Was Earth once covered in HELLFIRE? No – more like a wet Sunday night in Iceland

Geoboffins DEBUNK long-held scientific belief

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Early Earth may have been less like the hellish realm of molten magma previously suspected but also distinct from the sort of environment found in modern-day Iceland, according to research by geologists.

The Earth had already formed oceans, continents and an active crustal plates by the time it was 500 million years old, scientists reckon. This view of the Hadean, Earth’s first geological eon, has been refined by a fresh study comparing zircon crystals that formed during this time around four billion years ago with those formed in Iceland and zircon found in other contemporary environments.

The study concluded that although the early Earth was not covered in an ocean of molten magma, as thought as recently as 30 years ago, it wasn't quite like modern Iceland either, puncturing a favoured analogy among modern geologists.

Icelandic zircons grew from much hotter magmas than Hadean zircons, a new study has concluded.

Geologists led by Professor Calvin Miller of Vanderbilt University carried out the study. They put together a paper, titled Iceland is not a magmatic analog for the Hadean: Evidence from the zircon record, which was published by the journal Earth and Planetary Science Letters.

Up until around 30 years ago geologists generally agreed that conditions during the Hadean period were utterly hostile to life. A failure to find rock formations from the period prompted the belief that the early Earth was hellishly hot, either entirely molten or subject to such intense asteroid bombardment that any rocks that formed were rapidly remelted. Early Earth was imagined as being covered by a giant “magma ocean”.

The discovery of zircon crystals (a mineral typically associated with granite) with ages exceeding four billion years old and preserved in younger sandstones shattered this view.

Radiometric dating techniques revealed the ages of these ancient zircons before geologists used other techniques to extract information about the environment in which the crystals formed, including the temperature and whether water was present.

These studies provided evidence that the the early Earth had a solid crust and liquid water - at least at some of the time. Hadean Earth possessed an established crust cool enough so that surface water could form, possibly even on the scale of oceans. This perspective is these days generally accepted but geologists continue to debate the particulars, as a statement by Vanderbilt University explains.

Two schools of thought have emerged: One argues that Hadean Earth was surprisingly similar to the present day. The other maintains that, although it was less hostile than formerly believed, early Earth was nonetheless a foreign-seeming and formidable place, similar to the hottest, most extreme, geologic environments of today. A popular analogy is Iceland, where substantial amounts of crust are forming from basaltic magma that is much hotter than the magmas that built most of Earth’s current continental crust.

“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 Miller.

Vanderbilt doctoral student Tamara Carley, who has just accepted the position of assistant professor at Layfayette College, began collecting samples from volcanoes and sands derived from erosion of Icelandic volcanoes five years ago. Thousands of zircon crystals were separated from these samples.

Working with Miller and doctoral student Abraham Padilla at Vanderbilt, Joe Wooden at Stanford University, Axel Schmitt and Rita Economos from UCLA, Ilya Bindeman at the University of Oregon and Brennan Jordan at the University of South Dakota, Carley analyzed about 1,000 zircon crystals for their age and elemental and isotopic compositions. Analysis of Hadean zircon and representative analyses of zircon from other modern environments were compared against the Icelandic findings.

“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 Carley.

The comparative analysis found that Icelandic zircons grew from much hotter magmas than Hadean zircons. Although surface water played an important part in the generation of both Icelandic and Hadean crystals, in the Icelandic case the water was extremely hot when it interacted with the source rocks while the Hadean water-rock interactions took place at significantly lower temperatures.

“Our conclusion is counterintuitive,” said Miller. “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 study was supported by National Science Foundation grants and research grants from the National Geographic Society and the Keck Geology Consortium. ®

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