Interplanetary dust particles (IDPs) are routinely collected in the lower stratosphere (20km) by high-flying aircraft from NASA (Brownlee, 1985). Because of their small sizes (10-50µm) they typically experience less heating during atmospheric entry than mm to cm sized objects. For this reason IDPs are expected to have retained more original information. Additionally, according to orbit calculations one expect asteroidal and cometary particles within the set of IDPs.
Using PIXE (Proton Induced X-ray Emission) with the Heidelberg proton microprobe (Traxel et al., 1995) we analyze major, minor, and trace element concentrations down to the ppm level in individual IDPs. First we tested the reliability of PIXE-data on 100 micron sized fragments of the CI-meteorites Orgueil and Alais. By using the mean of all measured 64 samples we could reproduce the CI abundances with a sufficient accuracy but we found that (in general) one tends to overestimate the mean values of elements measured near the limits of detection. In further tests on layer lattice silicates we found that PIXE measurements cause no loss of volatile elements in contrast to measurements with the SEM-EDX. We also showed that we can avoid mineralogical destructions during PIXE-measurements (Maetz et al., 1995). Thus PIXE can be combined with further analytical techniques. In an additional test we compared the results from the two techniques, PIXE in Heidelberg and SXRF (Synchrotron X-ray Fluorescence) in Brookhaven, on the same three IDPs and found a convincing agreement for the elements from potassium to bromine (Arndt and Flynn, 1995) (in atomic order).
We compiled a complete table of major, minor, and trace element contents of all analysed IDPs (89) and ran a number of cluster analyses. As a result IDPs could only be divided into four not very well separated groups with a general enrichment of trace elements compared to the CI meteorites. Using STIM (Scanning Transmission Ion Microscopy) (Maetz et al., 1994) we determined mass distributions (area density) of IDPs with lateral resolutions of about 0.5 µm. The integration over the whole particle area lead to the total particle mass which could be determined down to the pg-scale. Combining the area density information from STIM with elemental composition from PIXE we can calculate absolute concentrations. We did this first for a 10µm sized IDP in combination with a multielemental PIXE-mapping. This particle showed an enrichment of Zn (10xCI) strongly correlated with Fe, Ni, and S. This observation complicates the discussed models, using low Zn values to determine possible atmospheric entry heating temperatures of a certain particle.
With PIXE we reproduced CI data on small fragments of the CI chondrites Orgueil and Alais. The comparison between PIXE and SXRF demonstrated consistency. PIXE measurements on layer lattice silicates produced no loss of the volatile elements Na and K in contrast to SEM-EDX. We can avoid mineralogical destruction during PIXE analyses. There is no obvious grouping in the complete data set of IDPs. Trace elements in IDPs are enriched compared to CI and in one IDP we found a strong enrichment of Zinc exlusively located in a FeNi-sulfide region.
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