Millimeter- to decimeter-scale compositional mapping using a scanning X-ray analytical microscope and its application to a reaction zone in high-grade metamorphic rock

A new method for quantitatively joining compositional maps measured by a scanning X-ray analytical microscope (SXAM) to visualize a larger scale element distribution (i.e., a joined element map) is proposed, and applied to the analysis of a 25-cm-long sample across a reaction zone from high-grade metamorphic rock. The method involves the in situ measurement of a standard material during a sample scan, which enables correction of the different sensitivities of multiple maps. The appropriate background intensity correction, spectrum processing, and X-ray intensity correction proposed in this study enable the production of a semiquantitative element map at a decimeter scale with relatively high resolution (~ 0.1 mm). The one-dimensional quantitative transect across the reaction zone has high resolution as well as high precision (e.g., relative standard deviation of <2% for Fe). The transect shows both a sharp boundary controlled by phase stability (as well as a millimeter-scale gradual reaction boundary) and a decimeter-scale gradual compositional gradient simultaneously, and these features are difficult to identify using conventional methods (i.e., electron probe microanalyzer, X-ray fluorescence analysis, or SXAM with prior data processing). These compositional gradients, which range from submillimeter to decimeters in length, provide a key to understanding the formation mechanisms of rock and/or mineral reaction zones.