Counting strontium atoms in rocks from northern Canada, researchers have discovered evidence that the Earth’s continental crust could have formed hundreds of millions of years earlier than previously thought.
Continental crust is richer in essential minerals than younger volcanic rock, which would have made it significantly friendlier to supporting life.
“Our evidence, which squares with emerging evidence including rocks in western Australia, suggests that the early Earth was capable of forming continental crust within 350 million years of the formation of the solar system,” says Patrick Boehnke, a postdoctoral fellow in the geophysical sciences department at the University of Chicago.
“This alters the classic view, that the crust was hot, dry, and hellish for more than half a billion years after it formed.”
One of the open questions in geology is how and when some of the crust—originally all younger volcanic rock—changed into the continental crust we know and love, which is lighter and richer in silica.
The task is made harder because the evidence keeps getting melted and reformed over millions of years. One of the few places on Earth where you can find bits of crust from the very earliest epochs of Earth is in tiny flecks of apatite imbedded in younger rocks.
Luckily for scientists, some of these “younger” minerals (still about 3.9 billion years old) are zircons—very hard, weather-resistant minerals somewhat similar to diamonds.
“Zircons are a geologist’s favorite because these are the only record of the first three to four hundred million years of Earth. Diamonds aren’t forever—zircons are,” says Boehnke, first author of the paper, which appears in the Proceedings of the National Academy of Sciences.
Plus, researchers can date the zircons. “They’re like labeled time capsules,” says coauthor Andrew Davis, professor and chair of the geophysical sciences department at Chicago.
Scientists usually look at the different variants of elements, called isotopes, to tell a story about these rocks. They wanted to use strontium, which offers clues to how much silica was around at the time it formed. The only problem is that these flecks are absolutely tiny—about five microns across, the diameter of a strand of spider silk—and you have to count the strontium atoms one by one.
This was a task for an instrument that came online last year: the CHicago Instrument for Laser Ionization, or CHILI. The detector uses lasers that researchers can tune to selectively pick out and ionize strontium. When they used CHILI to count strontium isotopes in rocks from Nuvvuagittuq, Canada, they found the isotope ratio suggested plenty of silica was present when it formed.
This is important because the makeup of the crust directly affects the atmosphere, the composition of seawater, and nutrients available to any budding life hoping to thrive on planet Earth. It also may imply there were fewer meteorites than thought pummeling the Earth at this time, which would have made it hard for continental crust to form.
“Having continental crust that early changes the picture of early Earth in a number of ways,” says Davis, who is also a professor with the Enrico Fermi Institute. “Now we need a way for the geologic processes that make the continents to happen much faster; you probably need water and magma that’s about 600 degrees Fahrenheit less hot.”
Other researchers from the University of Chicago, the Argonne National Laboratory, UCLA, and the Berkeley Geochronology Center are coauthors of the paper. NASA funded the work.
Source: University of Chicago
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