Unlike most achondrites, which can usually be classified according to a common origin, NWA 6693 does not belong to a specific group. Several bubble-like tracks are present inside this meteorite, some of which consist of various mineral phases. Our study at ID16B aimed to gather high-resolution elemental information on these structures, allowing a further expansion of our knowledge on the origin, formation and history of the meteorite.

An overview scan of 100 x 100 μm2 was made to locate the bubble stream inside the fragment of NWA 6693, using a fast-scanning procedure (50 ms dwell time, 100 nm step size). Significant amounts of Ca, Ti, Cr, Mn, Fe, Ni and As were detected within the investigated area. Using the chromium elemental map, several regions of interest (ROI) were selected for detailed, high-resolution analysis (25-40 nm steps) with longer measurement time (1 s per point). Figure 16 shows the Cr overview map, combined with an RGB composite image from the respective Fe, Cr and Ni elemental distributions obtained from one of the high-resolution scans. As observed, the matrix of the meteorite fragment is mainly composed of Fe-rich phases (i.e. pyroxene and olivine), while most inclusions are rich in Cr with trace amounts of Ti and Mn, a minor fraction of the inclusions consists of Ni combined with As and Ca. The mere size of the inclusions demonstrates the need for nanoscopic analysis techniques in the study of this kind of samples.

XRF overview map (Cr) of NWA 6693 fragment

Fig. 16: XRF overview map (Cr) of NWA 6693 fragment  (100 x 100 μm2, 100 nm step size, 50 ms per point) and  Fe/Cr/Ni RGB image of indicated ROI (7 x 6 µm2, 40 nm step size, 1 s per point) beam dimensions for both scans:  46 nm (v) x 50 nm (h), excitation energy 17.5 keV.

Nano-XRF spectroscopy was also used in the study of diamond inclusions. During the formation and growth of diamonds, fluids, rock fragments and minerals can be trapped within, thus being shielded from the environment when the diamond is transported to the surface of the Earth. By conserving materials which are unattainable by other means, diamond inclusions give a unique insight into the interior of our planet. Recently, scientists identified a diamond inclusion as ringwoodite (i.e. a hydrous Fe phase), which was its first discovery in natural, terrestrial material. The presence of this hydrous phase in the interior of the Earth might play an important role in terrestrial magnetism and plate tectonics. Preliminary studies using Raman spectroscopy indicated the possible presence of ringwoodite in diamond SL05. Nano-XRF spectroscopy was employed to image the inclusions in SL05 at higher resolution to gain insights into the elemental distributions in a non-destructive manner allowing for future studies using multiple analytical methods.

The analysis of the diamond inclusions followed the same pattern as the study of the meteorite fragments. A 100 x 100 μm2 overview map (Figure 17) of the region where preliminary studies indicated the possible presence of ringwoodite was used to select a ROI for detailed investigation. As observed, significant amounts of Ca, Ti, Cr, Fe, Ni, Cu, Zn and As were found to be present within the area investigated. The distribution of some of these elements is represented in Figure 17, revealing a localised aggregation of Cu and Zn, representing elements that are not usually linked to materials stemming from deep Earth. Detailed analysis using PCA and K-means clustering indicated these elements are present in a single, relatively large particle. This could have been a secondary phase that adhered to the diamond during its voyage to the surface of the Earth. In contrast, the presence of Fe-rich particles is an important observation with respect to the hypothesised presence of a ringwoodite inclusion in SL05.

XRF overview map (Fe) of diamond SL05

Fig. 17: XRF overview map (Fe) of diamond SL05 (100 x 100 μm2, 100 nm step size, 50 ms per point) with element distributions of Ti, Fe, Ni, Cu and Zn in indicated ROI (7 x 6 µm2, 40 nm step size, 1 s per point) beam dimensions for both scans: 46 nm (v) x 50 nm (h), excitation energy 17.5 keV.

The study of these rare geological samples has clearly proven the potential and importance of nanoscopic XRF analysis as a non-destructive imaging tool yielding valuable and otherwise unreachable information to add to our knowledge of the planet and even the universe.

Principal publication and authors

Nanoscopic X-ray fluorescence imaging of meteoritic particles and diamond inclusions,  B. Laforce (a), S. Schmitz (b), B. Vekemans (a),  J. Rudloff (b), J. Garrevoet (a), R. Tucoulou (c),  F.E. Brenker (b), G. Martinez-Criado (c) and  L. Vincze (a), Analytical Chemistry 86, 12369-12374 (2015), doi: 10.1021/ac503764h.
(a) X-ray Microspectroscopy and Imaging Group (XMI), Ghent University (Belgium)
(b) GeoScience Institute, JWG University (Germany)
(c) ESRF