Abstract
Well-crystallized groundmass fractions are commonly used to obtain reliable K-Ar ages for volcanic rocks, as phenocrysts potentially contain excess 40Ar. In general, phenocrysts are removed by magnetic separator, heavy liquid separation, and hand picking. Heavy liquid separation utilizes sodium polytungstate (SPT) solution, which replaced toxic heavy liquids such as bromoform in the 1980s. Recently, a vacuum pump has been used to reduce the filtration time for heavy liquid separation using SPT solution, which has a high viscosity. This note describes the separation procedure for gentle suction filtration using a hand vacuum pump, which can be safely completed in 50-60 minutes per sample. The procedure was tested on andesite and basalt samples from Hakusan, Miyakejima, and Etna Volcanos. Separation of the groundmass fraction from the phenocryst fraction was successful for all andesite samples, whereas separation was incomplete in some of the basalt samples, probably because the gravity of groundmass grains was heavier than the maximum gravity of the SPT solution (3.11 g/cm3).
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