Abstract

Introduction: Five ordinary chondrites collected in the El Médano dense collection area (DCA) of the Atacama desert, Antofagasta region, northern Chile, were classified using QEMSCAN® (Quantitative Evaluation of Minerals by Scanning Electron Microscopy), consisting of a Tescan Vega LHS Scanning Electron Microscope (SEM) combined with 4 Bruker XFlash 14010 Silicon Drift detectors (SDD), along with optical miscroscopy petrography, magnetic susceptibility and electron microprobe analysis (EMPA) on mineral chemistry. The use of this tool helps in the fast classification of chondrites, using a common technique for the mining industry in countries without access to microprobes. This technique also allows the quantification of terrestrial weathering by the general detection of alteration phases, although the exact identification of them still is part of undergoing studies. In this way, it is possible to complement the petrographic description and quantify the W grade definided by Wlotzka (1993)[1], and adding the silicates weathering as well, a parameter not included in Wlotzka classification. Methodology: The ordinary chondrites samples (DM-028, DM-40, DM-46, DM-132 and DM-145) were found during the 2017 Atacama Desert Meteorite Expedition carried out by the Chilean-French team, lead by MAS (Chile) and CEREGE (France). They were classified by petrographycal descriptions using optical microscopy, magnetic susceptibility using the hand sample device AtacaMet, developed by CEREGE, and, only for samples DM-028 and DM-145, mineralogical maps analysis using QEMSCAN®, at the University of Concepción, Chile. This was done after 5 samples already classified by microproble were classified correctly by QEMSCAN® [2]. Only one sample – DM-028 – was analyzed by microprobe at the NHM of London. Petrographic analysis of the five samples was carried out in 5 thin sections and 5 polished briquettes, allowing identification of textures and main constituents, evidences of shock and terrestrial weathering and mineral chemistry. In the case of samples DM-028 and DM-145, the optical petrography results were combined with field-scan images of the polished briquettes. Preliminary results: The analyzed samples were classified as follows: DM-028: H-L4/S3/W3, DM-40: H5/S3/W2, DM-46: L5/S3/W2, DM-132: L5/S3/W2-3 and DM-145: L-LL6/S3/W2-3. Classification was based on petrographic and magnetic susceptibility analysis, along with SEM and EMPA when possibly. The combined results allow the reassessment of the degree of terrestrial weathering, achieving a more accurate classification of the chon- drites. The 3 methodologies applied for the petrological and chemical classification yielded similar results but with a variability that depends on the technique used. In the case of sample DM-028, the estimated Fa-Fs (fayalite- ferrosilite) compositions of phenocrysts was similar to the SEM and EMP analysis, both methodologies yielding an intermediate chemistry between H and L. Discussion and conclusions: Complementing petrographic analysis with semi-automated techniques (QEMSCAN®) allows the description of textures and components, and subsequent classification, much faster and more accurately. In addition, it enables the detection and quantification of phases that are not easy to inspect under the optical microscope. However, it is necessary to review the limitations of the method and compare it with the work of other researchers who develop the methodology in the ordinary chondrites found at the Atacama Desert. Special attention is necessary to confirm the precision accuracy of semi-quantitative techniques for obtaining mineral chemistry in both equilibrated and non-equilibrated chondrites. References: [1] Wlotzka, F. (1993). A weathering scale for the ordinary chondrites. Meteoritics, 28; [2] Valenzuela M. et al. 2021 Abstract for the 84th Metsoc Meeting, Chicago, USA. Acknowledgements: FONDECYT project No11171090.