What are some of the limits of radiometric dating techniques?

Mar 1, 2016

There are many.

Explanation:

This question requires a very extensive answer to be able to cover all bases here but I'm going to attempt to explain the salient facts. Jump down to summary if you just want to know what both categories of limitations are.

The limitations of radiometric dating can be split into two general categories, analytical limitations and natural limitations.

Analytical limitations encompass the limitations of the machinery that is being used to date a material. For example, you may want to date a zircon $\left(Z r S i {O}_{4}\right)$ crystal using a secondary ion microprobe (SIMS). This technique bombards the sample, slowly drawing material out and then sending it through to an ion counter. This is then transformed into isotopic ratios and then used to date the material. The machinery you use has to be tuned and calibrated to which isotopes you want to measure and needs to be set with the correct running conditions. Think of it as making a roast dinner, you're going to need to set the oven at the correct temperature and leave it for the right amount of time to achieve the best results.

So you can never have perfect running conditions and certain parameters will change over time, this is just the nature of high-tech machinery. A small shift in a parameter can affect your final outcome. So some analytical limitations can be the beam intensity, counting statistics, dead-time and so on. These are parameters you can control and will affect how accurate and precise your age-dating is. (Don't worry what those parameters mean, just understand they are machine-based).

Natural limitations encompass those as a result of nature. For example, you may want to date the same zircon crystals using the U-Pb method. In order to do this, you need to measure various isotopes of uranium $\left(U\right)$ and lead $\left(P b\right)$. Though, when you come to do this measurement you find that uranium concentrations are very low in your sample (on the order of a few parts per million). This low concentration will mean your counting statistics will not be as robust and may result in decreased precision. Another limitation is the length of time a decay series can be used for.

Another example, you may want to use ${.}^{14} C$ (carbon-14) to date an old object. Lets say the object is a million years old (but as the scientist measuring this object we don't know that) and we go to measure it using the 14-C method. The age we come up with is around 50 000 years old. The reason it isn't 1 million year old is because the half-life of 14-C is about 5 730 years, which means after about 50 000 years there is no more 14-C to measure, hence the limit of that dating technique is about 50 000 years. All different decay series' have upper and lower limits for which they work effectively. So the million year old object was incorrectly dated using a decay series not suited to it.

Summary:

1. Analytical limit
One that you can control to some extent and will affect the precision and accuracy of the dating.
2. Natural limit
One that is not under your control and you must perform analyses accordingly and use the right decay series.