Copyright JCPDS-International Centre for Diffraction Data 2016 ISSN 1097-0002 85 COMBINED ELEMENTAL XRF AND PHASE XRD ANALYSES OF A METEORITE Frantessa Tañeza-Casco, Thomas Witzke, Nicholas Norberg, PANalytical BV, Almelo, The Netherlands ABSTRACT Meteorites are records of the solar system formation. Several meteorites have not gone through significant geological transformations over billions of years and have remained intact since the formation of the solar system. Knowing more about the composition and mineral phases of meteorites bring us a step closer to understanding how the solar system was formed. X -ray diffraction (XRD) is a good technique used to investigate phase composition of meteorites, but it does not provide elemental analysis. X-ray fluorescence (XRF) spectroscopy is a well recognized technique used to determine the elemental composition of rocks. Typically, the mineralogy and often fine-grained texture of meteorites requires analysis on a much smaller spot size than traditional bulk XRF and XRD techniques. In this study, small spot analysis and elemental distribution mapping of the Northwest-Africa 2086 (NWA 2086) CV3 meteorite were conducted. Qualitative mapping and semi-quantitative spot elemental composition analyses were compared to phase identification using X-ray diffraction for cross verification. The qualitative mapping results show that elements such as Al and Ca are abundant in the inclusion region while Fe and Cr are more prominent in the meteorite body. Combined XRF analysis and XRD phase identification show the presence of sodalite in addition to variations of olivine, pyroxene, and spinel in the rim and the core of calcium-aluminum rich inclusions (CAIs). The sodalite is an indication of early Na-Cl metasomatic processes in the solar nebula. This study was done to demonstrate the capability of these instruments for evaluation of mineralogy and chemistry of meteorites. INTRODUCTION The Northwest-Africa 2086 (NWA 2086) meteorite is the subject of this study and a slice of this meteorite is shown in Figure 1. The studied slice has a maximum dimension of 28 x 13 x 2 mm. NWA 2086 was found in 2003 with a total known weight of 780 g. The meteorite was classified as a carbonaceous chondrite of the CV3 group (Russel et al., 2005). This group of meteorites is characterized by the presence of chondrules and calcium-aluminum rich inclusions (CAIs) in a dark colored matrix. The CV3 meteorites contain ~ 0.6 wt.% of carbon and underwent the lowest degree of thermal or aqueous alteration on the parent body. With an age of 4.567 billion years, the CAIs represent the oldest known solid material formed in our solar system. They were
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Copyright JCPDS-International Centre for Diffraction Data 2016 ISSN 1097-0002 86 formed as condensation products at high temperatures, probably in a single event close to the proto-sun, and are rich in refractory elements such as Ca, Al, Si, and Mg. They can also contain small amounts or traces of Ti, Zr, Sc, and Re. The minerals commonly found in CAIs are spinel, pyroxenes, gehlenite-åkermanite, anorthite and Ca-aluminates. The size of a CAI can vary from microns to millimeters and on a rare occasion up to about 2 centimeters. The studied fragment of a slice contains a large CAI with maximum diameter of 7.5 mm. Figure 1. Photograph showing studied part of the meteorite Northwest-Africa 2086 Chondrule formation started at approximately the same time as the CAIs, but lasted over 3 million years during multiple shock/heating events. They were formed from molten droplets. The minerals commonly found in chondrules are olivine, Ca-poor pyroxene, glass, and sometimes plagioclase. The size of a chondrule varies from microns to millimeters and can be up to 4-5 centimeters. From chondrules, CAIs and dust, the planetesimals and protoplanets were formed by accretion processes. Some of these bodies survived as asteroids. Ejected material from impacts and collisions crossed the path of the Earth and fell as meteorites. Figure 2 shows the early processes in the solar system including the origin of meteorites from asteroids.
Copyright JCPDS-International Centre for Diffraction Data 2016 ISSN 1097-0002 87 Figure 2. Schematic illustrating early processes in the solar system EXPERIMENTAL SETUP X-ray fluorescence (XRF) analysis For small spot mapping (SSM) analysis, measurements were performed using a PANalytical Zetium WDXRF spectrometer equipped with a 4kW, Rh-anode SST R-mAX tube, a high performance ED core with a silicon drift detector, and SuperQ software. In this study, the ED capabilities of the spectrometer are used. For all measurements in this study, net intensities were derived from the acquired spectra by deconvolution. The NWA 2086 meteorite was mounted on a small spot sample holder and imaged using a high resolution camera. The surface of the slice was finely grounded creating a smooth surface finish. Figure 3 shows the mounting assembly used to hold the sample during the measurement. As shown in Figure 3, an aluminum plate was placed on top of the sample and then secured on the side with metal screws. Once the sample and the Al backing were secured, the whole sample assembly was then placed inside the sample cup. Another metal screw fixed the assembly inside the cup to prevent it from moving during the measurements. The sample was then loaded into the spectrometer for the measurements.
