The Partial Retention Zone

One of the most important concepts in thermochronology is the Partial Retention Zone (PRZ for short, referred to as the Partial Annealing Zone in the fission-track world). The term PRZ is used to refer to a variety of things, all related to the realities of diffusion and radiogenic decay. I’ve often included descriptions of how the PRZ forms, and what it can tell you, in introductory thermochronology seminars, but I’ve never been totally happy with the results. At a recent meeting, it hit me that I should make an animation that shows a PRZ forming. Now, my animation skills are awful, but here it goes, the PRZ.

The simplest way to interpret thermochronology dates is via the concept of the Closure Temperature, a simplified notion that there is some crucial temperature above which the daughter product (let’s say He for the sake of awesomeness) immediately diffuses out of the system when produced by radioactive decay, and below which the daughter product is entirely retained. We all know that the concept of the closure temperature is a gross oversimplification, but it is handy when discussing the relative temperature sensitivity of different thermochronometers, and very useful for introducing the fundamentals of thermochronology. The problem though is that the concept of a closure temperature (as originally defined by Martin Dodson) is for a very specific circumstance, and cooling histories in nature are often much more complex. Instead of there being a well-defined temperature where diffusion goes from super fast to nothing, there is actually a broad range of temperatures over which the diffusively of the daughter product goes from very fast to very slow. As a crystal cools, it therefore moves through a range of He diffusivities. There are temperatures, for example, where ~15% of the He produced may be retained, or if the system is slightly cooled, 35%.

So imagine a chunk of crust with temperatures starting at ~0°C at the surface, and increasing downwards (for sake of argument, at a rate of 30°C/km). In this situation all of the rocks will start out with a (U-Th)/He date of 0 Ma (no accumulated He). What would happen to samples at different depths (or temperatures) over time? Simply stated, the rocks that are very cool near the surface would accumulate as much He as was being produced, so if you measured their date at different times it would correspond exactly to how long the sample has been sitting around. Samples from very deep positions (and very high temperatures), will never accumulated any of the He they are producing, it diffuses out immediately, so they will always have a date of 0 Ma. Samples in between? Well, that is where it gets complicated.

So here is my attempt at an animation. This shows the dates you’d measure for samples at different positions in the crust over a 40 Ma timespan. Check out of technology.

output_XNc1U0After 40 million years of radioactive decay and He production, only the samples in the shallow (cold) levels of the crust would yield a date of 40 Ma. Samples kept at moderate temperatures would have dates of something in-between, notably, not necessarily related to any geologic event.

Now imagine that this whole chunk of crust is uplifted and exposed, and you have the ability to take all of these samples and measure (U-Th)/He dates. This could happen, for example along a normal fault. A superb example of this was published by Stockli et al. in 2000  using data from the White Mountains in California.

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Example of a cartoon chunk of crust exhumed along a normal fault. The dots represent samples from a variety of depths above, within, and below the partial retention (or annealing) zone. Figure from Stockli, D., Farley, K.A., and Dumitru, T., 2000, Calibration of the apatite (U-Th)/He thermochronometer on an exhumed fault block, White Mountains, California: Geology, v. 28, no. 11, p. 983.

Stockli et al. collected samples along the exposed footwall of a normal fault, calculated (U-Th)/He and apatite fission-track dates, and plotted them up against their reconstructed depth below the surface. The results will look familiar.

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Figure showing AHe or AFT date versus depth for samples from the White Mountains. Note characteristic sigmoidal pattern developed in a partial retention zone. Figure from Stockli, D., Farley, K.A., and Dumitru, T., 2000, Calibration of the apatite (U-Th)/He thermochronometer on an exhumed fault block, White Mountains, California: Geology, v. 28, no. 11, p. 983.

Of course, when you have access to a big suite of samples like this, you can do a  lot of excellent geology. This is the reason that the Holy Grail for many thermochronologists is the age-elevation, or vertical profile. When you have samples from a range of elevations you can reconstruct the whole partial retention zone. You can even reconstruct paleodepths and paleo-geothermal gradients.

Truth is, even the shape of the PRZ can tell you a lot about a geologic history, as described by Wolf et al., (1998). Because different geologic histories will create different thermal histories, sample patters can yield first order information about the overall geologic story.

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3 different thermal histories (solid line) showing the evolution of a (U-Th)/He date with time. Note that all 3 disparate thermal histories would all lead to the same He date because of changes in the retentivity of He with temperature. Figure after Wolf, R., Farley, K.A., and Kass, D., 1998, Modeling of the temperature sensitivity of the apatite (U-Th)/He thermochronometer: Chemical Geology, v. 148, no. 1-2, p. 105–114.

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Modeled shape of the PRZ for the three thermal histories described above. Note that although the shallowest samples all yield the same date, the different thermal histories all produce drastically different PRZ shapes. Figure after Wolf, R., Farley, K.A., and Kass, D., 1998, Modeling of the temperature sensitivity of the apatite (U-Th)/He thermochronometer: Chemical Geology, v. 148, no. 1-2, p. 105–114.

So if you can sample an entire PRZ, you are well on your way to figuring it all out. The trick though is what to do when you don’t have this big suite of samples.

What I mean is that we often describe the PRZ as I have above, by discussing what a whole suite of samples will look like, and how you can use the patterns to do all kinds of excellent geologizing. What the PRZ also does though, is illustrate why low temperature thermochronometer dates can be exceedingly difficult to interpret. Look at any paper that has a geologic interpretation based on an exhumed and characterized PRZ. Now imagine that all you had to work on was one sample from the middle. In the example from the White Mountains, imagine that you only had one sample from ~3 km paleodepth with a (U-Th)/He date of ~40 Ma. What would be your interpretation? Would that date actually constrain any specific event?

In thermochronology we are always looking for ways to constrain our data. Age-elevation transects are one of them, but there are many others (multiple chronometers, RDAAM, large sample arrays, etc). Understanding the PRZ, and questions where in the PRZ your particular samples come from, is a huge first step.

 

 

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This entry was posted in (U-Th)/He, dealing with data, geochronology, Primers, thermochronology, Uncategorized. Bookmark the permalink.

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