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Phosphate accessory phases contain a wealth of petrologic and chronological information, but each possesses particular properties that make accurate quantitative microanalysis difficult. This chapter provides an overview of the literature concerned with the main problems of electron microprobe (EMP) phosphate analysis. These include the volatility of fluorine in F-bearing apatite, and the mutual interference of L- and M-line X-rays from the major and trace elements in monazite and xenotime, along with consideration of standards, detection limits, absorption edges, and ZAF corrections in REE phosphates.
FLUORINE EXCITATION DURING APATITE ANALYSIS
The typical apatite in metamorphic parageneses is enriched in F− (e.g., Smith and Yardley 1999), though enrichments in OH− and Cl− have been noted, either as inherited grains (Smith and Yardley 1999), or original growth (Harlov et al. 2002). Diffusive volatility of light anions such as F and Cl, and light cations such as K and Na, is a well known problem in electron microprobe analysis (Goldstein et al. 1984), but the relatively high F content of typical metamorphic apatite combined with its particular crystalline structure generate a unique analytical problem, namely, time-dependent variation in apatite F X-ray intensity driven by surface-ward diffusion of F ions in response to the electric field produced by implantation of primary beam electrons at a depth below the analyzed region (Stormer et al. 1993).
Stormer et al. (1993) examined variations in apatite F and Cl X-ray intensity as a function of a number of factors including analysis time, analysis current, accelerating voltage, and crystallographic orientation. Under typical EMP operating conditions (15 keV, 15 nA), Stormer et al. (1993) found that for sections oriented perpendicular to the c crystallographic axis, F X-ray intensity doubles in the first 60–120 s of cumulative beam exposure (Fig. 1⇓). There is a strong crystallographic control over this F X-ray intensity …