A Practical Method for Accurate Measurement of Trace Level Fluorine in Mg‐ and Fe‐Bearing Minerals and Glasses Using Electron Probe Microanalysis

Abstract

Fluorine plays an important role in magmatic and hydrothermal processes, but due to its low abundance in geological samples determining F is difficult by electron probe microanalysis. By using a W-Si multilayered pseudocrystal as the diffraction crystal instead of thallium acid phthalate (TAP), count rates were considerably higher, which however introduced spectral interferences between FKa and FeLa and MgKb lines when normal integral mode is applied. In this study, we developed a protocol using a W-Si multi-layered pseudocrystal for measuring accurately trace level F in both minerals and glasses. First, we used differential mode with an optimised PHA (pulse height analysis) setting in signal processing, instead of normal integral mode, which completely eliminated the second-order MgKb line. Second, the overlap of the first-order FeLa on FKa peak, which cannot be filtered by modifying the PHA setting, was calibrated quantitatively using F-free minerals and silicate glasses. Applying this two-step method, F was determined in a number of reference glasses, as well as in glasses synthesised from powders of the rock reference materials AC-E, GS-N and DR-N. Our data are consistent within error with F concentrations determined by other methods, demonstrating the reliability of this method.Keywords: electron probe microanalysis, in situ techniques, reference materials, microbeam techniques, wavelength dispersive X-ray fluorescence spectrometry.Le Fluor joue un rôle important dans les processus magmatiques et hydrothermaux mais l'analyse quantitative de cet el ement dans les phases solides par microsonde electronique reste difficile. Le taux de comptage peutêtre augment e de mani ere significative en utilisant un pseudocristal de W-Si au lieu de thallium acid phthalate (TAP). Cependant, l'utilisation de W-Si entraine des interf erences spectrales des lignes FeLa et MgKb avec FKa, ce qui introduit des difficult es pour la d etermination de la position exacte a choisir pour la mesure du pic de FKa et du bruit de fond. Afin de quantifier le rôle de ces interf erences, nous proposons un protocole pour l'analyse du fluor dans des verres et des min eraux avec W-Si. Nous avons utilis e le mode diff erentiel pour le traitement du signal et optimis e la valeur de PHA (pulse height analysis), ce qui permet d' eliminer totalement la raie de second ordre de MgKb et par cons equent les interf erences spectrales avec la ligne MgKb. L'optimisation de PHA ne permet cependant pas de supprimer totalement la raie FeLa. Nous avons donc effectu e une s erie de mesures dans des verres et dans des phases solides d epourvus de Fluor pour calibrer les interf erences spectrales avec la ligne FeLa. Nous avons appliqu e cette m ethode pour l'analyse du Fluor dans des verres a teneurs en Fluor connues. En tenant compte de l'erreur de mesure, les teneurs en Fluor d etermin ees sont identiques aux valeurs de r ef erence. Il est important de souligner que la correction a appliquer pour tenir compte des interf erences spectrales avec la ligne FeLa d epe...

Halogens (F, Cl, Br, I) in Thirteen USGS, GSJ and NIST International Rock and Glass Reference Materials

“…() and Zhang et al . () and values previously obtained by pyrohydrolysis and ion chromatography by Marks et al . () (Table ).…”

“…Instead, we suggest that the low F mass fractions reported in the earlier studies of these glasses are the result of known difficulties in the determination of F by EPMA related to overlap with the Fe peak (Zhang et al . ) and/or element mobility under the electron beam and incomplete F extraction during pyrohydrolysis (Marks et al . ).…”

“…The I mass fractions of 51-91 ng g -1 measured in encapsulated duplicate aliquots of RGM-2, BHVO-2, BCR-2 and JR-1 included in two irradiations have intermediate precision's of 3-10% (Table 3). Literature values for rock reference materials (Imai et al 1995, Peterman and Cloke 2002, Michel and Villemant 2003, Shimizu et al 2006, Balcone-Boissard et al 2009, Wang et al 2010, 2014 and glasses (Hoskin 1999, Jochum et al 2006, Guggino 2012, Peate et al 2012, Zhang et al 2016, Marks et al 2017 are identified by superscript abbreviations. GeoReM preferred values downloaded from http://georem.mpch-mainz.gwdg.de/ Oct 2017. a Note that limited data are available for BIR-1a and RGM-2, and comparative values in brackets are for BIR-1 and RGM-1.…”

Geostandards and Geoanalytical Research Bibliographic Review 2016

“…Zhang et al . () report results for fluorine while Chen et al . () present Zn isotopic values for several rock RMs.…”

Trace element analysis by EPMA in geosciences: detection limit, precision and accuracy

“…Sometimes an empirical solution provides very good results, as in the case of fluorine (F) analysis in Fe-bearing minerals and glasses, where the peak of F-Kα position on the sensitive LDE1 (JEOL) PC1 (CAMECA) crystals is on the "shoulder" of the Fe-Lα 1 line [24,25].…”