Findings

Science  25 Nov 2011:
Vol. 334, Issue 6059, pp. 1039
  1. Earth's 'Time Capsule' May Be Flawed

    Zircons are some of the oldest minerals on Earth, some dating to just 150 million years or so after our world formed. Because zircons can contain tiny bits of minerals, or inclusions, geologists tend to view zircons as a kind of time capsule that may offer a peek at conditions on the early Earth, from the rise of early oceans to the movements of ancient continents. But a new study appearing next month in Geology suggests that zircon inclusions are not as pristine as scientists thought.

    Researchers led by geologist Birger, Rasmussen of Curtin University in Bentley, Australia, analyzed more than 7000 zircons from the Jack Hills of Western Australia, where rocks are between 2.65 billion and 3.05 billion years old. A total of 485 zircons held inclusions, and about a dozen or so of these contained radioactive trace elements that allowed the researchers to determine their ages. The zircons were between 3.34 billion and 4.24 billion years old—but the ages of their inclusions fell into two younger clumps: about 2.68 billion years and about 800 million years. The ages of the inclusions matched the ages of the metamorphic minerals surrounding the zircons, the researchers note: Mineral-rich fluids could have infiltrated hairline fractures in the zircons, or perhaps traveled along defects in the zircon's crystal structure caused by radioactive decay. http://scim.ag/zircons

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