Hafnium tungsten dating
Geologists can learn a lot from rocks with an unusually high or low amount of tungsten-182—which indicates how much hafnium-182 was present in the rock long ago.
"Nearly all of these anomalies formed within the first 50 million years after the solar system formed," Mundl said.
The rocks contain surprising geochemical anomalies—the "fingerprints" of conditions that existed shortly after the planet formed.The addition of volatile-rich material could have occurred in a single event, perhaps the giant collision between the proto-Earth and a Mars-sized object thought to have ejected enough material into Earth orbit to form the Moon.The results of the study support a 30-year old model of planetary growth called “heterogeneous accretion,” which proposes that the Earth’s building blocks changed in composition as the planet accreted.The researchers are not yet sure how Earth's mantle preserved these anomalies.But the group's results suggest that some of these rocks contain material that survived through all of Earth's history—and that the planet's interior may not be well mixed after all.
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“But these meteorites have compositions that are very volatile-rich, unlike the Earth, which is volatile-depleted.” The silver isotopes also presented another riddle, suggesting that the Earth’s core formed about 5-10 million years after the origin of the Solar System, much earlier than the date from the hafnium-tungsten results.