Thermochronologic Sampling

I’ve posted about thermochronology and mineral separations (twice), but I realized that I haven’t yet gone over the field work aspect of thermochronology.

Now, I’ll admit up front that the field work I do is not very hard core. There are plenty of thermochronologists who spend months in the classic style, mapping and sampling in some remote part of the planet, I am really not one of them. The most rugged field work I’ve done was when I was in college, and while the helicopter rides and grizzly bears gave me some geo-street cred, my field work since then has been rather tame. The truth is, as a thermochronologist, you rely on good maps. Your data is meaningless outside of geologic or structural context. Now, it is pretty commong for people doing thermochronology to provide the necessary structural and geologic underpinnings. For much of my work, I’ve been fortunate enough to work in places that have been mapped very well, and I’m often in field parties with the people who’ve done the mapping.

Anyways, what I am going to be blogging about assumes you either have or are making the maps necessary for actually understanding your data.

OK, first things first, you can take a thermochronologic sample anywhere there are rocks, but when you plan your sampling, there are some important things to consider.

First, although you can get important information from single samples, the most powerful sampling method involves collecting multiple samples from different structural positions. If cooling and exhumation have been vertical (as is the case with my current project, see this for a more in depth discussion), then this means collecting samples at different elevations. For reasons I’ll explain in a later post, it helps to cover as much structural relief as possible without moving too much laterally. The ideal sampling surface in a region with vertical exhumation would be a sheer cliff. In regions like the basin and range, where faults and structures have rotated lithologic units, structural relief might be almost horizontal (like Yerington). Again, this is why you need to already understand your field area.

So lets just say that exhumation has been vertical. Your sampling transect will try to cover the greatest amount of relief possible.

In almost every case, the best lithologies to sample for thermochronology are granites or granodiorites. So to summarize, you are looking for a big granite mountain.

Unless you are attempting some rather specialized analyses, you also want fresh and underformed rocks to sample. Obviously then, avoid altered rocks, weathering varnishes, and rocks that have been faulted or otherwise chewed on. You will also want to avoid the outer few cm’s of outcrop because of potential problems caused by forest fires (see Mitchell and Reiners, 2003, Geology v.31, n.12, pp.1025-1028; summary if you don’t have access, fires can reset apatites, so don’t sample the outer few cm’s. They do very interesting things with the data, but I won’t go into that right now).

OK, so find your enormous granite mountain made entirely of fresh outcrop (this sounds pipe dreamy, but actually describes some of my current field areas).

So you want to take a whole slew of samples from different elevations. It is pretty standard to sample every 100 meters or so of relief (structural relief or elevation). So, first step, find a place at the right elevation that has fresh outcrop. As below

If the rocks aren’t that well exposed or fresh, it helps to spend some quality time with your chisel. I love the ones with the hand guards. If you are sampling granitoids, bring a sledge hammer, not a pick hammer. Sledge hammers (3 or 4 lb head, metal or wood handle) make the job much easier.check out Dr. New Helvetia chiseling away at an oriented sample

If you are interested in mica, hornblende, or K-Feldspar, you usually don’t need much sample at all. You can often see the minerals you want to analyze with your hand lens, and can sometimes “field dress” your sample, and take out what you want with your knife (works well especially for books of mica). For apatite, zircon, and monazite, you have a lot more work ahead of you. since the first step in that mineral separation might mean feeding your sample through a jaw crusher, I find it useful to break your sample into bite sized chunks on the outcrop. If you wait to bash up a large sample into smaller pieces back in the lab, you have to spend all kinds of time cleaning up. You don’t have to clean up or worry about contamination if you bash things up on the outcrop. Bashing things up also lets you remove the weathered rinds you want to avoid.

Many rocks seem to be either feast or famine when it comes to apatite and zircon abundance. Some rocks I’ve worked on yield piles and piles of apatite, where I could have gotten away with much less sample. Others yield nothing, and it wouldn’t have mattered if I would have brought home a barrel’s worth. I usually go for about 3-4 kgs of sample.
So you’ve now got your mandarin orange sized chunks of amazingly fresh granite, time to bag them. I prefer heavy cloth bags, like those available here at Miners Inc. I don’t reuse them, so I label the bajeesus out of the bags with a Sharpie.

I write the sample name, elevation, and some basic info about the geologic context of the sample. I also like to take a picture of the sample bag (with the sample name readable) on the outcrop, kind of a way to help me remember the sample and where I took it. I also take a GPS reading, and plot myself manually on a topographic map. I’ll actually check my map location against the GPS whenever possible.

This is also a good way to trick otherwise camera shy members of a field party to have their picture taken. Again, Dr. New Helvetia.
Taking pictures like this also helps me keep my photos organized. For example, I’ll have pictures like this below, that I don’t write a ton about in my field book (because I am an idiot who is still acquiring skills), but I have it sandwiched in between pictures I have GPS coordinates for, so I’m all good. I was able to recreate the outcrop where I saw these enormous K-feldspars, even though it was towards the end of the day and my field notes weren’t as complete as they should have been.

The K-feldspars remind me of salmon swinning upstream.

Now, I usually have a back-and-forth sampling route. Meaning, I’ll hike up to the highest point in my field area, take a sample, and then head down, sampling as I go. This saves you from having to carry rocks up hill, which is usually a good thing. I also find it easy to plan time this way. I plan on eating lunch at the highest point, and I know that I can cover ~1300 m of elevation in a day (13 samples) with a good field party (at least 1 other person). You can also note good outcrops on your way up, making the way down more efficient.