Copyright JCPDS-International Centre for Diffraction Data 2016 ISSN 1097-0002 88 Figure 3. Sample cup and sample mounted with clamp assembly for SSM measurements. Qualitative analysis setup The data for the qualitative (intensity based) elemental mapping and distribution was obtained using the measurement conditions shown in Table 1. Table 1. Measurement condition used in the qualitative analysis NWA 2086 meteorite Condition kv ma Measurement time(s)/spot Medium Detector 1 60 66 60 Vacuum SDD The measurement spot size was 500μm with a step size of 250μm, producing a total of 600 spots to map the area of interest. Figure 4 shows the 5 x 7.5 mm area selected to perform mapping analysis. The total measurement time for mapping the sample was about 12.3 hours for high quality data. Figure 4. Photograph with the area selected for XRF small spot mapping analysis.
Copyright JCPDS-International Centre for Diffraction Data 2016 ISSN 1097-0002 89 Semi-quantitative analysis setup For the semi-quantitative (concentration-based) analysis, spots representing CAIs (spots 2 and 5), chondrules (spots 3, 4 and 6), and the matrix (spot1) were selected (see Figure 5). Figure 5. Selected spots for semi-quantitative analysis PANalytical's Omnian setup samples were used for the semi-quantitative analysis. The Omnian setup samples are composed of beads and pressed powder samples. All samples were measured via the ED-SSM component of the spectrometer. The Omnian software program makes use of fundamental parameters in setting up the calibration for the semi-quantitative analysis. Table 2 shows the spectrometer conditions used to measure the setup samples, the meteorite, and the validation sample. The Omnian software was used for the calculation of the concentration. All samples for the semi-quantitative analysis were measured using the ED-small spot (500um) component of the spectrometer. For the Omnian setup samples and validation sample, only a single location in the sample was measured. No averaging of results from different spots in the sample was done in the analysis. Table 2. Conditions for the semi-quantitative analysis on selected sample spots Condition kv ma Measurement time(s)/spot Medium Detector 1 60 66 100 Vacuum SDD 2 50 80 100 Vacuum SDD 3 32 125 100 Vacuum SDD X-ray diffraction(xrd) For phase identification, the complete surface of the fragment was measured simultaneously by X-ray diffraction using Bragg-Brentano geometry. Additionally, small amounts of the matrix,
Copyright JCPDS-International Centre for Diffraction Data 2016 ISSN 1097-0002 90 chondrules and a CAI of a neighboring fragment of the same slice were prepared, pulverized, and measured separately on a silicon zero-background sample holder. For the mapping of the minerals, a PANalytical Empyrean diffractometer (CoKα radiation, 40 kv, 40 ma) was equipped with a programmable xyz stage and a focusing lens (high intensity collimator, 300 micrometer pinhole). A PixCel 1D detector was used in 0D mode. For the intensity mapping, the 2theta positions of main peaks of different minerals (olivine, pyroxene, sodalite, and spinel) were used. The spot size was ~ 50 micrometer. The total measurement time for a 10 x 10 mm area (with 200 x 200 spots) was around 6 hours. EVALUATION X-ray fluorescence results Qualitative Analysis With just a measurement time of 60 seconds per spot (total 12.3 hours), 15 elements have been simultaneously analyzed and results are shown in Figure 6. It can be clearly seen that the intensities of these 15 elements have been distinctly mapped out in the sample. Figure 7 shows the 3D intensity contour maps for Al and Fe elemental distributions. It can be observed that the elemental composition of the CAIs is enriched in Al, Ca, Ni, and Zn, while these elements are less abundant in the dark matrix of the sample. Vice versa, the elemental composition of the dark matrix shows enrichment in Fe, Mn, and Cr, which are less prominent in the inclusions. These results illustrate that elemental mapping of the elements sample can be clearly shown in both 2D and 3D. Figure 6. Small spot elemental mapping of the NWA 2086 meteorite
