[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

Andor element contents in the lower ppm range

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and/or element contents in the lower ppm range synchrotron-based micro- XRF methods have to be applied. Sample Preparation For trace element analysis with XRF at high spatial resolution sample prepa- ration is of major importance. For positioning the beam exactly on the sam- ple and precisely defining the analyzed sample volume, its surface should be smooth or even polished. The X-ray beam penetrates deep into most geologic materials. For an exact quantification of the XRF spectra it is crucial to know the thickness of the analyzed sample volume. For those two reasons, double- sided polished samples of consistent thicknesses are most favored for micro- XRF analysis. Another consequence of the penetrative nature of the beam concerning the sample preparation is that the spatial resolution is controlled by the sample thickness. Any backing of the sample should be minimized or avoided since the backing material will contribute to the scattered radiation and will increase the background of the spectra leading to higher detection limits and potential additional peaks. Samples should be prepared as thin as possible while still mechanically stable. Typical thicknesses of geological samples for microanalysis vary between 30 and 100 µ m. U–Th–Pb Chemical Dating Radiometric dating provides absolute constraints on the timing and duration of (long-lived) geological processes and is therefore of fundamental importance within the geosciences. U–Th–Pb dating is one of most used tools for radio- metric dating. It is based on the decay of radioactive U and Th isotope to stable isotopes of Pb in three independent decay schemes (see later). Chemical dating, the calculation of the age from elemental abundances of U, Th, and Pb, is one of the earliest absolute geochronological techniques. It was even used on minerals containing abundant U or Th (e.g., uraninite – UO 2 ; [477]) before the development of mass spectrometry. Geochronology by isotope analysis is a
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670 D. Rammlmair et al. modern and more accurate technique allowing the comparison of the three dif- ferently calculated ages and corrections for nonradiogenic Pb in the mineral, but it is expensive and labor intensive. Renewed interest in chemical dating techniques arose from the applica- tion of relatively cheap and readily available EMPA [478, 479], which has the additional advantage of a spatial resolution in the range of 2–3 µ m. This spa- tial resolution allows the study of complexly zoned minerals. The application of nondestructive analyses on Th- and U-rich accessory minerals, especially monazite, became the focus of these studies because in situ microanalysis in geological samples allows the correlation of age data with the textural devel- opment of the main rock constituents.
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