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Plate tectonics got an early start

The chemistry of minerals preserved in Australian rocks suggests tectonic activity for Earth’s earliest eon

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11:56am, November 26, 2008
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The chemical composition of ancient crystals now found in Australian rocks bolsters the notion that tectonic plates may have jostled across Earth’s surface more than 4 billion years ago.

Scientists call the first 600 million years of Earth’s history the “Hadean eon” because of the presumably hellish temperatures on the freshly coalesced and largely molten planet. Also, radioactive isotopes, which generate heat inside Earth as they decay, were much more common then than they are now, says Mark Harrison, a geologist at the University of California, Los Angeles.

Few intact rocks survive from that period (SN: 10/11/08, p. 12), but previous studies suggest that the rate of heat flow from Earth’s interior during the Hadean eon was between three and five times higher than it is today, he notes.

Now, Harrison and UCLA colleagues Michelle Hopkins and Craig Manning have garnered clues about Earth’s early environment by analyzing zircons found in rocks from the Jack Hills of Western Australia. Those tiny crystals — hard, durable and chemically inert bits of zirconium silicate — are remnants of some of Earth’s first rocks, Harrison says. And the chemical composition of small mineral bits trapped inside those zircons as they crystallized more than 4 billion years ago suggests that Earth had tectonic activity at the time, the researchers report in the Nov. 27 Nature.

The most remarkable implication of this work, says Stephen Mojzsis, a geochemist at the University of Colorado in Boulder, is that the processes that made Earth habitable were established early in the planet’s history. On Venus, which is the same size as Earth but apparently lacked tectonic activity, carbon dioxide pumped into the atmosphere couldn’t be recycled back into the planet’s interior. On Mars, which also lacked tectonic activity, much of the atmosphere eventually became chemically locked in the Red Planet’s rocks — and without recycling of the crust, that atmosphere remained locked away.

To find clues of early tectonics, Harrison and his colleagues examined more than 400 Jack Hills zircons that had crystallized between 4.19 billion and 4.02 billion years ago. Of these crystals, about 85 had stray bits of mineral — called inclusions — exposed at the zircon surfaces. Of that fraction, an even smaller number of zircons — only seven, in fact — had mineral inclusions whose composition could be used to infer the temperatures and pressures at which their host zircons cooled.

Six inclusions were of the mineral muscovite. In these, the concentration of titanium atoms ranged between 3 and 9 parts per million, a sign that the host zircons crystallized at temperatures ranging between 665° Celsius and 745° C. The ratio of silicon to aluminum in the muscovite suggests that the zircons formed about 25 kilometers below ground, says Harrison. Different techniques used to analyze the single inclusion made of hornblende, a group of silicate minerals, produced similar results.

Together, these temperatures and pressures indicate that the rate of heat flow to the surface overlying the area where these zircons formed was about 75 milliwatts per square meter. That’s slightly higher than Earth’s average heat loss today but only one-third to one-fifth the heat loss expected for the planet during the Hadean eon, the team estimates.

The only regions on Earth today where the planet’s heat loss is so abnormally lower than average is above subduction zones, where one tectonic plate collides with and is shoved below another. As the solid tectonic plate is shoved down into Earth’s molten mantle, the plate cools the mantle material, allowing zircons to crystallize there and also stifling heat flow to the surface. The new zircon analyses indicate that similar processes were in play during the Hadean, the researchers say.

The team’s systematic look at the inclusions in these zircons “is a really good study,” says Steven Shirey, a geochemist at the Carnegie Institution for Science in Washington, D.C. The results, he adds, “are consistent with earlier studies” that indicate some sort of geophysical process near Earth’s surface was recycling the planet’s crust during the Hadean.

The Jack Hills zircons are evidence that Earth had granite early in its history, but the idea that the zircons include firm evidence of tectonic activity “is a scientific bridge too far,” says Robert J. Stern, a geoscientist at the University of Texas in Richardson. Other planets in the solar system, such as Venus and Mars, lose internal heat without having plate tectonics, he notes. Processes similar to the ones in place on those planets may have been active on early Earth as well, he notes.

Mojzsis disagrees: Using these zircons to infer the presence of tectonic activity “is not a stretch,” he says. “The mineralogy of the inclusions is compelling.” If the team’s results had come from zircons that were only 100 million years old, no one would question that they support the presence of tectonic activity, Mojzsis contends.

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