Copyright JCPDS-International Centre for Diffraction Data 2016 ISSN 1097-0002 91 Figure 7. 3D intensity contour maps of Al and Fe X-ray fluorescence results Semi-Quantitative analysis When a semi-quantitative analysis is done, the analysis method could be validated using a certified reference material (CRM). Since there are no certified materials for a meteorite that could serve as validation samples, a certified austenitic steel sample was analyzed to investigate the expected accuracy that could be derived from semi-quantitative analysis on small spots. For this section, a CRM sample BCS/SS 466 was used as a validation sample. This austenitic stainless steel sample is a solid tube of 38mm in diameter. The sample was directly placed in the SSM cup and was fixed in position by a spring. Table 3 shows the small spot semi-quantitative analysis of BCS/SS 466. It can be seen from Table 3 that there is a good agreement between certified and semi-quantitative measured concentrations. This result demonstrates the expected accuracy of the semi-quantitative method. Table 3. Semi-quantitative analysis results of certified reference material BCS*/SS** 466 Elements Certified value (wt.%) SSM Semi-quantitative (wt.%) Ni 8.7 8.9 Cr 17.6 17.7 Mn 0.66 0.88 Mo 2.21 2.22 Si 0.5 0.5 Fe balance 69.65 These BCS/SS austenitic stainless steel standards were from the 1977 series. *BCS (British Chemical Standard); **SS (Spectroscopic Standard)
Copyright JCPDS-International Centre for Diffraction Data 2016 ISSN 1097-0002 92 Table 4 shows the results of the semi-quantitative analysis performed with single spot measurements on the NWA 2086 meteorite. The normalization factor is also reported for each measurement in Table 4. A normalization factor that is close to 1 confirms that the accuracy of the semi-quantitative analysis is good. Table 4. Semi-quantitative analysis results of NWA 2086 Spot No. 1 2 3 4 5 6 Compound (wt%) Matrix CAI Chondrule Chondrule CAI Chondrule SiO 2 33.7 36.9 34.9 32.8 34.1 33.0 Al 2 O 3 3.1 24.2 4.1 4.0 19.3 4.4 P 2 O 5 0.31 0.06 0.27 0.17 0.13 0.13 SO 3 0.48 0.41 0.34 0.44 0.62 0.51 Cl 0.01 0.36 0.06 0.09 0.10 0.03 Na 2 O 0.43 2.9 0.48 0.15 2.4 0.21 MgO 25.8 12.6 34.5 24.9 17.9 24.6 K 2 O 0.08 0.12 0.07 0.09 0.24 0.05 CaO 1.85 13.21 1.90 2.32 6.92 2.29 TiO 2 0.18 0.49 0.18 0.21 0.50 0.13 Cr 2 O 3 0.66 0.22 0.58 0.59 0.29 0.49 Mn 2 O 3 0.23 0.07 0.25 0.24 0.16 0.21 Fe 2 O 3 * 32.7 7.5 22.1 33.5 15.9 33.5 NiO 0.47 0.94 0.32 0.56 1.39 0.44 ZnO 0.02 0.09 0.01 0.02 0.06 0.02 ZrO 2 0.004 0.004 0.003 0.004 0.01 0.004 Norm factor 1.02 1.04 0.97 1.01 1.02 0.99 *Fe 2 O 3 can be mixture of FeO and Fe 2 O 3
Copyright JCPDS-International Centre for Diffraction Data 2016 ISSN 1097-0002 93 X-ray diffraction results For phase identification by X-ray diffraction conventional measurements in Bragg-Brentano geometry of the complete slice as well as of separated materials from a CAI (Figure 8, including Rietveld quantification), chondrule and matrix were done. The matrix and the chondrules are composed of olivine, (Mg,Fe) 2 SiO 4, and Ca-poor pyroxene, (Mg,Fe,Ca)Si 2 O 6. The CAIs are comprised of Ca-rich pyroxene (diopside), CaMgSi 2 O 6, spinel, MgAl 2 O 4, anorthite, CaAl 2 Si 2 O 8, and sodalite, Na 8 (Al 6 Si 6 O 24 )Cl 2. Figure 8. Powder diffraction measurement of CAI material and Rietveld quantification The mapping of the minerals showed that the matrix is nearly homogenous. Chondrules and CAIs are clearly separated from the matrix, and some structures within them like rims or a zonation are visible. The large, rounded CAI with the rim shows clear mineralogical differences between rim and core (Figure 9) and is confirmed by the elemental analysis. Spinel is clearly enriched in the core and outer parts of the rim of the large, rounded CAI, compared to the inner parts of the rim, whereas in other CAIs with irregular shape the spinel is more homogeneously distributed. Sodalite shows a similar distribution like the spinel. The large, incomplete CAI with a more irregular shape (indicated by spot no. 5 in Figure 5) contains less sodalite than the large rounded one. For pyroxene regions, the separation between the core and rim is not so pronounced, higher amounts can be observed in the rim and the inner part of the CAI core. Internal structures are also visible in some larger chondrules. High intensity signals for olivine and spinel at some spots indicate larger single crystals, whereas dark chondrules in
Copyright JCPDS-International Centre for Diffraction Data 2016 ISSN 1097-0002 94 Figure 9 can indicate larger crystals in a non-diffracting orientation or the absence of olivine or pyroxene. Olivine (Mg,Fe) 2 SiO 4 Pyroxene (Mg,Fe,Ca)Si 2 O 6 CaMgSi 2 O 6 Spinel MgAl 2 O 4 Sodalite Na 8 (Al 6 Si 6 O 24 )Cl 2 Figure 9. Microspot XRD mapping 2D diffraction patterns at two positions, in the center of the large CAI (Figure 10) and in the inner, lighter part of the rim (Figure 11), confirmed the results of the phase identification and microdiffraction. Spotty diffraction rings, especially for sodalite, spinel and clinopyroxene, indicate coarse-grained intergrowth of the minerals. A Rietveld quantification indicate higher sodalite content in the core and higher pyroxene and anorthite content in the lighter part of the rim (Fig. 12). Figure 10. 2D diffraction pattern on a spot in the core of the CAI.
