Meteorites are records of the processes which occured in the early stages of the formation of the solar system. What makes them interesting is that they typically preserve the original chemistry and mineral phases over billions of years since they typically do not undergo geological transformations in space. By studying their elemental composition and mineral phases a better understanding of how the solar system was formed can be obtained.
X -ray diffraction (XRD) is a common technique used to investigate phase composition of meteorites but does not provide elemental analysis. X-ray fluorescence (XRF) spectroscopy is a well-recognized technique used to determine the elemental composition of rocks.
Meteorites are records of the processes which occured in the early stages of the formation of the solar system. What makes them interesting is that they typically preserve the original chemistry and mineral phases over billions of years since they typically do not undergo geological transformations in space. By studying their elemental composition and mineral phases a better understanding of how the solar system was formed can be obtained.
X -ray diffraction (XRD) is a common technique used to investigate phase composition of meteorites but 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 complex surface structure of meteorites requires analysis on a much smaller spot size than traditional bulk XRF techniques. The latest advances in floor-standing XRF instruments allow small spot analysis and element distribution mapping on spot sizes of 0.5 mm (FWHM) in diameter.
The Northwest-Africa 2086 (NWA 2086) meteorite was the subject of this study and is shown in Figure 1. It was found in 2003 with a weight of 780 g. The meteorite was classified as a carbonaceous chondrite of the CV3 group. This group of meteoritic bodies is characterized by the presence of chondrules and calcium-aluminium rich
inclusions (CAIs) as shown in Figure 1 in a dark colored matrix which contains ~ 0.6 % of carbon and has 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 are 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 rarely up to 2 centimeters.
The chondrule formation started approximately at 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 can cross the path of the earth and fall as meteorites. Figure 2 shows the early processes in the solar system including the origin of meteorites from asteroids.
Figure 2. Schematic illustrating early processes in the solar system
In this study, small spot analysis and elemental distribution mapping was conducted using a Zetium XRF spectrometer. Incorporating SumXcore technology, a combination of WDXRF and EDXRF core, Zetium is capable of both bulk analysis and small spot mapping. Elemental compositions obtained by XRF were compared to phase identification and peak intensity mapping obtained using an Empyrean X-ray diffractometer.
X-ray fluorescence (XRF) analysis For small spot mapping analysis, measurements were performed using a Zetium XRF spectrometer equipped with a 4 kW, Rh-anode SST R-mAX tube, a high-performance ED core, and state-of- the-art SuperQ software.
The NWA 2086 meteorite was mounted on a special small spot sample holder and imaged using a high-resolution camera. A 5x7.5mm measurement area was defined using Zetium’s high resolution camera (Figure 3). The area was mapped using a spot size of 500 µm and a step size of 250 µm in about 12.3 hours and with a total of 600 measurement spots.
Figure 3. Photograph with dashed area selected for XRF small spot mapping analysis
Both qualitative (intensity-based) and semi-quantitative (concentration-based) analyses were performed to determine the elemental distribution and the composition of the NWA 2086 meteorite.
The semi-quantitative analysis was set up using Malvern Panalytical's Omnian setup samples for standardless analysis.
Table 1 shows the measurement conditions used for the analysis of this sample.
Table 1. Conditions for the qualitative analysis used for mapping the sample
Table 2. Conditions for the semi-quantitative analysis on selected sample spots
For phase identification, the complete surface was measured simultaneously by X-ray diffraction using Bragg- Brentano geometry. Additionally, small amounts of the matrix, chondrules and a CAI were prepared and measured separately on a silicon zero-background sample holder.
For the mapping of the minerals, the Empyrean diffractometer (CuKα radiation, 45 kV, 40 mA) was equipped with a programmable xyz stage and a focusing lens (high-intensity collimator, 300 micrometer pinhole). A PIXcel1D detector was used in 0D mode. For the intensity mapping, the 2theta positions of main peaks of different minerals (olivine, pyroxene, and spinel) were used. The spot size was ~ 50 µm. The total measurement time for a 10 x 10 mm area (with 200 x 200 spots) was around 6 hours.
Qualitative
With a relatively short measurement time of 60 seconds per spot, data for 15 elements were collected simultaneously The distribution of these elements is illustrated in Figure 4. Figure 5 shows the 3D intensity contour maps for Al and Fe elemental distributions.
It can be observed that the elemental composition of the CAIs are enriched in Al and Ca, while these elements are less abundant in the matrix and the chondrules of the sample. Conversely, the matrix of the sample is enriched in Fe and Cr, which are depleted in the inclusions. The detail of the enrichment of the rim in Si, Ca and Zn of the CAls is also visible. The results illustrate that even with short measurement times elemental mapping of the elements is possible and provides valuable information.
Figure 4. Small spot elemental mapping of the NWA 2086 meteorite
Figure 5. 3D intensity contour maps of Al and Fe
Semi-quantitative
Selected spots (shown in figure 6) representing CAIs (spots 2 and 5), chondrules (spots 3, 4 and 6), and the matrix (spot 1) were selected for semi-quantitative analysis . Since no certified reference materials were available for this meteorite, a certified steel sample was analyzed to investigate what the expected accuracy might be at this spot size. A comparison of the certified and measured concentration for the steel CRM (SS 466) shown in Table 3 shows excellent agreement giving confidence in the accuracy of the data at this spot size.
The chemical compositions of the defined measurement spots are reported as oxides normalized to 100% (Table 4). The normalization factors reported for these measurements are close to 1 (better than 4% relative), giving confidence in the accuracy of the Omnian analysis, even at this spot size and measurement time.
Table 3. Semi-quantitative analysis of SS 466
Figure 6. Selected spots for semi-quantitative analysis
Table 4. Semi-quantitative analysis results of NWA 2086
Phase identification by X-ray diffraction of the complete slice as well as of the separated materials, showed that the matrix and the chondrules are composed of olivine, (Mg,Fe)2SiO4, and Ca-poor pyroxene, (Mg,Fe,Ca)Si2O6. The CAIs comprise Ca-rich pyroxene (diopside), CaMgSi2O6, spinel, MgAl2O4, anorthite, CaAl2Si2O8, and sodalite, Na8(Al6Si6O24)Cl2.
The mapping of the minerals showed that the mineralogy of the matrix is relatively uniform. 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 has a clearly defined rim, which is mineralogically different from the core (Figures 7 and 8).
Figure 7. XRD spinel mapping
Spinel minerals are clearly enriched in the core of the large, rounded CAI, compared to the rim, whereas in other CAIs with irregular shape the spinel mineral is more homogeneously distributed. For pyroxene regions, the separation between the core and rim is not so pronounced with the distribution generally more irregular. Internal structures are also visible in some larger chondrules. High intensity signals for olivine and spinel at some spots indicate larger single crystals in a special orientation.
Figure 8. XRD pyroxene mapping
Figure 9. XRD olivine mapping
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 1. These elements are prominent in the rim of the CAIs. The same behavior can be observed from Figures 7 and 8 of the XRD phase mapping.
The rim is mainly composed of anorthithe 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.
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)2SiO4 and pyroxene, (Mg,Fe,Ca) Si2O6. This can be seen on the XRF semi-quantitative results 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.
This study shows that Malvern Panalytical’s Zetium equipped with small spot mapping functionality is capable of small spot analysis and elemental mapping of a meteoritic sample. More than 15 elements were mapped across the surface with 600 spots in around 12.3 hours, while the XRD micro-diffraction analysis was performed using an Empyrean equipped with focusing lens, allowing mapping of 40,000 spots in 6 hours.
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 identify the 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.
