The Problem of Secondary Fluorescence in EPMA in the Application of the Ti- in-Zircon Geothermometer and the Utility of PENEPMA Monte Carlo Program

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 [1]. Watson and Harrison [2] 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. In cases where the level of Ti is extremely low (1-100 ppm), the preferred method of quantification is ion probe (SIMS). However, there are situations where EPMA is used to measure Ti: (1) for original validation of the abundance of Ti in zircons grown experimentally (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. However, as has been long recognized, the electron beam’s interaction volume is tiny relative to the volume excited by both characteristic and continuum x-rays generated in the primary electron beam volume. The question that has perplexed microanalysts is, exactly what is the amount of secondary fluorescence (SF) that is measured during EPMA— particularly critical for trace element levels. The problem potentially can be addressed experimentally for some systems, but is not trivial. An alternative approach is via Monte Carlo simulations. The PENEPMA program, based upon PENELOPE [3], has been shown to accurately predict the extent of SF [4] and has been recently modified to explicitly state SF intensity [5].