November 2014 – Mineral and groundmass separation

Sample preparation for 40Ar/39Ar dating continues in earnest! After Katy and Katie’s marathon rock crushing session in October, sample splits were taken back to UEA for the next stages of preparation for dating. We want to analyse sanidine crystals from the pumice samples, and fragments of groundmass from the lavas and these have to be separated. After crushing, the next step was to sieve the crushed samples into different size fractions to make the grains easier to separate. The sieved fractions were then washed in water in a sonic bath to clean them and dried overnight in an oven. For the next steps of the process, Katie made another trip to East Kilbride to work with the SUERC-based members of the team (Darren, Ben, Jim and Ross). Here, the samples were then run through a magnetic separator, which separates different materials based on their how magnetic they are. After several runs through the magnetic separator, set at different conditions, crystal and groundmass concentrates were separated!

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The magnetic separator at SUERC

However, this is not the end of the process! In order to get the best data, the samples need to be refined even further. Crystals from the pumice were leached in hydrofluoric acid, which attacks and removes any groundmass glass stuck onto the crystals, and the groundmass was leached in hydrochloric acid and then nitric acid. The crystal separates from the pumice were then refined even further by carrying out density separation. The crystals were dropped into a heavy liquid, lithium metatungstate (LMT), which is denser than the sanidine, allowing the sanidine crystals to float and the other crystals to sink to the bottom.

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Density separation underway at SUERC

After all this preparation, the samples are now ready to be hand-picked under the microscope, in order to choose the very best crystals and groundmass for analysis.

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