Hidden worlds: the power of the humble thin section

Anyone who studies geology ends up looking down a microscope at some point in their degree. For some it's a moment of joy and wonder as they peer down at an incredible, complex, colourful world of interlocking frozen chemical reactions called minerals. Lucky them.

For the rest of us it involves moments of terror as we realise that we have no idea what mineral is what, and it all just looks like a complex jumble, and why didn't I try harder in maths, and maybe I should have changed to botany.

Er, just me?

Thin section photograph of a 205million year old granite from eastern Tibetan Plateau. Plagioclase in the centre is zoned, a common property of this mineral. Photo by A.Reid.

This week I've been spending time peering into the microscopic world again, and I'm loving it. I've been logging cores for a company and now I'm reviewing some of their thin section collection with a view to understand the minerals that make up the deposit and testing their idea on how it formed.

I'm certainly not the world's best petrologist, but over the years I have tried to keep up my mineral ID skills and challenge myself to learn more as I go. There are lots of helpful resources out there online to make this easier. For example, I love the work of Alex Strekeisen. His website and social media posts have been helpful for keeping my skills up in basic mineral ID and are a great resource for images of minerals and rocks.

There is so much value in the humble thin section. At this scale you can see the difference between a plagioclase and K-feldspar when sometimes it's impossible to see in hand specimen. You can see if those garnets grew with biotite, or after. The sulphides too have textures that reveal the timing of their interactions with silicates. Are they inclusions in quartz or do they replace or engulf it? Were they early or late? Syn-genetic or epi-genetic? A key and controversial question in most mineral deposits.

Thin sections from Anthony's PhD collection from eastern Tibet, showing structures formed during Triassic orogenesis.

Don't let those thin sections gather dust

There is an emphasis these days on the numerical aspects of geology or geophysics. Things like geochemical values, or density measurements and so on. Things that enable us to quantify rocks and minerals. This abundance of information is incredibly useful for building our understanding of the complex geological systems.

In contrast to these techniques, the humble thin section looks, well, boring. A bit archaic. Something they did in the "old days", when there were few other techniques available. Geologists have been using this technique since the 1870s, so I guess it is old technology.

But that shouldn't put us off.

These days thin sections are made by sticking a small slab of rock to a glass slide then polishing it until it is around 30 microns thick at which point it becomes almost transparent. A good polish will then enable you to use reflected light to distinguish the various opaque minerals such as most sulphides.

Plus we also have access to a range of other techniques such as the scanning electron microscope and the electron microprobe with which to delve deeper into the composition of the minerals in the thin section. I'm also really excited about using the slide scanning technology, like that available at Adelaide Microscopy, where an entire slide can be imaged in one go in plain and cross polarised light to enable the petrology to be done offline, when you don't have a microscope handy. An example is the slide image below.

So my point is don't let those thin sections gather dust. Get them out, have a good look at them. Even better if you have a petrologist's report to go with the samples, then you can be more confident in the mineral identification and have more time to concentrate on understanding the textural relationship between the minerals. This will help you understand things like the host rock types, the relative timing of sulphide introduction into a system, or alteration facies of a series of samples. All of these will mean exploration decisions can be improved to account for the actual geology that is in your samples. Knowledge is a powerful tool, and thin sections are a reliable and cost effective way to gain this power.

Transmitted light scan of an entire thin section, 76mm by 25mm, using facilities at Adelaide Microscopy. From Reid et al, 2022.