Picking Apatites for (U-Th)/He Thermochronology

Since its reintroduction in the mid 1990’s, apatite (U-Th)/He thermochronology has become a widely used and reliable method for understanding upper crustal thermal histories. The technique has been used on a wide range of lithologies to help understand an enormous amount of geologic processes. As such, earth scientists who primarily identify as structural geologists, tectonicists, petrologists, geomorphologists, and sedimentologists (among others) are now collaborating with He thermochronology labs to help place temporal and thermal constraints on the rocks and features they study.

In our lab, we work with a lot of people who are just learning how to do He thermochronology. Most of it isn’t that difficult, or is done by the lab managers. But often, scientists want to pick their own samples for analysis. This can save a lot of money because this is easily the most time-intensive part of the analysis, especially for apatite. You can pay me (or any other lab manager) to do the work, but then you have to pay a premium, and work within the timeframe we provide. In addition, many scientists dislike relying on others to do work for them. Sure it makes things easier, but it also becomes a little too black-boxy for many.

So how do you pick apatites? What material and equipment do you need? How do you chose good grains?

I thought I’d write up a little guide for those interested in doing the work. This is by no means exhaustive, and I highly suggest actually working with an experienced He thermochronologist to help you get started. If you plan on working with a particular lab, I also highly suggest going over procedures, some of these recommendations will vary from lab to lab. This post does not deal with mineral separation, but assumes you are starting with a relatively pure separate of apatite. If you’d like some guidance on mineral separation, please see my posts here and here.

The Equipment

Picking Microscope: All mineral picking is done under a binocular microscope, usually one that has the capacity for both transmitted and reflected light . You also want a digital camera, rotatable stage, and the ability to polarize the light (this requires a filter on the transmitted light source as well as the lens). Many scopes now have the option to have image capture and measurement software. The best scopes have  digital outputs so the software knows exactly what zoom is being used any given time. This saves a lot of hassle and time, once you calibrate the software, grain measurement (which is crucial) is simple and reliable. You also want a scope and lens setup that has sufficient working distance. You will need to have grains in a petri dish, and you will need enough room to get tweezers in for picking and manipulating grains. You need a minimum of a few inches for this type of work.
The scope we use for picking is a Leica M165 C with a Leica EC 3 camera attached to it. The scope has a zoom of 12X, and the eyepieces are 16X. We have a 1X main lens because we like the working distance, more magnification makes it difficult to work with tweezers under the scope. The base is rotatable, and has polarized transmitted light. We have a second polarizer on the lens, which allows for viewing minerals under cross-polars. This isn’t the same type of polarization you might be familiar with from thin section petrography, these are instead circular polarizers, but they work well for viewing and picking minerals. Canon and Nikon (as well as others I am sure) make similar set-ups, my suggestion is always to contact local sales people and ask them to bring out some demo models for you to test.

Tweezers: These are my picking tweezers. There are many like it, but these are mine. Tweezers are the primary tool you use to select and prepare grains for analysis. Depending on the method you use there are a variety of tweezer styles that you may need. The workhorse of your tweezer collection will be ultra-fine, stainless steel picking tweezers. I like the EMS Style 5 tweezers myself. I don’t bother with the ultra-fine, I have found that it the normal style 5 are too ham-fisted to pick up a grain, then the grain is too small. These tweezers are also delicate, so anything finer just seems to be inviting danger. The tips of these bad boys are 80 microns or so.

Petri Dishes: We do all of our picking and packing under alcohol. This isn’t required, but many people prefer it. Grains are easier to see and manipulate in ethanol, and if a grain slips or flies out of your tweezers, it won’t go very far. This means that you need petri dishes to hold both your grains and your alcohol. I prefer smaller, taller glass petri dishes, 60 mm in diamter and 15 mm tall, with lids. Fisher Scientific sells them (here). I stress the need to use glass dishes. Plastic dishes are cheap, but you are using metal tweezers that scrape up the plastic, and you are looking under cross polarized light, which makes the plastic all kinds of rainbow madness colors. The glass dishes can be washed and re-used forever. They are too pricey to store grains, but fantastic for working on them. We even pack the grains in these dishes (under alcohol).

Metal Tubes: Once you have selected the grain you want to analyze, you will need to pack it into a metal tube for analysis. We use these for a variety of reasons. I won’t link to a place to buy these, because even though most everybody used the same Pt or Nb tubes, they often need to be cleaned, and you should check with the lab you want to do the analysis before buying or using any tubes.

Measurement Skills: This will likely come with your microscope, but it is required that you be able to accurately measure your grains. If your scope doesn’t have something built in, you’ll need a digital camera and a micrometer.

