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Earthly riches may be heaven-sent
Meteorites possibly peppered planet with precious metals
By Devin Powell
A meteorite maelstrom that pummeledEarth and pocked the moon with cratersbillions of years ago may have had a silver lining — and linings made of othermetals, too.
New evidence from old rocks suggeststhat many of the precious metals minedtoday were delivered to the planet bystony meteorites called chondrites in abombardment that lasted for hundredsof millions of years.
“Adding a tiny amount of chondriticmaterial could explain where many present-day metals came from,” says MatthiasWillbold, a geochemist at the Universityof Bristol in England, whose team reportsthe findings in the Sept. 8 Nature.
Within 50 million years of Earth’s formation, much of its iron sank to the middle to form the core. Gold, platinum andsome other precious metals followed.
Searching for a source that could havelater replenished these metals, Willboldand colleagues looked for different forms,known as isotopes, of the metal tungstenin rocks from the 3.8-billion-year-oldIsua greenstone belt in Greenland. Thebelt probably formed before the massiveinflux of meteorites that geologists callthe late heavy bombardment.
Compared with the Greenland specimens, newer rocks contain lower ratiosof light tungsten to standard tungsten,the team found. Chondritic meteoritesdon’t have much of the lighter tung-sten-182 compared with Earth’s mantle,
Gold priceper ounce onSept. 6, 2001
Gold priceper ounce onSept. 6, 2011
272.05
dollars
so meteorites mixed into the mantle overgeologic timescales could have dilutedEarth’s younger rocks. The amount ofincoming material needed to explainthe tungsten discrepancy would bringenough metals to the planet to accountfor the quantities of those elementsobserved today, says Willbold.Meteorites that pummeled Earthbillions of years ago may have deposited the precious metals mined today.
