John H. Fournelle

1. The Problem of Secondary Fluorescence in EPMA in the Application of the Ti-in-Zircon Geothermometer And the Utility of PENEPMA Monte Carlo Simulations John H. Fournelle Eugene Cameron Electron Microprobe Lab Department of Geology and Geophysics University of Wisconsin Madison, Wisconsin

2. Taking the Earth’s temperature • geologists can directly measure temperature of lava flows • but direct methods are not possible for most of Earth conditions … • use various geochemical evidence -- element partitioning between coexisting minerals -- to infer conditions deep within the earth • these geothermometers and geobarometers have been developed with, and utilize, electron and ion probes for microanalysis of minerals and glasses.

3. Zircon…takes a licking, keeps on ticking… Zircon (ZrSiO4) is a small mineral highly resistant to chemical changes; this robustness has led it to be used for estimating earth conditions eons ago. It has been used to date geologic events, measuring its radiogenic Pb, U and Th isotopes. The oldest dated mineral on Earth is a zircon from Australia, and its oxygen isotope value suggests Earth’s crust was cool and wet as long ago as 4.3 billion years.

4. Zircon…takes a licking, keeps on ticking… • ZrSiO4, small ~100-200 microns, accessory mineral (~granites, rhyolites) • highly resistant to chemical changes • Th, U and Pb present: radiogenic decay • used for estimating earth conditions eons ago • oldest dated mineral on Earth is a zircon from Australia • its oxygen isotope value suggests Earth’s crust was cool and wet as long ago as 4.3 billion years.

5. A zircon thermometer? Watson and Harrison* and Watson et al** experimentally determined that the amount of Ti incorporated in zircon (~1 to 1000s of ppm), coexisting with a high-Ti mineral (e.g., rutile TiO2 or ilmenite FeTiO3), was proportional to the temperature at which the zircon crystallized and could be used as a geothermometer. * Watson and Harrison, 2005, Science, 308, 841 ** Watson, Walk and Thomas, 2006, Contrib Mineral Petrol, 151, 413

6. EPMA SIMS Measuring tiny levels of Ti ….. Where the level of Ti is very low (1-100 ppm), the preferred method is ion probe (SIMS). However, there are situations where EPMA is used: for original validation of Ti in synthesized zircons (i.e., for SIMS standards),

7. EPMA SIMS Measuring tiny levels of Ti ….. Where the level of Ti is very low (1-100 ppm), the preferred method is ion probe (SIMS). However, there are situations where EPMA is used: for original validation of Ti in synthesized zircons (i.e., for SIMS standards), by geologists who do not have ready access to SIMS, or

8. EPMA SIMS Measuring tiny levels of Ti ….. Where the level of Ti is very low (1-100 ppm), the preferred method is ion probe (SIMS). However, there are situations where EPMA is used: for original validation of Ti in synthesized zircons (i.e., for SIMS standards), by geologists who do not have ready access to SIMS, or where the large SIMS spot size (~25 microns) is prohibitive.

9. However For EPMA at trace element levels, Secondary Fluorescence may be an issue

10. Secondary Fluorescence … • the electron beam’s interaction volume is small relative to the volume excited by both characteristic and continuum x-rays generated • what is the amount of secondary fluorescence that is measured during EPMA??? • critical for trace element EPMA measurements

11. One alternative: model the effect The PENEPMA Monte Carlo program, based upon PENELOPE, has been shown to accurately predict the extent of secondary fluorescence and has been recently modified to explicitly state SF intensity and strip out continuum contribution. Energy total I error fluor I error Ti Ka

12. 2 Cases Modeled with Penepma Both 15 kV, 40° take off angles, with only continuum secondary fluorescence Presence of rutiles (TiO2) in experimental runs for measuring Ti in zircon solubility Zircons in rock samples with nearby ilmenite and biotite (= Ti-bearing minerals)

13. Watson et al (2006) experimentally grew zircons where rutile (TiO2) were also present, and tried to minimize/account for secondary fluorescence (SF) …. I have tried here to model the SF effect magnitude by Monte Carlo

14. Case 1: Zircon with Rutile — 7 Geometries Modeled Geometry 1: 30 um zircon, no rutile, only Ti in surrounding silicate glass (6 wt% Ti) => 452 ppm SF Ti 30 um Incident beam (15 kV) impacts center of round zircon here and in all cases Cross Section Plan View

15. Geometry 2: 30 um zircon, 5 large rutiles 15 um away, Ti in surrounding glass (6 wt% Ti) => 948 ppm SF Ti Each rutile contributes ~100 ppm Ti to the level already present from the matrix glass. Silicate glass with Ti

