M diameter with the approximate cal culated ages

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m diameter, with the approximate cal- culated ages given in million years Bruhn et al. [486] used a proton microprobe (PIXE) to date monazite, with lateral spot size of 20–30 µ m but high penetration depth (30 µ m), and they were able to reproduce isotopic ages of monazite standard material. A 1.5-nA sample current and acquisition times of 5–10 min yielded detection limits of approximately 10–20 ppm for Pb, Th, and U and < 1000 ppm for the REE. Frei et al. [487] have shown the feasibility of micro-PIXE chemical dating on a 1 Ga old high uranium (265 ppm) titanite. Structural damage to the minerals due to the large mass of the protons in PIXE analyses has not been investigated but cannot be ruled out. Analytical techniques using X-ray induced excitation of fluorescence (XRF) also have good potential in providing lower detection limits due to the better peak-to-background ratio of this technique. A micro-XRF method was devel- oped by Cheburkin et al. [428] with a set-up using monochromatic X-ray radi- ation. Scherrer et al. [488] modified this technique, using it to date monazite
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672 D. Rammlmair et al. with a lateral resolution between 38 and 90 µ m in doubly polished sections. These methods use EDX detection, enabling the acquisition of the complete XRF spectrum at once in a relatively short time. However, at the energies needed (approximately 17–20 keV), the beam transects the complete thick- ness of a 30- µ m-thick thin-section (i.e., thickness of the sample determines the analyzed volume) and therefore spatial control is limited. Detection limits of approximately 10 ppm were achieved with a beam of 50 µ m on whole grains at 40-min data collection times, sufficient to date monazite grains as young as 15 Ma [489]. Additional improvements may be achieved by the use of synchrotron radiation XRF (SRXRF). The higher photon flux and brilliance of such X-ray sources provide the possibility of focusing the beam down to 2–3 µ m lateral diameter or less. The high photon flux of the beam still ensures very low detection limits down to ppm range or better, thus pushing the theoretical limits for the youngest datable monazite below 1 Ma even for monazite with low contents of ThO 2 (4 wt%). In other words monazite with an average ThO 2 of 10 wt%, yielding 20 ppm of radiogenic Pb within approximately five million years could be dated relatively precisely. However, small amounts of nonradi- ogenic Pb may cause some concern at this level of detection. The lower detection limits in PIXE, micro-XRF, and SRXRF analyses make the investigation of other phases or minerals with lower concentrations of U and Th that accumulate less radiogenic Pb viable (e.g., zircon, titanite), of course only if nonradiogenic Pb is negligible. Microanalysis Using Synchrotron Radiation XRF The use of SR as a photon source in contrast to a lab source has enabled studies on trace element contents and compositional changes of tracements in minerals and glasses on the micrometer scale. The high flux of synchrotron ra-
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  • Spring '14
  • MichaelDudley

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