New group paper: QEMSCAN as a method of semi-automated crystal size distribution analysis: Insights from Apollo 15 mare basalts

The journey a magma takes through the crust and during eruptive processes has been well studied on Earth in places such as in Iceland and Hawaii. However, magmatism is a fundamental planetary-scale process that occurs on many rocky bodies throughout the Solar System, including the Moon. The Apollo 15 mission returned many mare basalt samples which have been studied to try and understand the magmatic history at this landing site.

Understanding complex magmatic histories is difficult enough for terrestrial samples, but understanding these processes on other planetary bodies in the Solar System has additional challenges. Extra-terrestrial rock samples, including the Apollo 15 mare basalts, are often small and need to be preserved for future generations. Therefore, analysis of these samples may be limited to non-destructive techniques (i.e. those which cause no damage to the sample).

Crystal size distribution analysis is a non-destructive method of investigating the crystallisation histories of magmas. Collecting data for crystal size distribution analysis often requires the manual tracing of crystal boundaries from a digital image. This method requires very little equipment, however, the process is very time-intensive.

Originally developed for the mining industry, Quantitative Evaluation of Minerals by SCANing electron microscopy (QEMSCAN) is a system that works in conjunction with a scanning electron microscope, and is capable of producing mineral maps of a rock sample. The QEMSCAN software also contains a variety of features which allows you to view the occurrence of different minerals and separate touching crystals of the same mineral. As such, QEMSCAN showed potential to be a faster alternative to crystal size distribution analysis compared to manually tracing around hundreds if not thousands of crystals. We investigated if QEMSCAN could be used as a semi-automated alternative to manual crystal tracing.

In our new paper, we present crystal size distribution analysis of pyroxene, olivine and plagioclase crystals in four Apollo 15 mare basalt samples. Each sample was analysed using both the manual crystal tracing method and the QEMSCAN method of data collection. This allowed us to compare the two datasets to see if QEMSCAN was able to produce reliable crystal size distribution plots.

Picture1

Backscattered electron maps of the samples in this study (a-d) used for manual tracing of crystals, shown alongside QEMSCAN mineral maps of the same samples (e-h). Whereas minerals appear in various shades of grey in a backscattered electron map, QEMSCAN is able to identify and colour code each mineral using a predefined mineral database. Image from: https://doi.org/10.1093/petrology/egaa047.

In most cases, there was an offset between crystal size distribution plots produced by QEMSCAN compared to the manually collected data. Samples with the largest differences in results, were those which had elongate crystals that overlapped with other crystals in multiple places. Such textures make it harder for the QEMSCAN software to correctly separate touching particles. We found that elongate crystals were being incorrectly segmented into multiple shorter crystals which had a detrimental effect on the crystal size distribution plots produced. Despite these limitations, QEMSCAN was able to reproduce broadly similar crystal size distribution trends compared to the manual data, and was up to 13 times faster than manual crystal tracing.  The suitability of using QEMSCAN for crystal size distribution analysis on future samples must be considered on a sample-by-sample basis, as the reliability of the outcomes are dependent on sample texture.

Picture2

An example of a group of pyroxene crystals before (a) and after (b) processing using the QEMSCAN software. Lines are used for reference to match the same points on the large central crystal in (a) and (b). The elongate crystal in (a) is clearly segmented into multiple crystals (b) by QEMSCAN. For reference, (c) shows a manual segmentation of the same group of crystals. Image from: https://doi.org/10.1093/petrology/egaa047.

The full citation for the new paper can be found below, along with a link to the Journal of Petrology webpage:

S K Bell, K H Joy, J F Pernet-Fisher, M E Hartley, QEMSCAN as a method of semi-automated crystal size distribution analysis: Insights from Apollo 15 mare basalts, Journal of Petrology, https://doi.org/10.1093/petrology/egaa047

About Sam Bell

I am a PhD student at the University of Manchester studying lunar basalts. I use the chemistry and textures of crystals to better understand processes which occurred as magma made its way through the lunar crust and erupted onto the surface of the Moon.
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