"According to these results, the gold in your jewelry can be viewed as a relative latecomer to our planet," geochemist Richard J. Walker of the University of Maryland wrote in the July 27 issue of the journal Nature.
Walkers commentary -- titled "The extraterrestrial wedding ring" accompanied the new study, which was conducted by Astrid Holzheid of the University of Koln, Germany, along with colleagues in Germany, Canada and Australia.
Walker said the bombardment probably occurred within 50 million to 100 million years after Earth first formed or accreted from smaller pieces of rock and dust some 4.6 billion years ago. The meteorites likely were dozens of miles (kilometers) wide much larger than asteroids and comets blamed for some later mass extinctions and they dumped enough material on Earth to equal the
Holzheid and colleagues used furnaces with piston or anvil devices to apply tremendous pressures and heat to capsules containing platinum or palladium alloys. The conditions were meant to simulate depths of 300 miles (500 kilometers) beneath Earths surface, where pressures in a magma ocean were up to 160,000 times atmospheric pressure and temperatures reached 2,000 to 2,900 degrees Fahrenheit (1,100 to 1,600 degrees Celsius).
The experiments showed that even under such conditions, palladium and platinum accumulate in iron-rich metallic magma, or molten rock, rather than in silica-rich magma. That indicates that the iron-loving elements concentrated at the bottom of a subterranean magma ocean and from there migrated into the core as the core separated from the overlying mantle, Walker said.
Such a separation process should have left the mantle poor in the iron-loving or "siderophile" elements: gold, platinum, palladium, iridium, osmium, rhenium, rhodium and ruthenium.
In fact, mantle rocks erupted by volcanoes show such elements are present in the mantle in concentrations orders of magnitude greater than expected. So the findings are consistent with a meteorite bombardment that replenished gold, platinum and related elements in Earths mantle after the original supply ended up in the core, Walker said.
Primitive meteorites
Indeed, relative amounts of gold and other iron-loving elements in the mantle are comparable to their relative abundances in primitive meteorites left over from the solar systems formation, he added.
However, Walker and Holzheid conceded both processes might have occurred: meteorite impacts may have brought iron-loving elements to Earths mantle, while other metals with a lower affinity for iron-rich magma were more likely to remain in the mantle when it became distinct from the core.
Walker said impacting meteorites hit early Earths crust, which eventually was recycled into the mantle; when mantle rock later was made into crust, gold and other iron-loving metals "came along for the ride" to form ore deposits.
The new study "tells us that the accretion of the Earth was a fairly drawn-out process," Walker said. "People like planets we live on one and it is interesting to learn how they formed, how long they took to form, why they formed and why the different planets of the
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