comparison of sem-eds and epma-wds analysis of rare earth … · comparison of sem-eds and epma-wds...
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Comparison of SEM-EDS and EPMA-WDS Analysis of Rare Earth Element
Containing Minerals from Bokan Mountain, Alaska
H. A. Lowers1 and D.B. Stoeser
1
1Central Minerals and Environmental Resources Science Center, U.S. Geological Survey, Denver, CO
80225 USA
The Bokan Mountain Complex consists of peralkaline granitic intrusions and an associated "vein-dike"
system that is enriched in LREEs, HREEs, Y, Zr, Nb, Th, and U and is mineralogically and
paragenetically extremely complex (Fig. 1) [1,2]. Throughout the course of the work, SEM-EDS and
EMP-WDS were employed to identify REE-containing minerals to create a REE budget for the prospect
and determine the relative timing of the mineralizing events. To date, over 25 minerals have been
identified which contain significant amounts of rare earth elements (REEs) including fergusonite,
various Ti-Nb-Y-oxides (mainly polycrase), pyrochlore, gadolinite, hingganite, Y-Mg-fluosilicate
(magnesiorowlandite?), xenotime, iimoriite, bastnaesite, parisite, synchysite, allanite, monazite, and
multiple types of unidentifed Ti-Y-HREE-silicates, LREE±F±Ca±Na±Al-silicates, and an unidentified
HREE-Nb-Y-oxide [2].
EDS analyses were acquired at 20kV, approximately 1 nA beam current, and 30 seconds live count
time. Data was processed with the ThermoFisher Scientific Noran System Seven software package using
vendor supplied peak profiles and all data are normalized to 100%. Examination of spectra with known
composition suggests it is best to include all REE in the analysis. Once a potential mineral identification
was made based on the ratio of elements present, simulated EDS spectra were created with DTSA-II for
comparison [3-5].
WDS analyses were acquired with a JEOL 8900 electron microprobe with five spectrometers operated at
20kv, 30 nA current, and a focused spot. The complexity of the mineral assemblage at Bokan Mt
required the following elements to be acquired: F, Na, Mg, Al, Si, P, K, Ca, Ti, Mn, Fe, Zn, Y, Zr, Nb,
the REEs (La through Lu), Pb, Th, and U. Natural and synthetic mineral standards, the Edinburgh REE
glasses, and Smithsonian REE orthophosphates were used for calibration and accuracy checks. A
comparison of multipoint background acquisition and mean atomic number background corrections
show good agreement down to 1 wt% for most elements. The use of MAN background corrections
considerably reduces the analysis time.
The accuracy of SEM-EDS analysis of the Drake and Weill REE glasses [6] for each REE relative of the
given value is: La (4%), Ce (10%), Pr (8%), Nd (8%), Sm (8%), Eu (8%), Gd (8%), Tb (8%), Dy (8%),
Ho (8%), Er (8%), Tm (8%), Yb (8%), Lu (8%). These glasses were produced to have no interfering
peaks. SEM-EDS analyses compared to EMP-WDS of the minerals identified at Bokan Mountain show
good agreement for REEs that are a major component of the mineral (>10 wt%) and less agreement
when the REE is a minor component (<3 wt%) (Fig. 2). Generally, the SEM-EDS analysis is high
compared to WDS analysis, which is likely explained by poor handling of peak overlaps. However,
694doi:10.1017/S1431927614005194
Microsc. Microanal. 20 (Suppl 3), 2014© Microscopy Society of America 2014
https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1431927614005194Downloaded from https://www.cambridge.org/core. IP address: 54.39.106.173, on 07 Mar 2021 at 11:06:53, subject to the Cambridge Core terms of use, available at
SEM-EDS analysis has been essential in the reconnaissance of the mineralogical and paragenetic study
of the Bokan Mt complex because of the ease with which all major and minor elements can be acquired
relative to the time required to acquire 30 elements with EPMA.
References:
[1] J Dostal et al, Canadian J Earth Sci. 50, (2013) p. 678.
[2] D Stoeser and H Lowers, Geo Soc Am, Abstracts with Programs, 45 (2013).
[3] N Ritchie, Microsc Microanal, 13, (2007), p. 160.
[4] N Ritchie, Microsc Microanal, 15, (2009), p. 454.
[5] N Ritchie, Microsc Microanal, 16, (2010), p. 248.
[6] M J Drake and D F Weill, Chem Geolog, 10 (1972), p. 179.
Figure 1. BSE image showing the complex intergrowth of REE minerals. Scale bar is 250 μm.
Figure 2. Comparison of SEM-EDS to EMP-WDS data for the minerals fergusonite, polycrase, and
magnesiorowlandite.
695Microsc. Microanal. 20 (Suppl 3), 2014
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