Berry, FJ

Bland, PA

Cressey, G

Menzies, ON

English

Spiral - Imperial College Digital Repository

AN X-RAY DIFFRACTION STUDY OF INCLUSIONS IN ALLENDE USING A FOCUSSED X-RAY MICROSOURCE.  O. N. Menzies1, P. A. Bland2, G. Cressey3 and F. J. Berry4. 1Planetary and Space Sciences Re-search Institute, The Open University, Milton Keynes MK7 6AA, UK, o.n.menzies@open.ac.uk; 1Department of Earth Science and Engineering, Exhibition Road, Imperial College London, South Kensington Campus, London SW7 2AZ, UK.; 3Department of Mineralogy, Natural History Museum, London SW7 5BD, UK; 4Department of Chemistry, The Open University, Milton Keynes MK7 6AA, UK   Introduction: We have analysed milligram samples of dark inclusion, CAI, and matrix from Allende using a novel X-ray powder diffraction (XRD) system that employs a high-brightness X-ray source. The Mi-croSource X-ray generator produces a finely focussed X-ray beam with an output intensity at least x30 greater than that from sealed-tube sources, allowing rapid data acquisition from small samples. Our XRD system utilises a position sensitive detector (PSD) - a fixed diffraction geometry means that mechanical movement is not necessary, so irradiated volume re-mains constant for each experiment. Consequently, diffraction patterns from standard phases and multi-phase mixtures can be compared directly. This allows not only characterisation of phases, but also quantifica-tion of their abundance [1,2,3]. When our database of standards is complete it will be possible to quantify mineral abundance in milligram samples of extraterres-trial materials [3]. XRD-PSD is a well-established technique that has provided a rapid means of characterising and quantify-ing the mineral phases in bulk meteorite samples in a non-destructive manner [1-5].  The addition of a Mi-croSource to our XRD-PSD system significantly ex-pands the range of analytical targets available to us. The MicroSource employs precision electron optics to focus electrons into a 10µm spot on the target. The X-rays emitted from this area are focussed by specular reflectance along a capillary with an internal mirror coating. This ensures that almost all X-rays generated are concentrated into a fine (almost parallel) beam. A portion of this beam can be selected to deliver a mi-crobeam as small as 10µm at the sample. An initial trial study was carried out, utilising the MicroSource to analyse inclusions in situ [6]. This allowed identifica-tion of phases, but not quantification of abundance. The intention of the current study is to apply a meth-odology that will allow quantitative mineralogical analysis to be carried out on samples 10,000 times smaller than was possible using our earlier system.  Experimental Procedure: The samples were hand-picked from a specimen of Allende and, in the case of white inclusions, any obvious matrix contami-nation removed. We also analysed a sample of Orgueil that had previously been studied using a conventional source. Samples were crushed to a powder and placed in a well of 0.07mm3 volume. The microwell was placed in a spinning goniometer stage where all angles can be adjusted so that a focussed 500µm beam is aligned with the centre of rotation of the spinning mi-crowell. XRD patterns were collected over 30minutes and calibrated with an external silicon standard.  The samples were then fitted as described by [1-5]. In addition to Bragg scattering from crystalline phases, CuKα1 radiation interacts with Fe in the sample caus-ing Fe-fluorescence radiation to be emitted. This fluo-rescence is detected by the PSD and the resultant pat-tern is a superimposition of Bragg diffraction peaks on a fluorescence background signal. This characteristic fluorescence signal is of use in the fitting process and in determing the abundance of Fe in a sample. Results: Analysis of the Orgueil sample revealed that a pattern recorded previously from a ground bulk sample in a 180mm3 well, quantified with a suitable standard set, was essentially identical to the pattern recorded from a 0.07mm3 well using the MicroSource.  