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

Abstract

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.