Following a lot of sample prep including crushing, sieving, crystal picking, polishing, the first set of crystals and glass from the pyroclastic units were ready to be imaged using a Scanning Electron Microscope (SEM) at the University of East Anglia. Following imaging, analyses of major element (elements which have a concentration greater than ~ 0.5 wt.%) variations within single crystals (and glass) was quantified using an Electron Probe MicroAnalyser (EPMA) whilst Katy was visiting the Victoria University of Wellington in New Zealand.
Epoxy block with glass fragments ready to be imaged and analysed.
Electron microprobe university of Wellington
The brightness of back-scattered electron (BSE) images is proportional to the total atomic number of elements in the substance being analysed, therefore we can use these BSE images as proxies for chemical changes within the single crystals, which can record changes in magmatic conditions whilst the crystals were residing in the magma chamber. We then used the images to help us locate our analysis points on the EPMA to make sure we got analyses of all chemically different zones within crystals. We obtained BSE images of the crystals and glass from both mafic and silicic fall units, and one pyroclastic flow unit, and the crystals show surprisingly little chemical changes from the interior of the crystals to the rims of the crystals- showing the lack of changes in the magmatic system as the magma evolved. However, there are significant differences in chemical compositions between different units sampled, which we will use to help track the evolution of the magmatic system underlying Ascension through time.
Zoned olivine crystal from scoria fall unit
BSE image of young lava flow from Letterbox showing different crystal types by their different greyscale intensities.