XRD patterns were also obtained from the three white inclusions using the microwell (Figure 1). OM3        05001000150020002θ (CuKa1)IntensityOM1OM2OM30 4000Figure 1. XRD patterns for three white inclusions from Allende. OM3 has a typical CAI mineral as-semblage while OM1 and OM2 have elevated Fe fluorescence backgrounds indicative of a high pro-portion of Fe-containing minerals. Olivine compo-sitions in these inclusions centre on Fo80. has a low Fe-fluorescence background and peaks indi-cating the presence of grossular garnet and gehlenitic melilite: a typical CAI assemblage. We are currently Lunar and Planetary Science XXXIV (2003) 1734.pdfAN XRD STUDY OF INCLUSIONS IN ALLENDE: O. N. Menzies, P. A. Bland, G. Cressey and F. J. Berry. searching for standards of suitable composition to al-low phase abundance to be quantified. The inclusion is likely to be an altered type A CAI (e.g. [7]). The other two inclusions (OM1 and OM2) are in-triguing. Apparently similar to CAIs under binocular microscope, their XRD patterns are dramatically dif-ferent to OM3 (but closely related to each other). Peaks associated with typical CAI minerals are absent, and they show elevated Fe-fluorescence backgrounds indicating the presence of Fe-rich minerals. These in-clusions were best fitted with a range of olivine com-positions (Fo70-90) with a mode at Fo80. In order to compare the unusual white inclusions with the surrounding matrix a small sample of Allende matrix was analysed. Figure 2 shows that matrix has an elevated Fe-fluorescence compared to the anoma-lous white inclusions. Both matrix and inclusions are dominated by the presence of olivine although matrix has a wider range of olivine compositions. In contrast, the white inclusion has more sharply defined, narrower peaks, centred on a composition at Fo80.   In addition, we sub-sampled a dark inclusion in Al-lende to investigate its mineralogy and compare it to the mineralogy of adjacent matrix. The patterns ob-tained are shown in Figure 3, and are almost identical. Discussion: The work described here is a prelimi-nary study to explore the application of MicroSource XRD-PSD in characterising and quantifying the min-eralogy of components in primitive chondrites, includ-ing fine-grained CAIs, matrix, and DIs. We have shown that this is an appropriate technique for such an investigation. One of the ultimate aims of this study is to characterise and quantify the mineralogy of fine-grained CAIs, work that may help to determine whether there is a sequence of mineralogical alteration that can be linked to oxygen isotopic variation. The next phase of the work will proceed in this direction.    The unusual white inclusions, OM1 and OM2, re-quire further analysis with SEM to verify the XRD results. At this point it is clear that they are distinct from typical CAIs [7]. It is possible that they may be altered amoeboid olivine aggregates (AOAs), although AOAs are not known to contain high proportions of fayalitic olivine. Their mineralogical similarity to the matrix is interesting, although visually they are distinct entities and compositionally more uniform than matrix. 05001000150020002θ (CuKa1)IntensityDarkInclusionMatrix0 4000Figure 3. XRD patterns showing that matrix and DI in Allende are mineralogically almost identical. 05001000150020002θ (CuKa1)IntensityMatrixOM10 4000Several formation mechanisms have been proposed for dark inclusions ranging from size-sorted fragments of matrix to aqueously altered and metamorphosed CV fragments [8,9]. Our preliminary study shows that mineralogically the difference between the dark inclu-sion analysed and adjacent matrix is negligible. Conclusions: This study emphasises the potential of the combined MicroSource XRD-PSD system, al-lowing for the mineralogical investigation of a wide variety of meteoritic components. Fine-grained, het-erogeneous samples can be studied rapidly and their component mineral phases characterised and quanti-fied. The results should help in elucidating genetic relationships between meteoritic components. Figure 2. XRD patterns showing the anomalous white inclusion OM1 compared to Allende matrix. References: [1] Cressey G. and Schofield P. F. (1996) Powder Diff., 11, 35-39. [2] Batchelder M. and Cres-sey G. (1998) Clay and Clay Min., 46, 183-194. [3] Bland P. A., Cressey G. and Menzies O. N. (2003) MAPS, 38, (accepted). [4] Bland P. A. and Cressey G. (2001) LPSC XXXII, #1853. [5] Menzies O. N. et al. (2002) LPSC XXXIII, #1500. [6] Bland P. A. et al. (2000) MAPS, 35, A27. [7] MacPherson et al. (1988) In Mets. and the early Sol. Syst. (ed. J. F. Kerridge and M. S. Matthews), pp. 746-807. [8] Fruland et al. (1978) LPS, 9, 1305-1329. [9] Krot al. (1997) MAPS, 32, 31-49.  Lunar and Planetary Science XXXIV (2003) 1734.pdf

An X-ray diffraction study of inclusions in Allende using a focused X-ray MicroSource.

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An X-ray diffraction study of inclusions in Allende using a focused X-ray MicroSource.

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