Primordial crystals give insight into Earth’s first rains

A new study has found that the Earth's first rains happened about 4 billion years ago - as much as 500 million years earlier than previously thought, Science Alert reported.

A team of researchers from Australia and China used oxygen isotopes trapped in ancient minerals to determine when the first signs of fresh water wetted the skin of our newborn planet.

The Jack Hills in Western Australia contain the oldest surviving material from the Earth's crust. For 4.4 billion years, the primary minerals have remained relatively unchanged by heat and pressure.

The dry, red, dusty landscape doesn't get much water these days, but scientists have found evidence of Earth's oldest rains trapped in Hadean zircon crystals in the rock. It's a big shift in our understanding of the planet's hydrological history.

"By examining the age and oxygen isotopes in small crystals of the mineral zircon, we have found unusually light isotopic signatures as early as four billion years ago," says geologist and lead author Hamed Gamaleldien of Curtin University in Australia.

Gamaleldien and his colleagues used secondary ion mass spectrometry to analyze the tiny zircon grains and determine which oxygen isotopes were present in the magma from which the crystals formed.

The Jack Hills zircons had an "extremely light isotopic" composition, possible only if they formed beneath the mantle and were exposed to fresh water - specifically meteoric water - such as had recently fallen from the sky. Consequently, locked within these crystals may be evidence of the Earth's first rains penetrating the shallows of its newly solidified crust.

"Such light oxygen isotopes are usually the result of hot, fresh water altering rocks a few kilometers below the Earth's surface," Gamaleldien said.

"The evidence of fresh water so deep in the Earth challenges the existing theory that four billion years ago the Earth was entirely covered by ocean," he added.

Curtin University geologist and co-author of the study Hugo Olieruk pointed out that this research has implications for many fields of science.

"This discovery not only sheds light on Earth's early history, but also suggests that land masses and fresh water created the conditions for life to flourish within a relatively short period of time - less than 600 million years after the planet formed," he said.

Until now, the Earth's crust was thought to have been submerged under an ocean at the time; some of the earliest terrestrial life forms we've found are 3.48-billion-year-old microbial reefs known as stromatolites, found just over 800 kilometers north of the Jack Hills, in the Pilbara Craton.

But this new research suggests that land, freshwater reservoirs, the water cycle and possibly even life on Earth emerged much earlier than we had thought.

It also reinforces the "cool early Earth" theory described by University of Wisconsin-Madison geologist John Vallee, whose 2014 paper listed Hadean zircons as the oldest material on Earth.

The theory suggests that soon after the planet's sea of molten rock coalesced into crust, the Earth was cool enough to accept liquid water, oceans and a hydrosphere.

"The discoveries mark a significant step forward in our understanding of Earth's early history and open the door to further research into the origins of life," Olieruk said. | BGNES