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Fluorescence effects at phase boundaries: petrological implications for Fe-Ti oxides

MATRlX corrections in electron probe microanalysis, commonly called the ZAF procedure, comprise the atomic number (Z), mass absorption (A) and fluorescence (F) corrections. Both atomic number and mass absorption corrections have recently been improved, in particular with respect to light element analysis (Ugarte et al., 1987; Scott and Love, 1989; Bastin and Heyligers, 1989). The magnitude of the fluorescence correction is generally far less than the atomic number and mass absorption terms. In the data collection of 430, mainly binary, systems used for the evaluation of correction programs, the characteristic fluorescence factor is less than 1% in 90% of the systems studied and very rarely exceeds 5% (Scott and Love, 1989). It is, therefore, not surprising that the mathematical expression of Reed (1965) is still widely used for the correction of characteristic fluorescence. The continuum fluorescence term is still of less importance, and is usually not incorporated in online ZAF computer programs on commercial WDS and EDS systems because of its cumbersome, lengthy and questionable nature (Heinrich, 1981). Low concentrations of heavy elements in a relatively light silicate matrix (for example: Fe in Mg-rich olivine, pyroxene or garnet), however, give fluorescence correction factors of 0.97 and 0.98 (Sweatman and Long, 1969). Correction factors are highest at low accelerating voltage and high take-off angles. A special case in which fluorescence corrections must be taken into consideration, concerns the situation at phase boundaries. The influence of fluorescence effects is often underestimated here or even neglected, in particular in the study of Fe-Ti oxides. The composition of coexisting magnetite-ulvrspinel and ilmenite-hematite solid solutions may provide equilibrium temperature and oxygen fugacity estimates (Buddington and Lindsley, 1964). These phases (Fe304Fe2TiO4 and FeTiO3-Fe203 respectively) are frequently intimately intergrown following exsolution accompanied by oxidation. The resultant texture consists of ilmenite lamellae in the (111) planes of magnetite and hematite lamellae in the (0001) planes of ilmenite (Lindsley, 1976). Accurate microprobe analysis of both phases is normally considered only to be possible when the primary X-ray excitation volume lies well within a single phase. The spatial resolution for quantitative analysis, d-quant, is given by the expression of Reed (1975):