The element is scant in zircons formed on the ocean floor from magma but abundant in zircons formed in Earth's continental crust.
The Jack Hills zircons contain high levels of lithium, the researchers found, suggesting that the tiny crystals formed from continental crusteven though some of these rocks formed when our planet was less than 300 million years old.
Further analysis showed that the zircons were low in heavier forms of lithium atoms. Such low ratios occur only in heavily weathered rocks, said study co-author John Valley, a geochemist also at the University of Wisconsin.
The new study supports prior research suggesting the Jack Hills zircons formed from other rocks that had originally formed at relatively low temperatures and in the presence of liquid water.
"Once you have liquid water on the surface [of the Earth], then you'll have precipitation," which can weather the rocks, Valley said.
The team's findings suggest that very early in its history, Earth had crust that was being eroded by a harsh climate.
Bruce Watson, a geochemist at Rensselaer Polytechnic Institute in Troy, New York, who was not involved in the study, said its findings were "important."
"They definitely bolster the enticing picture of an early Earth with continental crust and possibly water."
But other researchers are more skeptical.
Thorsten Geisler-Wierwille of the University of Münster in Germany, said: "I think that most of the [changes in] lithium is indeed due to weathering, but from recent times."
He believes radioactivity from uranium in the zircons could have damaged them over eons, allowing lithium to move in or out of the rocks after they formed.
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