16. Geometry 3: 30 um zircon, 5 large rutiles 15 um away, NO Ti in surrounding glass, with Pb instead => 390 ppm SF Ti This simulates what Watson et al did, dissolved the original Ti-bearing glass, replaced with Pb-bearing glass, to reduce SF — with moderate result Silicate glass with Pb

17. Geometry 4: 30 um zircon, 5 large rutiles 15 um away, in epoxy => 1179 ppm SF Ti Thought experiment: what if no surrounding glass, only epoxy? Result suggests a balance of fluorescence and absorption involved — here fluorescence enhanced and absorption strongly eliminated epoxy

18. Geometry 5: 30 um zircon, 5 large rutiles 60 um away, in Pb-glass => 25 ppm SF Ti Silicate glass with Pb

19. 7 Experimental Geometries Modeled Geometry 6: 30 um zircon, 5 tiny 4 um rutiles 0-1 um away, in epoxy => 61 ppm SF Ti

20. Geometry 7: 30 um zircon, 10 tiny rutiles 0 um away, in epoxy => 120 ppm SF Ti

21. Conclusions from Penepma 7 geometries • easy to get several hundred ppm of Ti if large Ti-rich phases are within tens of microns of analysis point • conceptually, “solid angle” that Ti-rich phases present to analysis point • continuum SF is not insignificant

22. Case 2 examines the potential for secondary fluorescence in a rock where zircons are surrounded by ilmenite, hematite and biotite (suggested by C. Morisset, Univ. British Columbia). Here it is less easy to model a realistic geometry as the zircons are irregular in shape.

23. FeTiO3 Fe2O3 Biotite(Ti) FeTiO3 Biotite(Ti) The actual EPMA measurements of Ti in these zircons and temperatures from Watson et al’s geothermometer: Zr5 2559 ppm - 1679°C Zr1 162 ppm - 1064°C Zr2 189 ppm - 1142°C Zr3 264 ppm - 1197°C Zr4 645 ppm - 1314°C

26. Summary

28. Size Can Matter in EPMA Or ….

29. Complications of Secondary Fluorescence in EPMA: The “Size Discrepancy Issue”

30. Electron beam Electron beam Standard not to scale with unknown, would be much larger if true scale. Difference between small sample and large standard Sample = 10 um polished sphere Cr2O3 embedded in plastic Standard = 2 mm polished sphere PMM (plastic) PMM (plastic) In troubleshooting low totals, the question arose: if there is a several order magnitude size difference between unknowns (small grain separates) and standard (large), what could result?

31. Electron beam Electron beam Difference between small sample and large standard Sample = 10 um polished sphere Cr2O3 embedded in plastic Standard not to scale with unknown, would be much larger if true scale. Standard = 2 mm polished sphere PMM (plastic) PMM (plastic) If the primary electron “interaction volume” is confined within the material, and therefore the primary x-ray generation is also confined therein, is the lack of “additional” Cr x-ray counts resulting from secondary fluorescence outside the primary electron interaction volume of any importance in “normal” EPMA???

32. z Sample = 10 um polished sphere embedded in plastic Standard = 2 mm polished sphere Cr2O3 PMM (plastic) Set up a Penepma Monte Carlo simulation: Standard of “huge size”, 2 mm Unknowns of much smaller size Accelerating voltage of 20 kV, takeoff angle 40°

33. 4 Yes, Secondary Fluorescence can cause problems Standard=2000 mm Cr2O3 Unknown = smaller Cr2O3 Electron range (K-O): 1.7 micron Cr Ka X-ray range (A-H): 1.6 micron A 100 mm grain of pure Cr2O3 will have 1% low Cr K-ratio, and a 10 mm grain will have a K-ratio 2.5% low. (plots show K-ratios produced in centers of various discrete sized diameter cut-off spheres imbedded in epoxy simulations)

34. Conclusion Discrepancies in size between unknown and standard can lead to small, but noticeable errors, because secondary fluorescence yields • additional x-rays beyond the primary electron impact-x-ray production volume in the same phase if the phase is large, • or • a lack of additional x-rays if the phase is small and mounted in epoxy.

36. PENELOPE • created to model high energy radiation in bodies of complex geometries • simulates x-ray generation and x-ray absorption/secondary fluorescence • a new version developed for EPMA, with EDS-like spectral output • a FORTAN program, runs with G77 compiler under OS X, Linux, Windows • developed by Salvat, Llovet et al. of Universitat de Barcelona … and free