Dr Steve Hood explains that, by viewing a thin section of limestone rock through a petrographic microscope, detailed information about its origin and composition is revealed. Further information can be obtained by using a technique called cathodoluminescence. In seep carbonate work, it allows the calcium carbonate minerals, aragonite and calcite, to be identified by the colours that they emit.
Point of interest
Look for this micrograph
- Micrograph a: Thin section image of a cold-seep carbonate as seen under cathodoluminescent light. The blue regions indicate the presence of aragonite and the red/pink regions calcite.
DR STEVE HOOD
We initially start at the field, we describe what we can see, certainly the big fossils that are visible.
We then need to take a sample – so we can only do so much in the field – we bring it back to the lab. We then use a saw to slab it or cut it open, and from that, we get a lot more detail visible to us on a fresh unweathered surface.
We mount it on a glass slide, a microscope slide, and we grind it down so it’s very thin so light can pass through it. And we’d put that on a traditional petrographic microscope to be able to see what’s inside the seep limestone in fine detail – what are some of the cement fabrics, crystals, mineralogies, timing of different growths of cement and some of the organisms that are there.
We can establish a sequence of events. Just like a tree grows with a logical sequence of bands, often you can discern the first event and work outwards if you like, through the rock section and pick up the different events that have occurred throughout the history of the rock.
Cathodoluminescence is a specialist petrographic technique. We put our little glass slide, our thin section as we call it, in a vacuum chamber which sits on a traditional petrographic microscope, and instead of using a traditional sort of light source like a bulb and a lamp, we fire an electron beam at our sample in the vacuum chamber and different minerals respond to this energy source by emitting visible light, and some minerals emit different colours. Therefore, particularly for seep carbonate work, it’s easy to identify where aragonite is versus calcite, for example, which is very important in terms of unravelling the history of cementation of these deposits.
Associate Professor Kathleen Campbell, University of Auckland