Monday, February 24, 2014

Earth's oldest crystal

Magnified image of a zircon crystal dated at 4.4 billion years,
making it the supposed oldest earth mineral ever discovered.
Credit: John Valley, University of Wisconsin.  Image source.
The old-earth timeline places the formation of the earth at about 4.55 billion years ago, but exact timing of the hardening of earth's crust has been debated.  Some modern rocks have been dated at about 4.03 billion years, but many scientists believe that such rocks were not formed during the initial cooling of the earth.  In 2001, a paper was published in Nature detailing a single zircon crystal found in  Western Australia that was dated to 4.4 billion years ago.  The date, which was obtained by uranium-lead (U-Pb) dating, was presented as evidence that the earth's crust formed within 150 million years of the formation of earth itself.  However, the finding was not universally accepted, as some scientists suspected that the U-Pb date could be thrown off by movement of lead atoms within the crystal.

Yesterday, a follow-up study on the zircon was published in Nature Geoscience.  Using a new method called atom-probe tomography, the authors of the study "confirmed" the date previously assigned to the crystal.  Atom-probe tomography uses a specialized microscope, called an atom probe, to map out and weigh individual atoms within a specimen.  This method allowed the researchers to account for every lead atom in the sample, eliminating the effect that migrating lead would have on the dating technique.  Technically, they didn't actually map out every atom, but they measured distinct lead "clusters" and used the isotope ratios to show that the age was not thrown off.  The details of the process are not revealed in the abstract, and I do not currently have access to the full article, but I will give the authors the benefit of the doubt on their conclusions.

Unfortunately, the wording of the study and of the news articles covering it gives the impression that two independent dating techniques were used on the sample, thus confirming its age of 4.4 billion years and validating the techniques.  That is not the case, however.  The only dating method used on the crystal was U-Pb dating, which has been discussed previously.  The atom-probe tomography used in the new study did not attempt to assign an age to the crystal.  Rather, it was a more accurate method of measuring the amount of lead in the sample from which an age could be calculated using standard U-Pb methods.  This was by no means an independent dating method, and it does nothing to confirm the assumptions used in radiometric dating (with the possible exception of the crystal being a "closed system").  The historically constant decay rate of radioactive elements has yet to be proven by the scientific community.

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