HR: 11:50h
AN: V42A-07 INVITED     [Abstracts]
TI: Electron Microprobe Analysis of Fe$^{2+}$/Fe$^{3+}$ in Minerals With low Total Iron Concentrations
AU: * Creighton, S D
EM: Steven.Creighton@ualberta.ca
AF: Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Science Building University of Alberta, Edmonton, AB T6G 2E3 Canada
AU: Matveev, S
EM: smatveev@ualberta.ca
AF: Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Science Building University of Alberta, Edmonton, AB T6G 2E3 Canada
AU: Stachel, T
EM: tstachel@ualberta.ca
AF: Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Science Building University of Alberta, Edmonton, AB T6G 2E3 Canada
AU: Luth, R W
EM: Robert.Luth@ualberta.ca
AF: Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Science Building University of Alberta, Edmonton, AB T6G 2E3 Canada
AB: The development of the `flank method' by H\"{o}fer et al. (1994) has made it possible to quantify ferrous and ferric iron concentrations in minerals using the electron microprobe. The flank method makes use of the changes in both the wavelength and intensity of soft Fe L$\alpha$ and Fe L$\beta$ X-ray emission lines of minerals containing Fe$^{3+}$ and Fe$^{2+}$.By measuring at energies off the peak maxima (on the peaksAŸA›A›ƒ_sAªA›ƒ_zA› flanks) the differences due to variable ferric iron ratios are maximized, thus making the flank method much more sensitive than methods relying on either peak shifts or peak area ratios. Using a correction for self-absorption, the Fe$^{3+}$/Fe$^{2+}$ ratio of minerals may be accurately and precisely determined. The original flank method was developed for minerals with high total iron concentration e.g. Fe-rich garnet end-members (almandine, andradite, and skiagite), and Fe-oxides (w\"{u}stite). To make it applicable to minerals with total iron concentrations of less than 10 wt.% as is common in mantle-derived minerals, we have modified, in three ways, the flank method to significantly improve the precision. Firstly, we have increased the number of analyses per mineral grain to 400 thereby providing a far more representative mean. Secondly, because it is necessary to accurately reproduce the exact position of the spectrometer crystal for each flank measurement, we have eliminated the need to reposition the spectrometer by serially measuring each flank position. Thirdly, we compensate for instrumental drift by measuring two 10 $\mu$m by 10 $\mu$m grids (1 $\mu$m spot size and spacing) for both flank positions. Each mineral grain is measured sequentially, collecting 200 analyses on the first flank position. The procedure is identically repeated for measurements on the second flank position. The elapsed time between each grid measurement is the same for all samples and drift for either flank position is eliminated by averaging. Using these modifications to the flank method, we have been able to precisely measure ferric iron concentrations of less than 1 wt.% in mantle-derived pyrope garnets with true microprobe spatial resolution. The accuracy of the flank method is currently being verified with spectrophotometric wet-chemical analysis of ferric iron in pyrope garnet standards. References H\"{o}fer, H.E., Brey, G.P., Schulz-Dobrick, B., and Oberh\"{a}nsli, R., 1994. Eur. J. Min., 6: 407-418
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting