This zircon crystal from Australia is currently the oldest piece of the Earth’s crustJohn Valley
Water first fell on the Earth’s surface approximately 4 billion years ago — 500 million years earlier than previously thought. Evidence of these primordial rains was obtained through analyses of zircon crystals from Jack Hills, a range of hills in western Australia that represent the oldest records of the Earth’s crust. Because it is resistant to major climate variations and chemically very stable, zircon is seen as something of a time capsule. By determining the different forms (isotopes) of certain atoms found in the crystals, it is possible to date the oldest rocks on the planet and infer the conditions under which they were formed. That was the approach taken by a team from Australia and China, with samples of the mineral taken from Jack Hills. In zircon samples dated at 4 billion years old, they found oxygen isotopes at a ratio associated with contact with fresh water. The discovery suggests that the crystals were formed deep within the Earth’s mantle in an environment containing moisture from rainfall, according to geologist Hamed Gamaleldien of Curtin University, Australia, lead author of the study (Nature Geoscience, June).
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