How does helium diffusion effect the radiometric dating of a zircon crystal?

Answer 1

Generally speaking, helium diffusion doesn't really affect the radiometric dating of a zircon crystal. The reason that helium diffusion isn't a factor is that helium diffusion isn't a reliable method of dating the crystal, and it usually isn't used. Other radiometric means are applied to this purpose, and uranium-lead is probably the most common. This dating method (U-Pb) has been around for a long time, and it is highly refined. Let's dig around a bit and see what's up.

It was just after the turn of the 19th century that investigators considering the nature of radioactive materials thought of the method of radiometric dating. (Yes, that's right. Radiometric dating has been around for a hundred years or so.) The uranium-lead dating method has been a favorite because the uranium and lead (as well as the products of their decay) aren't really able to "escape" the confines of the zircon crystal matrix. Helium, which is formed during the alpha decay of uranium and its daughter products as they transform into lead, is also produced. But helium diffusion can generally be ignored when doing U-Pb dating. It's a good thing, too, and here's why.

Helium is an inert gas and is not "bound" into the crystal structure of the zircon crystal. Further, it has a pronounced ability to diffuse, and it can move through and actually escape the zircon. Lots of variables contribute to the diffusion of helium through and out of a zircon, and thermal conditions will dramatically affect helium diffusion. As a zircon crystal lies entombed in rock that is heating and cooling in response to geologic processes, helium will diffuse (increasingly and decreasingly) in response. But the uranium and its daughters produced in radioactive decay can't really do that. They're stuck. Most investigators will ignore helium when doing radiometric dating because there are too many things that can affect its concentration. Further, these things (save the very highest thermal conditions) don't significantly affect the elements in the U-Pb decay chain.

Variability in the helium concentration in zircons is subject to many difficult-to-model variables to make it a top notch radiometric dating tool. On the other hand, U-Pb dating works extremely well for most applications where the age of a zircon needs to be determined. We should also note that the decay chains of U-238 and U-235 are different, and each can be considered separately when running a mass spectrographic analysis of a zircon. That gives us a two-in-one way to calculate the age of a sample by looking at the U-238 and U-235 decay chains independently. We can compare the results of U-238 - Pb-206 and U-235 - Pb-207 side by side and state with good accuracy the age of zircons millions and even billions of years old. Variables that affect helium diffusion (and therefore helium concentrations) can generally be ignored with U-Pb dating.

There is some talk recently about the use of helium concentration in zircons to determine age, but the diffusion of helium within zircon is not that well understood. Additionally, it is difficult to quantitatively assess the variables to obtain an accurate age of the crystal. It is far easier (owing to the "confinement" of the uranium and its daughters as well as the lead) to use U-Pb dating techniques. The recent to-do over helium dating is primarily being supported by those who wish to overturn the estimates of the age of earth that have been broadly accepted by almost all scientists. Very few investigators doubt the veracity of the earth's age (set at about four and a half billion years) which has been estimated using radiometric decay in gelogic samples. Those few who believe in a young earth have grabbed onto the terrible science (it's worse than just bad) offered by a few investigators (who should know better) that casts doubt on the general view of the age of the earth. Helium diffusion doesn't in zircon generally doesn't affect the calculation of the age of a sample using standard radiometric techniques. Links have been provided below to related questions and other articles directly relative to this issue.