The zircons are the only known minerals older than four billion years.
"We know continental crust is incredibly hard to get rid of. It's a very durable object," Fedo said. "We have relics that go back up to four billion years, and yet why are there not whole strings of it earlier than that?"
Harrison admits his team's findings are controversial, but he says they paint a picture of an early Earth where active and rapid geological processescalled plate tectonicswould have recycled most of the earliest continental crust.
"This research is supporting a view put forth nearly 40 years ago, but seen as the lunatic fringe, that the Earth was essentially completely differentiated 4.5 billion years ago and has simply been in the process of recycling the surface ever since," he said.
To reach this conclusion Harrison and his colleagues examined the ratios of two rare elementshafnium and lutetiumin zircon crystals from the Jack Hills in Western Australia.
One element is associated with the continental crust, and the other with the layer below the crust, called the mantle.
"Lutetium loves the mantle, and hafnium is a continent lover," Harrison said.
The signature in the zircons is that of a "continent lover," meaning that by nearly 4.4 billion years ago Earth must have had continental crust.
In addition, the zircons show signs that Earth's crust had remelted through plate tectonics and come back to the surface.
"Everything we're learning about the early Earth tells us in fact that almost immediately the Earth formed significant amounts of continental crust and there was plate boundary interaction going on pretty much like today," Harrison said.
"In fact the continents were recycling at a substantially greater rate early on than today," he said. "That makes more sense. You've got a lot more heat to drive the recycling."
Too Much Heat?
Balz Kamber, a geologist at Laurentian University in Sudbury, Ontario, Canada, said the radioactive heat 4.4 billion years ago would not have permitted continental crust to stabilize.
"The heat would have simply melted the continental crust," he said.
The zircons, if accurately dated, must represent a minority of the rocks constituting early Earth's surface, Kamber said.
Fedo, the University of Tennessee geologist, said the suggestion that only these Australian zircons survived the geological processes that created the original continental crust is a stretch.
Nevertheless, he added, the zircons demand an explanation.
"These are incredibly rare grains and the only things we have from this window of time on Earth," he said. "As much as we can study them, the better off we are. You've got to applaud any kind of attempt to continue to study these."
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