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

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. Zircon…takes a licking, keeps on ticking…

ZrSiO 4, small ~ 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. Zircon…takes a licking, keeps on ticking…

Taking the Earth’s temperature geologists can directly measure temperature of geothermal springs and 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. Scan slide of ? At Kileaua re Therm

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 TiO 2 or ilmenite FeTiO 3 ), 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 A zircon thermometer?

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

However, there are situations where EPMA is used: 1.for original validation of Ti in synthesized zircons (i.e., for SIMS standards), 2.by geologists who do not have ready access to SIMS, or Measuring tiny levels of Ti ….. Where the level of Ti is very low (1-100 ppm), the preferred method is ion probe (SIMS). SIMS EPMA

However, there are situations where EPMA is used: 1.for original validation of Ti in synthesized zircons (i.e., for SIMS standards), 2.by geologists who do not have ready access to SIMS, or 3. where the large SIMS spot size (~25 microns) is prohibitive. Measuring tiny levels of Ti ….. Where the level of Ti is very low (1-100 ppm), the preferred method is ion probe (SIMS). SIMS EPMA

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 (SF) that is measured during EPMA??? critical for trace element EPMA measurements Secondary Fluorescence may become an issue

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. One alternative: model the effect

2 Cases Modeled with Penepma Both 15 kV, 40° take off angles, with only continuum secondary fluorescence 1.Rutiles (TiO 2 ) in experimental runs for Ti in zircon solubility 2.Zircons in rock samples with nearby ilmenite and biotite (= Ti-bearing minerals)

Watson et al (2006) experimentally grew zircons where rutile (TiO2) were also present, and tried to account for secondary fluorescence….

7 Geometries Modeled

7 Experimental Geometries Modeled Geometry 1: 30 um zircon, no rutile, only Ti in surrounding glass (6 wt%) => 452 ppm SF Ti Case 1 examines the potential for SF in experimentally grown zircons that have crystals of rutile either touching them or present in the vicinity.

Geometry 2: 30 um zircon, 5 large rutiles 15 um away, Ti in surrounding glass (6 wt%) => 948 ppm SF Ti

Geometry 3: 30 um zircon, 5 large rutiles 15 um away, NO Ti in surrounding glass, with Pb instead => 390 ppm SF Ti

Geometry 4: 30 um zircon, 5 large rutiles 15 um away, in epoxy => 1179 ppm SF Ti

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

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

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

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

Case 2 examines the potential for SF 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.

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

Difference between small sample and large standard Sample = 10 um polished sphere Cr2O3 embedded in 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? Standard = 2 mm polished sphere PMM (plastic) Standard not to scale with unknown, would be much larger if true scale. Electron beam

Difference between small sample and large standard Sample = 10 um polished sphere Cr2O3 embedded in 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??? Standard = 2 mm polished sphere PMM (plastic) Standard not to scale with unknown, would be much larger if true scale. Electron beam

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

Yes, Secondary Fluorescence can cause problems Standard=2000  m Cr2O3 Unknown = smaller Cr2O3 A 100  m grain of pure Cr2O3 will have 1% low Cr K- ratio, and a 10  m 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) Electron range (K-O): 1.7 micron Cr Ka X-ray range (A-H): 1.6 micron 4

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.

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