Micro-laser induced breakdown spectroscopy of an iron meteorite

The study has been conducted on two samples of the iron meteorite Dronino in the framework of a campaign aiming to validate planetary data on terrestrial and extra-terrestrial materials by portable techniques. The samples were studied by a portable double-pulse micro-laser-induced breakdown spectroscopy (DP-μLIBS) system coupled with optical microscopy [1]. The two fragments of Dronino have very different features, i.e. one sample is highly alterated, while the oth- er one does not show any superficial alteration but has sulphide inclusions rounded and elongated along the banding. Ac- cording to the Meteoritical Bulletin Database (MBD) Dronino meteorite belongs to an ungrouped ataxite meteorites and consists of kamacite (7.0 ± 0.5 wt% Ni and 0.75 wt% Co) and rare taenite (26.5 ± 0.5 wt% Ni and 0.35 wt% Co). The altered sample has been found in a swamp and suffered of marked weathering effects that modified the mineralogy and chemistry of the whole sample. The quantitative chemical composition measured by DP-μLIBS with One Point Calibration method [2] showed that the two samples were very different. The scanning electron microscopy (SEM) EDS analyses confirmed the DP- μLIBS results and showed that the unaltered slab consisted mainly of taenite (26.6 wt% Ni and 0.6 wt% Co), whereas kama- cite (8.2 wt% Ni and 1.1 wt% Co) was present in only one point. Differently, the altered sample consisted mainly of kamacite (6.2 wt% Ni and 1.02 wt% Co), as described in the MBD. The differences found between DP-μLIBS data and SEM results were ascribed to the heterogeneity of the sample and to the different sampling areas. In fact, the SEM chemical map showed a very fine grade texture of elongated precipitates (duplex structure) in the unaltered sample. In the altered sample DP- μLIBS indicated the presence of a Li peak on several spots, whereas Li was absent in the unaltered sample. In particular, Li is a light trace element not detectable by routinely techniques such as SEM EDS or X-ray fluorescence. The importance to evaluate Li both qualitatively and quantitatively has been already highlighted by previous studies made by ChemCam LIBS analysis on the Martian surface [3-4]. In fact, this element can provide evidence of alteration processes that might have af- fected extra-terrestrial rocks, i.e. the presence of Li in the meteorite sample would be related to the weathering processes due to aqueous solutions occurred in the swamp. [1] Senesi et al., 2016, Geostandards and Geoanalytical, Volume 40, Issue 4, Pages 533–541 [2] Cavalcanti G.H., Teixeira D.V., Legnaioli S., Lorenzetti G., Pardini L., Palleschi V., 2013, One point calibration for calibration-free laser-induced breakdown spectroscopy quantitative analysis, Spectrochim. Acta Part B, 87, 2013, 51–56. [3] Maurice et al., 2016, Journal of Analytical Atomic Spectrometry Volume 31 Number 4, Pages 823–1050 [4] Ollila et al., 2014, JGR-Planets, Volume 119, Issue 1, Pages 255–28