Western Australia. The age of the oldest Jack Hills zircon has previously been assessed using the uranium-lead radio-isotopic dating technique; according to these measurements, the zircon is 4.4 billion years old. Yet lead isotopes can move around inside the mineral, potentially leading to erroneously old estimates. However, the researchers show that although lead isotopes do move and cluster into small groups, this heterogeneity occurs on such a small scale that the radio-isotopic age measurements will not be affected. By confirming the antiquity of the grain, the results imply that continental crust began forming on Earth about 100 million years after the Earth was struck by a large impact that probably formed the Moon.
In an accompanying News and Views article, Samuel Bowring writes: “The results show that single grains of ancient zircon can yield a rich history, the implications of which date back to the very earliest history of our planet.”