The Method – Picking the Right Grain

Typical apatites, if you can’t find grains like this it is all your fault

The ideal apatite grain is large, euhedral, and inclusion-free. Like many ideals, these grains are often hard to come by. Some lithologies have better apatites, and there are instances where picking grains is simple. However, if you are trying to understand the thermal history of some region, you often can’t decide to only sample Cenozoic granites.

To start, some rocks don’t yield apatites that are good enough to reliably analyze. That is just the reality of He thermochronology. It is pointless to measure crappy grains, really just a waste of money. Some people are more flexible with the minimum quality of the grains that they will analyze, that can depend on a variety of factors. What I am describing is how we go about picking grains in our lab.

1. Is it apatite? This sounds silly, but even good mineral separates can often have things that are not apatite. It is important to learn how to identify apatite, and to not pick anything that is ambiguous. So how do you identify apatite? Well, apatite looks like apatite. That is annoying and circular, but start with easy to pick samples. In our lab we have clean separates with great apatites that we use to help train people. Here are some guidelines though. Apatite has medium relief,  is typically clear, and has blunt ends.  The grains are hexagonal in cross section, and form little barrels. Zircons have much higher relief, pointier ends, a slight color, and a rectangular cross section. Sphene is wedge-shaped and honey-brown. Your separate will also likely be full of nondescript shards of mystery minerals. My rule is that if it isn’t identifiable apatite, I don’t pick it.

Nice little barrel shaped apatite crystal, about 150 microns long

Cross sectional view of a nice hexagonal apatite

2. Is it large enough? When He is produced, the kinetic energy of the decay shoots the He atoms through the crystal. The atoms travel 15-20 microns or so, meaning that some of them are expelled from the grain. We correct for this using a geometric method, which requires knowledge of the crystal size. As a rule, we prefer that this isn’t too large of a correction. The size of the correction is controlled by the surface area to volume ratio of the grain, basically the larger the grain, the smaller the percentage of the He is expelled, so the smaller the correction. As a default, we have a maximum alpha-ejection correction of 0.65. We’ll push it down to 0.60 in a pinch, but prefer to not go any lower than that (in alpha-ejection terms, the smaller the factor, the larger the correction – a factor of 1 would be no correction at all). The smallest dimension of the crystal is the main control on this factor, meaning that we aim for average minimum widths of around 70 microns or so. You can often find pretty little apatites, even in crappy separates, that are just too small.

3. Is it clear? Inclusions are the bane of the apatite He thermochronologists. Not all inclusions are a problem, but since we have no way of determining what they are, we have to just avoid them all. Finding grains without inclusions is what takes up most of your time in sample preparation. The problem with inclusions is that they can contain U and/or Th. The He is extracted during heating of the sample, but when we dissolve the grains to measure the U and Th, the weak nitric acid we use to dissolve the apatites isn’t strong enough to dissolve inclusions like zircon and monazite. We therefore have an overabundance of daughters, and a too-old age. Anyways, avoid inclusions, that is the general rule.

Some inclusions are easy to see, you can even see the crystal faces of the included phase. Most inclusions, though, are much harder to see. They can appear as small black dots, sometimes solitary, sometimes concentrated in a zone or a planar field. They might just look like specks of dust, but by rotating the grains you can tell if they are actually inside the crystal. You can also use the cross-polarizing filters to identify inclusions. If you make the apatite extinct, the inclusions will often stick out like bright pin-pricks of light. This will help identify inclusions that are otherwise invisible. Here are some pictures of grains with inclusions, although to be honest the pictures rarely do justice to their tininess and difficulty to see.

Apatite crystal with tiny, dark inclusions

Apatite crystal with small mineral inclusions.

4. Is it euhedral? The alpha-ejection correction I mentioned above is geometric, meaning that it is based on an idealized apatite crystal shape. Rounded or fractured grains that deviate from that idealized geometry introduce uncertainty into the calculation. Broken grains are a problem, but many people will still run grains that are rounded or broken.

The Method – Packing grains

To pack grains into the tubes you need some more burly tweezers. The Style 7, for example, or any of the fatter ones. I prefer to use one style 7 curved tweezer and one fat tipped flat pair. The curved tweezers are great because they are strong enough to crimp the tubes, but you can then rest the tweezers on the bottom of the petri dish while you do the rest. The basic method is to crimp the bottom of the tube, dump in the grain, and crimp the top. This is tough to explain in a post, but here at least are some pictures.

Tube and apatite ready for packing

Tube with the bottom crimped, top open, ready to have an apatite dropped in

Tube with an open top, sitting comfortable in a pair of Style 7 curved tweezers

Complete! A little Pt pillow with a grain inside

View into an open packet with a grain sitting in it.