Privileged Planet: more on early zircons (Introduction)

by David Turell , Friday, May 03, 2024, 18:05 (204 days ago) @ David Turell

They always tell us a lot:

https://www.sciencemagazinedigital.org/sciencemagazine/library/item/03_may_2024/4192470...

"Tiny grains of a mineral called zircon might have witnessed the fall of rain on Earth’s earliest dry land some 4 billion years ago, when oceans likely covered most of the planet. The chemical composition of the crystals, plucked from rocks in Australia, hint that they formed from magmas doped with freshwater, a team of scientists argues. That would only have been possible on terra firma, they say.

“'We found evidence for two things: There was land above sea level and, at the same, that this land interacted with freshwater,” says Hamed Gamaleldien, a geochemist at Khalifa University who presented the results last month at a conference of the European Geosciences Union. “This means you start to have the hydrological cycle, and you started to have the recipe for the start of life.”

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"The zircons represent a rare report from the mysterious Hadean, the geological time period that ended about 4 billion years ago, 500 million years after Earth’s formation. The planet, originally a ball of magma, had cooled off and formed a crust. Somehow, perhaps from a bombardment of water-rich asteroids, it had accumulated a global ocean. Earth may have remained watery for quite some time— at least until tectonic processes began to recycle Earth’s crust into its interior, and magma bubbled up in chains of island volcanoes that eventually fused into continents.

"Much of this is guesswork, because almost no rock survives from the Hadean. The oldest rock with a reliable age—a gneiss from Canada—is 4.03 billion years old. The only surviving material from before then are zircons, found embedded in younger rock, which are as much as 4.4 billion years old. “Just about any information that we can get from these Hadean zircons is useful because it’s our singular record of the Earth’s first 500 million years,” says geologist Stephen Mojzsis of the HUN-REN Research Centre for Astronomy and Earth Sciences.

"Zircons are tough little minerals, often outliving the original rock they formed within. For example, Hadean zircons found in Australia’s Jack Hills sit in rock that’s about 3 billion years old—having eroded out of even older rocks that have long since disappeared. Zircons contain small amounts of elements such as uranium that allow them to be precisely dated, as well as other clues about ancient Earth. Researchers have used zircons to try to date the onset of plate tectonics and the origin of continental crust. They even offer hints of the origin of life: In 2015, Bell and her colleagues found bits of graphite in a 4.1-billion-year-old zircon that might have been derived from biological carbon.

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"Before Gamaleldien and his colleagues could make the measurements, they had to collect, polish, and individually inspect thousands of zircons from two rock chunks pulled from the Jack Hills. They analyzed about 1400 zircons that were between 1.85 billion and 4.28 billion years old.

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"To be certain, the researchers performed tens of thousands of computer simulations of zircons forming from magmas mixed with seawater, rainwater, or some combination of both. Only with at least some freshwater could the team reproduce the exceptionally light isotopic signature of their zircons.

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"If Gamaleldien and his colleagues are right, however, the late Hadean might have been habitable long before the appearance of the first fossils. Not all hypotheses for the origins of life require dry land. But some invoke freshwater hot spring environments. Gamaleldien hopes his discovery will spark renewed interest in a search for life before 4 billion years. “Whether there’s life or not, we don’t know,” he says. “But you had the recipe.'”

Comment: fossilized stromatolites from 3.8 byo support the zircon theory. What is apparent is the Earth was barely formed before life tried to appear. The whole story looks like a designed process following the purpose of creating life, ending with humans. Looked at as a series of necessary contingencies, think of what the odds are if it is all by natural chance. Impossibly an enormous set of odds.