Earth's Oldest Mineral Grains Suggest an Early Start for LifeDecember 24, 2001 / Posted by: Shige Abe
Scientists are drawing a portrait of how Earth looked soon after it formed 4.56 billion years ago, based on clues within the oldest mineral grains ever found.
Tiny zircons (zirconium silicate crystals) deposited in ancient stream deposits appear to indicate that Earth developed continents and water — perhaps even oceans and environments in which microbial life could emerge — 4.3 billion to 4.4 billion years ago.
The findings by two research groups, one in Australia and the other in the US, indicate “liquid water stabilizes early on Earth-type planets,” said geologist Stephen Mojzsis, a member of the NASA Astrobiology Institute’s University of Colorado, Boulder, team. “This increases the likelihood of finding life elsewhere in the universe” by suggesting that conditions conducive to life can develop faster and more easily than previously thought.
It also “gives us a new view of the early Earth, where the Earth cooled quickly” after gas and dust in the newborn solar system congealed to form planets, said geologist William Peck, of Colgate University in Hamilton, New York. “There were continents and water really early — and maybe oceans and life — all to be obliterated later by meteorites, with almost no record left except these zircons.” That contrasts with the old view that Earth stabilized more slowly, with delayed formation of oceans and, ultimately, continents.
Until roughly 3.9 billion years ago, swarms of comets and meteorites whacked the young Earth often enough to occasionally vaporize the surface zones of the oceans and erase any life residing there. The earliest known evidence of microbial life on Earth comes from carbon isotope patterns investigated by Mojzsis and colleagues in 3.85-billion-year-old Greenland sediments.
Now, the zircons from Western Australia demonstrate that continents and water existed 4.3 billion to 4.4 billion years ago, which suggests “life could have had the opportunity to start 400 million years earlier than previously documented,” Mojzsis said.
“Life could have arisen many times, only to be smashed, and it only gets a hold once the meteorites taper off,” Peck added.
Mojzsis and Peck belong to separate research teams that published studies in the Jan. 11, 2001, issue of the British journal Nature.
Discovery of a 4.4-billion-year-old zircon was reported by Peck, Simon Wilde at the Curtin Institute of Technology in Australia; John Valley at the University of Wisconsin, Madison; and Colin Graham of the University of Edinburgh in the United Kingdom. Wilde found the 4.4-billion-year-old grain in 1999 while dating zircons from a rock collected in 1984, Peck said.
Mojzsis and colleagues say they found a pair of 4.3-billion-year-old zircons last year from the same area of Western Australia’s Jack Hills rock formation. Mojzsis worked with geochemist Mark Harrison of the University of California, Los Angeles, and Robert Pidgeon of the Curtin Institute of Technology.
The 4.4-billion-year-old zircon is “our earliest record of the earliest crust” on Earth, Peck said. That zircon and the slightly younger zircon grains measure roughly 250 microns wide — less than one one-hundredth of an inch.
“These zircons have really been through the ringer,” said Peck.
Their history began sometime after Earth formed, when “liquid water interacted with rocks,” he said. That interaction can happen in one of three ways: when water exchanges with minerals in rocks, when crystals grow out of solution in ground water, or when mineral veins are deposited. The interaction with water increased the rocks’ normally low proportion of the uncommon isotope oxygen-18 to the more-common isotope oxygen-16, he said.
Later, the rocks melted underground — or perhaps during a meteorite bombardment — and the zircons formed as crystals within molten granite that was cooling to form solid rock.
The zircon-laden granite eventually was thrust upward to form mountains, which later eroded. The granite vanished, but the zircons ultimately came to rest 3 billion years ago in sandy Australian stream sediments. These sediments later hardened into rocks that subsequently were altered by heat and pressure.
Both research teams used instruments called ion microprobes to date and analyze the zircon crystals, which often contain uranium, rare earth elements and other impurities. Uranium decays to lead at a known rate. Uranium-lead ratios in the zircons showed they formed as early as 4.4 billion to 4.3 billion years ago when they crystallized in molten granite.
Continental crust is different than crust that underlies the oceans. Granite is a common rock in continents. And zircons commonly crystallize in granite.
So the zircons indicate granite was present 4.3 billion to 4.4 billion years ago, while the granite means continents existed at that time. Such old granitic rock has not been found; it all has subsequently been eroded away or otherwise recycled. The ancient zircons are surviving vestiges of crustal granite from Earth’s early years.
“The fact you have a 4.4-billion-year-old zircon from granite suggests there had to be the rock of the continental crust,” said geologist Sam Bowring of the Massachusetts Institute of Technology.
Ion microprobe analysis of rare-earth elements within the zircon crystals also found levels typical of continental rocks, Peck said.
The presence of water on the young Earth was confirmed when both groups analyzed the zircons for oxygen isotopes and found the telltale signature of rocks that have been touched by water: an elevated ratio of oxygen-18 to oxygen-16.
As a result, “we know there was liquid water at some point before 4.4 billion years ago,” Peck said. Liquid water had to collect somewhere, raising the possibility of oceans, he added.
He said it also is likely oceans existed because “to make continents, you need to have water.”
Peck said that before there were oceans, giant plates of Earth’s crust already could have started moving and colliding with each other, causing large blocks of rock to dive downward in a process called subduction. Without oceans, that rock could not have melted to form continental rock like granite, he said.
Once there were oceans, however, seawater would have reacted with and hydrated lava erupting from undersea volcanoes at the mid-ocean ridges. The lava would then have cooled and formed new seafloor, which later subducted. The water trapped in minerals within the sinking rock lowered its melting point, triggering volcanic eruptions that probably produced island chains made of granitic rocks. It is thought that such “island arcs” ultimately clumped together to form continents.
“Oceans, atmosphere and continents were in place by 4.3 billion years ago,” said Mojzsis.
According to Peck, the first oceans might have formed from water brought to Earth by comets or have been emitted during early volcanic eruptions from what became mid-ocean ridges.
The zircons suggest that life could have existed on Earth 4.3 billion years ago, said Mojzsis, because three key factors necessary for life to take hold were present: energy, organic material (from incoming comets and atmospheric reactions) and — according to the zircons — liquid water.
Peck and his colleagues will continue the search for ancient zircons “to confirm what we discovered or discover new insights about the early crust and early Earth.” Mojzsis is actively involved in a newly formed NASA Astrobiology Institute Focus Group named Mission to Early Earth that plans a multifaceted study of Earth’s earliest rocks, including zircons.
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