Lecture 4a Common Error in EPMA: Secondary Fluorescence from Outside Primary Excitation Volume John Fournelle, Ph.D. Department of Geoscience University of Wisconsin Madison, Wisconsin, USA December 1, 2015 Nanjing

Common EPMA needs/wants Need to measure low ppm of some element Need to analyze small crystals in volcanic glass) Need to analyze small inclusions in minerals Need to determine if there is diffusion profile between two phases But simple EPMA can produce errors if SECONDARY FLUORESCENCE is not understood

“Simple EPMA” assumes…. That all X-rays detected come from the electron beam interaction volume

Fluorescence Problems UW- Madison Geology 777 Fluorescence Problems Alloy: 90 at% Ni 10 at% Fe Goldstein et al. p. 142 Secondary fluorescence is an important issue that must be appreciated. Generated X-rays are not scattered nearly as much as incident electrons, and thus the generated X-rays can travel relatively long distances (50 um in Fig 3.49) within the specimen and produce a second generation of X-rays. If the specimen (and standards) are relatively large (=homogeneous), this is not a problem.

Fluorescence Across Phase Boundaries UW- Madison Geology 777 Fluorescence Across Phase Boundaries Reed 1993, p. 258 However, if minor or trace elements are being analyzed in small grains (Phase 1) and the host phase (2) has high element abundance, an error can be made in the EPMA analysis.

Fluorescence across boundaries UW- Madison Geology 777 Fluorescence across boundaries “false Co” “false Cu” Cu Co

Secondary Fluorescence Correction UW- Madison Geology 777 Secondary Fluorescence Correction An American Mineralogist 2003 article (Llovet & Galan) reports an innovative approach to correcting the secondary fluorescence (SF) in diffusion couples and from adjacent phases. This utilizes a complex Monte Carlo program called PENELOPE that permits complicated geometric models of electron and X-ray behavior in materials. SF can be simulated in a model that represents the actual specimen.

Secondary Fluorescence Correction UW- Madison Geology 777 Secondary Fluorescence Correction SF can be simulated with Monte Carlo program PENELOPE in a model that represents the actual specimen (e.g. Fig 1).

Secondary Fluorescence Correction UW- Madison Geology 777 Secondary Fluorescence Correction Correction for SF in Ca in olivine using PENELOPE. The corrected Ca-in-olivine temperatures matched closer the 2 pyroxene thermometer temperatures than when uncorrected. Llovet and Galan, 2003.

Ti in Quartz Geothermometer Quartz fill of rutile needles Sato and Santosh, 2007, Mineralogical Magazine, 71, 143. This published study used the Ti measured in the quartz at ~10 microns from the rutile as the ‘true’ Ti content of the quartz. It gave minimum temperatures of 1190°C.

Ti in Quartz Geothermometer Sato and Santosh, 2007, Mineralogical Magazine, 71, 143. I simulated the quartz with a rutile, at various distances from the interface in the quartz. The simulation matches the published experimental data; my conclusion is that virtually of the Ti measured by EPMA is due to Secondary Fluorescence from the rutile.

UW- Madison Geology 777 An SF real story Another lab reported 10 wt% Nb (below) in what should have been Nb-free phase (by EDS at 30 kV). The issue was small grain size and nearby Nb (right image) When I ran WDS (18 kV) I found essentially zero Nb -- what is the problem? The original researchers used Nb Ka because, with EDS, it is impossible to resolve Nb La (it sits between Al Ka, Hf Ma and Pd La). Above: EDS spectrum on Pd2HfAl 5 um away from Nb. Pd Ka is very efficient at traveling across the border and exiting the Nb Fournelle, Kim and Perepezko (2005)

And I could reproduce their error by WDS – at 28 kV! Using Nb Ka And we see that also at 28 kV, there is minimal Secondary Fluorescence for Nb La line

Experimental Validation of Nb Ka Secondary Fluorescence in Nb-free Pd2HfAl

PENELOPE/PENEPMA Rigorous Monte Carlo program that tracks both electrons and photons Complex geometries possible Steep learning curve on original software Newer interfaces easier to use: PyPenelope (Philippe Pinard)- full app (go to SourceForge to download) Probe for EPMA – simplified app for secondary fluorescence (included in CALCZAF/Standard). A tutorial is at the ProbeSoftware Forum.

A GUI for the calculation program can be downloaded from http://epmalab.uoregon.edu/calczaf.htm)