Copyright JCPDS-International Centre for Diffraction Data 2016 ISSN 1097-0002 95 Figure 11. 2D diffraction pattern on a spot in the lighter, inner part of the rim of the CAI. Figure 12. Quantification of the 2D diffraction patterns. Combined XRF and XRD results Combining XRF and XRD techniques show that the rim and the core of the CAIs are of different mineralogy and chemistry. For the XRF elemental mapping, this can be clearly seen on the elements Ca, Si, Ti, and Cl in Figure 6. These elements are prominent in the rim of the CAIs. Similar results can be observed from Figure 9 of the XRD phase mapping. The rim is mainly composed of anorthite, pyroxene and sodalite. The presence of sodalite has only been reported a few times for CAIs. This is of special interest because sodalite is regarded as a product of Na-Cl metasomatic processes which occurred in the early solar nebula before accretion into protoplanets or is a product of aqueous alteration on the
Copyright JCPDS-International Centre for Diffraction Data 2016 ISSN 1097-0002 96 parent body (Wasserburg et al., 2011; Ross et al., 2012). Both techniques also show that the core of the CAIs is a fine-grained mixture of spinel, diopside, and sodalite. It was observed that there is an enrichment of Ni and Zn in the CAIs which are probably present as minor and trace elements in the spinel. XRF and XRD also show correlating analysis results on chondrules which are mostly composed of olivine, (Mg,Fe) 2 SiO 4 and pyroxene, (Mg,Fe,Ca)Si 2 O 6. This can be seen on the XRF semiquantitative result of spots 3, 4, and 6 shown in Table 4. Spot number 3 shows a Mg rich olivine. Results also show an enrichment of Mn and Cr in the in chondrules and matrix. Manganese is probably a trace in the olivine and chromium in the pyroxene. CONCLUSION This study shows that the Zetium wavelength dispersive spectrometer equipped with an ED and small spot mapping functionality can perform practical elemental mapping in the analysis of meteorites. Combining ED with the small-spot optics provides a mapping capability that can distinguish important compositional variations in a relatively short period of time, with minimum effort and without calibration setup. The XRD microdiffraction analysis was demonstrated using a PANalytical Empyrean equipped with focusing lens, allowing mapping of 200 x 200 spots in 6 hours. Qualitative XRF mapping of the meteorite confirms the enrichment of Ca and Al in the inclusion region and elements such as Fe and Cr enriched in the meteorite matrix. Combined XRF semi-quantitative and XRD phase identification analyses show correlating results on the mineralogy of CAIs, chondrules, and the matrix of the meteorite. The analyses also show the uncommon occurrence of sodalite in CAIs. Combined XRF and XRD provide elemental composition and mineral distribution in a meteorite. The two technologies used in this study are complementary, and contribute to the identification of distinct features in the CAIs and chondrules. This information is essential to investigate early processes which occurred in the solar system. These combined methods provide an easy, rapid and nondestructive approach in analyzing extraterrestrial materials. REFERENCES Ross, D.K.; Simon, J.I.; Simon, S.B. & Grossmann, L. (2012): Ca-Fe and Alkali-Halide Alteration of an Allende Type B CAI: Aqueous Alteration in Nebular or Asteroidal Settings? 43rd Lunar and Planetary Science Conference, LPI Contribution No. 1659, id.2466.
Copyright JCPDS-International Centre for Diffraction Data 2016 ISSN 1097-0002 97 Russel, S.S.; Zolensky, M.; Righter, K.; Folco, L.; Jones, R.; Connolly, H.C.; Grady, M.M. & Grossmann, J.N. (2005): The Meteoritical Bulletin, No. 89, 2005 September. Meteoritics & Planetary Science 40, Nr 9, Supplement, A201 A263. Wasserburg, G.J.; Hutcheon, I.D.; Aléon, J.; Ramon, E.C.; Krot, A.N.; Nagashima, K. & Brearley, A.J. (2011): Extremely Na- and Cl-rich chondrule from the CV3 carbonaceous chondrite Allende. Geochimica et Cosmochimica Acta 75, 4752-4770.