Trace element concentrations in rocks and their components have long been of geological interest, but most techniques provide only data that reflect the bulk composition of a sample. Furthermore, as these techniques are highly destructive, they do not take into consideration textural relationships and they disregard frequent chemical zonations. Electron microprobe as an in-situ microanalytical tool is restricted to the measurement of major and minor elements in the >500 ppm range (e.g. Riciputi et al., 1994). In the past decade, development of sophisticated non-destructive trace element tools, such as Laser Ablation Microprobe (LAM)-ICP-MS, Secondary Ion Mass Spectrometry (SIMS) or Proton Induced X-ray Spectrometry (PIXE) have contributed considerably towards tackling the above deficiencies. PIXE in particular has been utilized for trace element investigations in many fields of geological research, such as diamond exploration (Sie et al., 1991), ore genesis (Campbell et al., 1990), diagenesis (Bruhn et al., 1995) reconstruction of the chemical and isotopic composition of ancient oceans (Bruckschen et al., 1995).
Trace element analyses using the Bochum PIXE probe are performed using a 3 MeV proton beam generated by a dynamitron tandem accelerator. The considerably lower "Bremsstrahlung" background of protons compared to electrons enables detection limits at the <10 ppm level for a great number of elements with atomic numbers >13. A lateral resolution of <10 µm is achieved using quadrupole magnets for beam focusing. With the penetration depth of the proton beam in the range of 30-50 µm, sample masses as low as 10 ng can be analyzed. The samples can be observed with a polarization microscope with a maximum magnification of 120x. Trace element measurements can be carried out as point analyses, linescans (distribution profiles), and elemental maps of up to 1 x 1 mm2. The acquisition time for a single point analysis is about 10-15 minutes, and with typical beam currents of a few nA the accumulated charge for each point analysis is in the range of 1-5 µC. Quantitative PIXE linescans require longer acquisition times, usually in the range of a few hours. Due to statistical constraints, elemental maps, consisting of up to 100 x 100 points, can only yield qualitative information on trace element distributions. Quantitative data can be obtained from subsequent point analyses of the areas of interest. The evaluation of absolute concentrations from the measured X-ray spectra is performed by a least square fitting procedure using the GUPIX software package (Maxwell et al., 1989). The calibration of the experimental setup is currently based on X-ray fluorescence analysis of NBS standards fused in Spectromelt®. Tests on more suitable homogenous glass standards are in progress. Repeated measurements of standard reference material AGV-1 yielded an internal precision and external accuracy of 10 % (1s).
The Bochum Proton microprobe has been used mainly for trace element analyses in calcite and quartz and is intended to be used for analysis of more complex mineral matrices in the future. Analyses of cement calcite provide important supporting information on the conditions of mineral precipitation (pH, Eh, chemistry of pore water) in the course of diagenesis. A comparison of Mn and Fe concentrations with calcite cathodoluminescence (CL) has clearly demonstrated the influence of both absolute element concentrations and their ratios on the cathodloluminescence intensity. In isotope stratigraphy, diagenetically unaltered material of brachiopod shells has been detected, based on the Mn, Fe, and Sr concentrations, even for extremely small samples. Applications to quartz geochemstry involved correlation of CL colour with Ti and Fe concentrations in detrital quartz grains that, in turn, led to applications in provenance studies. Ongoing projects involve trace element microanalyses of conodonts, in order to ascertain their degree of preservation, and a quantitative comparison of trace element concentrations with CL spectra in carbonates and quartz.
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