Quantitative EPMA methods assume that the sample volume from which X-rays are emitted hashomogeneous composition. While this volume is of the order of several cubic micrometres forX-rays generated by the electron beam, it can be one-to-two orders of magnitude greater forsecondary X-rays fluoresced by characteristic and continuum X-rays. This means that even for anelectron beam impacting quite a distance away from a phase boundary, there can be X-rays emittedfrom the adjacent phase that may reach the detector. Although secondary fluorescence (SF) acrossphase boundaries can generally be disregarded, it may become a significant source of error whenanalysing for a minor/trace element next to a phase containing the element of interest. For example,temperature estimates based on trace Ti, Zr and Cr contents in minerals and glasses affected by SFin nearby phases (e.g., rutile, zircon and chromite) can be severely overestimated, in some cases byhundreds of degrees Celsius.
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