Jailbait zircons

As the company SHRIMP driver, I do a fair bit of demonstration analyses for potential customers.  One thing that has become increasingly common over the past two years is demonstration of the ability to successfully date jailbait zircons.  For those of you unfamiliar with the term, a jailbait zircon is a zircon so young that dating it while making all the usual assumptions will get you into all sorts of trouble. 

The chief problem is that for deep geologic time, we assume that the ²³⁸U to ²⁰⁶Pb decay is a simple process. In actuality, there are eight alpha decays and more beta decays than I can remember in this process, but most of the intermediate daughter products are short-lived relative to the age of the analyst, much less the Earth.

However, if you are dating a phase that is much, much younger than the Earth, then these intermediate decay products can become important.  Corrections need to be made relating to whether or not now-extinct intermediate species were incorporated into the target mineral more or less efficiently than uranium. 

For minerals which are a few hundred thousand years old, or younger, you can abandon the uranium-lead system entirely, and use uranium-thorium dating instead.  This simply looks at how close to secular equilibrium ²³⁰Th and ²³⁴U have grown after their initial incorporation into the target mineral in a unequilibrated ratio.  The linked wikipedia explanation is good (at least qualitatively).  Check it out.

Of course, even for targets old enough for uranium-lead dating, in addition to the theoretical problems above, there is the practical problem of measuring a statistically significant amount of very low levels of radiogenic lead, while somehow keeping common Pb contamination to an absurdly low level.  Because one of the nasty things about the disequilibrium species is that they disrupt many of the assumptions that are needed to accurately and precisely correct for common lead.  Which means that if you can’t keep the blank down, you’re screwed.