Instruments like an electron microprobe or laser ablation mass spectrometer allow us to analyze materials to determine their major and trace element compositions. However, many of these analyses are based on single spots, missing perhaps the larger story that a material, or in this case a mineral, can store. To gain a more complete story, another neat feature of the electron microprobe involves X-Ray elemental maps of the entire surface (see Figs. 1 & 2). The process is much like the name implies; the entirety of the object in question is analyzed utilizing characteristic X- rays excited by an electron beam, with the end product is represented as a color map of a specific element, for instance magnesium, iron, or phosphorus. The spatial distribution of elemental abundance within your sample is shown as variations in that color (Fig. 1) and provides a qualitative look at the compositional variability in that element. This can then allow you to observe things like heterogeneity in a sample. In the case of minerals, heterogeneity typically represents a growth or diffusion zone, which are indicative of changes in magmatic conditions.
Above is an X-ray map of an olivine phenocryst from the 1751 eruption of Volcán Llaima. Although barely visible, there are slight zones of lighter blue in the left portion of this olivine grain, signifying higher concentrations of phosphorus (P). P is an interesting element to map in olivine as its presence is typically attributed to periods of rapid growth in a magmatic system, and it's large size makes it a slow diffuser. Thus you can learn a lot about an olivine's older history through X-mapping in P.
Complete x-ray map for this 1751 olivine grain. 10 elements were mapped, and although variations in the crystal interior are absent, late stage zoning on the rims of this olivine grain provide evidence for the processes that occurred just prior to it's eruption.
During the summer of 2015 I was awarded the amazing opportunity to spend two months in Singapore through the EAPSI program, working at the Earth Observatory of Singapore with Dr. Caroline Bouvet De Maisonneueve on some samples from Volcan Llaima.
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