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Confirmed: Earths oldest rock

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  • It's a pretty damn amazing discovery, really interesting results to come out. I read in Bowring's synopsis (attached) of the Valley paper (attached) that geologists have looked at the sandstones of the Jack Hills in Western Australia which contains these old grains in abundance. They've dated more than 100,000 grains and come to ages that are older than 4.0 billion years old. The provenance of the grains would suggest that they're derived from the continental crust which could in itself be more than 4.37 billion years old. The problem with dating (and what makes the Valley paper so important) is that when you're looking at zircons (really tough, really resistant to modification) is that when you have the decay of uranium isotopes which form lead (daughter isotopes) within the zircon crystal, they emit alpha particles during this process. This then results in some of the radiogenic isotopes moving around the grain and diffusing along the small fissures in the crystal itself. As you're only looking at part of the grain itself, these diffusion processes may lead to poor calculation of the actual isotopic composition (can't measure something without knowing where it all is). The problem then propogates when you do determine the crystal age and it's at something like 4.4 or 4.3 billion years old as then the material reflects the age of the Earth with a difference of between 4.4 and 4.5 billion years which can be a difference of something like 0.1 and 0.2 billion years.

    So, what Valley et al. (2014) did was to use atom-probe tomography which can then map the distribution of lead isotopes within the grain, in this case the oldest known grain on the planet with nano-resolution. What was found was that the radiogenic lead atoms are distributed in regularly spaced clusters that accumulate in isolated damaged zones in the crystal, in something like a few nanometres in diameter. With this, the redistribution of the lead isotopes is only over a small area and clustered, which now means that the age can be determined with greater accuracy and precision. Thus, the age quoted by Valley et al. (2014) for this particular zircon from the Jack Hills in Western Australia is at 4.374 ± 0.006 billion years.

    Not bad!:D

    I've attached the original paper and a summary paper that I read in order to write this post.


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