Secondary fluorescence effects in microbeam analysis and their impacts on geospeedometry and geothermometry

Abstract

Characteristic and bremsstrahlung X-ray emission during electron-specimen interactions in electron microprobe (EPMA) and scanning electron microscope (SEM) instruments causes secondary fluorescence X-ray effects from adjacent (boundary) phases. This is well-known, yet the impact of such effects in microbeam analysis of natural mineral-hosted inclusions and adjacent to mineral-mineral and mineral-glass boundaries are frequently neglected, especially in geospeedometry and geothermometry applications. To demonstrate the important influence of the secondary fluorescence effect on the measured concentration of elements and its consequences for geochemical applications, we consider the effect of mineral-mineral and mineral-glass boundaries in microanalysis of Cr, Zr and Ti both experimentally, using electron probe measurements on cold-pressed material couples, and computationally, using the software suite “CalcZAF/Standard” and its Graphical User Interface (GUI) for the semi-analytical model FANAL (Llovet et al., 2012). We demonstrate, for example, that apparent Cr contents of the order of ~3000 to 5000 ppm in chromite-hosted glass inclusions at 6 μm from the inclusion boundary can be entirely due to secondary fluorescence in the Cr-rich host phase. Because the spatial gradient in secondary fluorescence-induced X-ray emission superficially resembles a diffusion profile, we emphasize the need to quantitatively correct for such effects in any geospeedometry application involving measurement of diffusion profiles adjacent to grain boundaries with large concentration contrasts. We also provide a scheme for estimating analytical errors related to the secondary fluorescence effect when applying geothermometers such as Ti-in-zircon, Ti-in-quartz (TitaniQ) and Zr-in-rutile. Temperature estimates based on trace Ti, Zr and Cr contents in minerals and glasses affected by secondary fluorescence in nearby phases (e.g., rutile, zircon and chromite) can be severely overestimated, in some cases by hundreds of degrees Celsius.