Conditions during the first 500 million years of Earth's existence may not have been as "hellish" as researchers previously believed.
New research suggests the early Earth may have been rich with oceans, continents, and active crustal plates, Vanderbilt University reported. The new view of Earth's first geological eon, called the Hadean, was made through the comparison of zircon crystals that formed about four billion years ago and those that formed relatively recently in Iceland.
From the early 20th century to the 1980s geologists thought the Hadean period was hostile and inhospitable to life; the lack of rock formations from this period led scientists to believe the early Earth was either "hellishly hot" or consistently bombarded by asteroids.
About 30 years ago researchers discovered zircon crystals believed to be older than four billion years preserved in younger sandstone. Studies on these crystals have revealed crust conditions during the Hadean were not uniformly hellish but at times cool enough so that surface water could form.
The crystal analysis has led to two theories; the first is that the early Earth had similar conditions to today, the other suggests while it was less hostile than previously believed it was still extremely hot and formidable. Iceland has commonly been known as an analog to the Hadean.
"We reasoned that the only concrete evidence for what the Hadean was like came from the only known survivors: zircon crystals - and yet no one had investigated Icelandic zircon to compare their telltale compositions to those that are more than [four] billion years old, or with zircon from other modern environments," said Calvin Miller, the William R. Kenan Jr. Professor of Earth and Environmental Sciences at Vanderbilt University.
The researchers analyzed over 1,000 zircon crystals for their age and elemental isotopic compositions.
"We discovered that Icelandic zircons are quite distinctive from crystals formed in other locations on modern Earth. We also found that they formed in magmas that are remarkably different from those in which the Hadean zircons grew," said Tamara Carley, who has just accepted the position of assistant professor at Layfayette College and worked to collect the extensive crystal samples.
The analysis also suggests Icelandic zircons grew from much hotter magmas than Hadean zircons.
"Our conclusion is counterintuitive," Miller said. "Hadean zircons grew from magmas rather similar to those formed in modern subduction zones, but apparently even 'cooler' and 'wetter' than those being produced today."
The findings were recently published in the journal Earth and Planetary Science Letters.
