High-Resolution Spectroscopy Analysis of the Lunar Meteorite NWA 10495: Comparative Study of Fusion Crust and inner zone Alterations

The fusion crust of meteorites is a thin, thermally altered surface layer formed during atmospheric entry. In stony meteorites, this layer typically comprises olivine, silicate glass, wüstite, and other iron oxides from the magnetite series, and generally does not exceed 1–2 mm in thickness. The meteorite NWA 10495, discovered in southern Morocco in 2015, is classified as a lunar feldspathic breccia. This specimen comprises several brown-gray stones with a total mass of 15.6 kg and according to the Meteoritical Society, it lacks a fusion crust. All pieces display a fine-grained texture, with larger white clasts visible within a dark gray matrix [1,2]. For this study, 2 grams of the NWA 10495 meteorite were analyzed to perform a multianalytical characterization of its primary mineral phases and secondary alteration products, aiming to determine the presence or absence of a fusion crust. For elemental characterization, a Bruker M4 TORNADO (Bruker Nano GmbH, Germany) X-ray fluorescence spectrometer was used. For the molecular characterization, the equipment used was an inVia confocal micro-Raman instrument (Renishaw, UK), provided with 532, 633 and 785 nm excitation lasers and a Peltier-cooled charge-coupled device detector. The instrument is coupled to a Leica DMLM microscope (Bradford, UK) and has different lenses (5x, 20x, 50x and 100x) for the visualization and focusing of the sample. To obtain Raman images, the high-resolution StreamLine technology (Renishaw) and the map image acquisition tools were used. The measurement was made using 532 and 785 nm excitation laser with 5% laser power and 50x or 100x lenses. The measurement conditions were 5–15 s of exposure time, 1 or 2 accumulations and a spatial resolution of 0.5–0.75 μm depending on the laser and the objective used. The results obtained through µ-EDXRF (Figure. 1) have shown that the outer part of the meteorite exhibits a significantly higher silicon (Si) content when compared to the data from the inner parts of the meteorite, where the Si content decreases and is replaced by other elements, such as calcium (Ca) and iron (Fe), which become more abundant.

Figure. 1 µ-EDXRF results obtained from the inner part (upper)  and the outer part (bottom) of the meteorite NWA 10495.

Regarding the results obtained through Raman spectroscopy, it was difficult to assign specific bands in the outer regions of the meteorite due to the high fluorescence associated with each spectrum. This is a common occurrence in the analysis of volcanic glasses or fusion crusts. Nonetheless, some main mineral phases such us piroxene and secondary alteration phases are identifiable, such as calcite (CaCO₃), which is the main alteration mineral observed in NWA 10495.

The inner area of the meteorite, in contrast, exhibits a wide mineralogical diversity. As shown in Figure 2, calcite stands out as a prominent phase. Although it is presumably a secondary alteration product and not native to the lunar surface, its distribution throughout the meteorite is evident, as nearly all the analyzed spectra show characteristic signals of this mineral at 153, 281, 714, and 1086 cm⁻¹. Another secondary alteration phase identified is barite (BaSO₄), characterized by its main Raman band at 988 cm⁻¹. Regarding the primary mineralogy, plagioclase ((Na,Ca)(Si,Al)₃O₈) with main bands at 486, 504 cm^-1  is notably abundant, which is consistent with the classification of the sample as a feldspathic breccia. Other commonly observed minerals include Ti-rich hematite, Fe₂O₃ (672 cm^-1), augite-type pyroxene ((Ca,Mg,Fe)₂(Si,Al)₂O₆), and olivine ((Mg,Fe)₂SiO₄).

Figure. 2 Raman spectroscopy results obtained in the inner part of the meteorite

It seems obvious that at least this small fragment of the meteorite has been affected by terrestrial alteration processes, as evidenced by the detection of calcite or barite within it. However, if we look at Figure 3, although there are concentrations or hotspots of elements that could be associated with further alteration—such as Cl or Mn—others, such as Fe, Ti, Si, or K, are more likely related to a fusion crust.

Figure. 3 µ-EDXRF image highlighting the signal intensity of elements such as Si, Fe, Mn, Cl, K, and Ti in the outer crust of the meteorite

Potassium is particularly interesting in this context because the heterogeneous enrichment in potassium (on the exterior and not the interior), together with the detection of shifted feldspar bands, stands as strong evidence for the presence of  high-pressure feldspar phases such as maskelynite. The formation of maskelynite  (Figure. 4) is linked to melting followed by rapid quenching under high pressure. It is important to note that when maskelynite coexists with any crystalline phases in the area sampled by the excitation laser beam, the Raman signal from maskelynite tends to be masked (Figure. 4) by the spectral background of the crystalline phases and is difficult to extract [3].

Figure. 4 Image obtained through Raman spectroscopy highlighting the presence of feldspars altered under high pressure and temperature.

In conclusion, the results suggest that an external fusion crust may indeed be present on the NWA 10495 meteorite, contrary to previous assumptions. Raman spectroscopy and µ-EDXRF analyses have revealed clear mineralogical differences between the interior and exterior surfaces. These include the identification of high-pressure, high-temperature minerals typically associated with fusion processes, as well as elevated concentrations of elements such as Si and K, commonly found in glassy materials within fusion crusts of other meteorites.

Acknowledgements: Work supported through the PAMMAT project (Grant No. PID2022-142750OB-I00), funded by the Spanish Agency for Research (through the Spanish Ministry of Science and Innovation, MCIN, and the European Regional Development Fund, FEDER).

 

References:

[1] Korotev, R. L., & Irving, A. J. (2021). Lunar meteorites from northern Africa. Meteoritics & Planetary Science, 56(2), 206-240.

[2] Bouvier, A., Gattacceca, J., Grossman, J., & Metzler, K. (2017). The meteoritical bulletin, No. 105. Meteoritics & Planetary Science, 52(11), 2411-2411.

[3] Wang, A., Kuebler, K., Jolliff, B., & Haskin, L. A. (2004). Mineralogy of a Martian meteorite as determined by Raman spectroscopy. Journal of Raman Spectroscopy, 35(6), 504-514.