Dating Stumbles

So Paul Braterman was asking the other day about some advance knowledge of how the new infatuation with detrital zircons and HeFTy plots might be misread in some circles. This is really a question for a real card-carrying geochronologist, but GG will take a swing at it because he’s kind of worried a bit about this and is not staking out any particular terrain.

For the most part both of these are simple variants on classical age-dating techniques (uranium-lead dating and potassium-argon dating), and though rooted in geochronology, most of the applications are elsewhere (e.g., detrital zircons are mainly used as a means of identifying the sources (provenance) of clastic sedimentary rocks, while HeFTy (Helium-Fission Track analysis) is dominantly a means of assessing the thermal or unroofing history of some body of rock through application of multiple geochronometers). But they start pointing at things that could be misleading, so let’s look a bit more….

Detrital zircons are arguably the simplest and most prone to produce trouble in coming years. The application most likely to cause issues is that the youngest of the detrital zircons in a sedimentary rock is telling you the maximum age of that rock. But problems can arise.  For one, this work is at present mostly done with a laser-ablation inductively coupled plasma mass spectrometry, or LA-ICP MS, machine. These can chug through lots of samples fairly quickly, but their intrinsic precision is well below that of more traditional  thermal ionization mass spectrometry (TIMS) more commonly used for precision dating. Basically, LA-ICP MS can deal with small amounts of materials necessary to get an age date, but the small amount limits the precision of the date that emerges.

One of the things that has emerged with tools capable of looking at small parts of individual grains is that you can measure now overgrowth rims and such not.  If you fail to identify the presence of an overgrowth on a detrital zircon, you could end up with a metamorphic date instead of a primary igneous date from a zircon–and so in metamorphic environments, care must be used.  In the old days, folks dating rocks with zircons would divide the zircons into separates by characteristics like having euhedral crystal edges or different signs of abrasion–and they often would abrade the crystals some to remove any really late stage material (and also to guard against any chemical exchanges with surrounding minerals). While it is common still to note the different characteristics of crystals, such explicit division is not required and so in reporting everything found, the occasional unusual crystal might come through with an odd date.  At this point, most workers are kind of suspicious of a single zircon date that is unusual, recognizing that it could just be an error that was not caught, but outsiders might happily misinterpret such a find.

Another aspect of such sensitive equipment is that contamination is a more severe problem in the past.  If you were melting down dozens of zircon crystals to get one date, one oddball crystal could only bias the result a little.  But now, it might appear all on its own as a special date. If this is a crystal that got caught in a sample bag that was reused, or if there was bioturbation of sediment that brought younger zircons down into a particular horizon, or if a sample was being acquired on a slope where younger material might be getting mixed in with the stuff being sampled, you could get some numbers that had nothing to do with the rock you thought you were sampling.

It is almost inevitable that some blunders like these will make it into the literature, which  is getting buried under (literally) millions of such analyses. So occasionally we will see some numbers that seem to violate the age of the rock in question.

How about HeFTy?  This program in and of itself doesn’t pose much of a risk, but the issues in geochronology that underpin it might. This is mostly the domain of the low-temperature end of the world, and so the issues here come from attempts to push to the very edges of what minerals might record.

The scariest issue has to do with fission track dating.  The basic idea, if you don’t know, is that fissionable elements (mainly uranium and thorium) will throw off debris that can damage the surrounding crystal structure, leaving behind (ahem) a fission track. These tracks will heal at high temperature but stay present at low temperatures. If you know the concentration of uranium (and thorium, if relevant) and know the decay constants, you can compare how much uranium remains and how many fission tracks you can find in a given chunk of crystal to figure out how long the crystal was cold enough to preserve the tracks.  This part of fission track dating is fairly robust, but etching techniques that have varied over time have led to dating errors even here.

The main problem arises with the use of the length of the fission tracks.  There are issues with measuring track length to start with, and then there is the question of annealing over geologic timescales.  The idea is that when you are near the closure temperature for fission tracks (the temperature where they rapidly heal and vanish), the rate of annealing slows and your end up with a lot of short fission tracks. By itself, this doesn’t give you an age, but what it does do is provide some information about how long the sample sat near that closure temperature in what is termed the partial annealing zone. This is where codes like HeFTy come into play: they incorporate estimates of how tracks will decay with time at different temperatures, and so as you try different temperature-time paths, you get some level of misfit to the observations. But there is a suspicion that the annealing timescales in use are only weakly supported by empirical evidence and more difficult to derive in some analytical way. Depending on the situation, you could get a result indicating a sample was buried at a moment when from other considerations you know it was exposed.  [As it so happens, though, most practitioners will put such a geologic constraint into HeFTy from the start, so you are unlikely to see such a result emerge].

The other main player is a newer entry, the use of U-Th/He dating (counting helium atoms within a crystal and comparing to the uranium and thorium concentrations to get the time since helium readily diffused out of the crystal–ironically the first attempt to date a rock used this but failed to account for the diffusion problem). Numerous complications are present here ranging from early work that overlooked the contribution from thorium contamination to the variation in diffusion constants with crystal damage. Newer approaches irradiate a sample to make He-3 and then gradually extract the helium through heating, using the resulting information to get at even lower temperature parts of the mineral’s history. These temperatures are low enough that workers have worried, for instance, about getting dates reset by forest fires. Another tack being explored is using the low-temperature behavior of damaged crystals (using the uranium concentration as a proxy) to look at the low-temperature end of the technique.

As a still growing technique, it seems likely there are still a few unrecognized caveats to the use of U-Th/He, and so seeing some of these inconsistencies arise shouldn’t be a surprise. For instance, a large part of the controversy about the age of the western Grand Canyon centers on what constitutes a good and robust age measurement; that there are nearby measurements with very different values points to some of the issues in the field.

Where all this stuff comes together is in a code like HeFTy, where a collection of estimates of how track lengths anneal and how helium diffuses with uranium content and other sorts of details of the physics underlying these techniques is embedded. We’ve discussed how some of those details might be troubling, but the greater risk of misinterpretation probably comes from our limited understanding of the shallow thermal structure of the earth. These data are often interpreted with the assumption of a constant geotherm, or a fixed thermal conductivity or other simplifying assumptions that might or might not apply. Strong lateral variations in shallow thermal structure abound in the heat flow literature, and even simpler and more subtle effects were well known in that community for a long time, but that knowledge is for the most part either absent or unapplied in the low-temperature geochronological community. It is likely that some of the interpretations resting on this low-temperature data and incorporating such simplifying assumptions will produce an earth history inconsistent with other geologic constraints (indeed, some of the problems discussed above were first recognized because of such problems). HeFTy is, by all accounts, a very easy to use and flexible tool, and so its widespread acceptance should not surprise.  But it is not open-source and much of how the code works is hidden from most users, which in the long term is a recipe for misapplication and the misinterpretation of data.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

%d bloggers like this: