Lavas

In addition to the geothermal fluids, the rock samples can also be analysed for the volatile contents and their isotopic compositions. However, the volatiles of interest are contained in phenocrysts — crystals such as olivine and pyroxene which precipitate out of the magma — and are caught by the lavas. Therefore, the first task is to crush the rock — down to the grain size of the phenocrysts (using 1-5 mm) so that we can separate out the crystals. We can do this either magnetically or through the use of heavy liquids or even by hand-picking under a microscope. In this way, we select only the most pristine crystals for analysis.

Click to enlarge

Once we have the phenocrysts we need to release the volatiles trapped in their matrices and get the gas into the mass spectrometers. This can be done in one of two ways. Crushing or pulverizing the phenocrysts under vacxuum is one approach — in this way, we attach a crusher device to out inlet line, and use a solenoid magnet to operate a slug inside the crusher to hammer the crystals into tiny pieces. This releases the trapped volatiles. Alternatively, we have bought a laser system whereby we can melt samples or drill holes into them to search for volatile-rich inclusions (small bubbles). Our laser system represents an exciting new approach to the processing of samples for their volatile contents and the Costa Rica samples will be amongst the first to be processed in this way.

Mass Spectrometry>>