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Taking a chemical look at the early Earth.

Geologists believe that soon after Earth took shape some 4.6 billion years ago, it consisted of a conglomeration of minerals jumbled together. Later, molten material rose to the surface, altering the uniform composition of minerals below and forming Earth's first crust.

Until now, researchers had only been able to date this key evolutionary sequence to sometime during the first half-billion years of Earth's history. But last month, two Harvard University scientists presented evidence suggesting that the episode occurred very soon after the Earth formed -- within the first 100 million years of our planet's existence. They base their finding on measurements of the relative abundances of neodymium isotopes in an ancient rock.

Geologist Richard W. Carlson of the Carnegie Institution of Washington (D.C.) cautions that the study relied on just one ancient rock sample and used a less-than-optimum reference sample for comparison. But, he adds, "What I find exciting about this work, if it proves true, is that the Earth was a very hot, chemically active body soon after its formation."

In providing a peek at very early geo-chemical processes, the new work suggests that Earth formed an extensive crust -- some 40 percent of the volume of the present-day continents -- more than 4 billion years ago, says study coauthor Charles L. Harper Jr. That primordial crust appears to have vanished long ago in the tumult of tectonic activity, so scientists cannot study it directly. But the ancient terrestial rock examined by Harper and colleague Stein B. Jacobsen may represent a complement to the material that formed the ancient crust - a lost terrain of the early Earth, he asserts.

Harper reported the new findings in March at the annual Lunar and Planetary Science Conference in Houston.

The researchers measured the abundances of two isotopes of neodymium, a rare-earth element, in a Greenland rock that dates back about 3.8 billion years. One of the isotopes, neodymium-142, represents in part the radioactive decay product of a form of the element sumarium, which was present only during Earth's earliest epochs. Because this form, known as sumarium-146, has a half-life of only 103 million years and has essentially been extinct for billions of years, a relative excess of its decay product must reflect chemical changes that took place very early in Earth's history, Jacobsen and Harper assert.

Although other researchers have examined the amount of neodymium-142 in meteorites, Harper says he and Jacobsen are the first to report on its abundance in terrestial rock.

Using a mass spectrometer, the researchers examined with unprecedented precision the amount of the decay product neodymium-142 relative to a stable form of neodymium in the rock. They then compared that value with the abundance in a standard neodymium sample believed to have come from a rock formed long after initial chemical differentiation had taken place on Earth. Combining those data with measurements of neodymium-143, a decay product of a much longer-lived form of sumarium, Jacobsen and Harper found that the ancient rock had a small excess of neodymium-142 -- 33 parts per million -- dating to within 100 million years of Earth's formation.

The researchers link the excess to early terrestial evolution because neodymium, along with some other materials, binds to only a few other elements and would have left Earth's mantle readily during melting -- the result of radioactive decay or the impact of a giant object. When a significant amount of neodymium departed, it would have left behind a higher ratio of elemental sumarium to elemental neodymium. As sumarium-146 decayed, it would have produced a relative excess of neodymium-142 in the mantle -- and a matching underabundance in the material that left the mantle, Harper says.

Although the new study pinpoints the time when parts of the Earth began to differentiate chemically, it doesn't prove that material that left a region of the mantle billions of years ago formed Earth's first continental crust. The material might have migrated elsewhere in the mantle, Harper notes. But earlier studies of neodymium in rocks from Mars and the moon support the crustal hypothesis, he says. Another possibility: Harper thinks the study may shed light on the conjecture that a Mars-size object once struck Earth, melting the mantle and blasting enough material to form the moon. The findings may indicate when this massive collision could have occurred.
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Author:Cowen, Ron
Publication:Science News
Date:Apr 4, 1992
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