Abstract

Introduction: Atacama Desert, located in Northern Chile, South America, has proved to be an excellent place for meteorites to accumulate due to the long lasting hyper arid conditions of its old surfaces (~25 Ma [1]). For this reason, too many foreign expeditions and a few national ones, have been searching for meteorites there since 2017, after the discovery of some of the highest meteorite densities per km2 on Earth [2], and the datation of meteorite populations as old as having an average terrestrial age of ~710 ky [3]. One of the problems that Chilean meteoricists are facing, is the lack of laboratories with microprobes to perform the standard classification of meteorites samples by themselves, reason why most foreign expeditions take the whole amount of finds to their own countries, giving just little samples to the Chilean repositories. In the other hand, as mining activity is the main economic domain, exist other techniques to study ore deposits that are easily available, including Qemscan® (Quantitative Evaluation of Minerals by Scanning Electron Microscopy), a great technique that combines an image analysis system that measures mineralogical variability based on chemistry at micrometric scale. The technique generates a particle map with semi-quantitative non-volatile element abundances from which the modal abundance and occurrence of mineral phases are estimated. Methodology: Qemscan® equipments used in this preliminary study include a ZEISS EVO 50 Scanning Electron Microscope (SEM) combined with Bruker Series 4 energy dispersive spectrometer (EDS) detectors at Mainí-UCN facility in Antofagasta, and Tescan Vega LSH (SEM) combined with 4 Bruker XFlash 14010 Silicon Drift Detectors at GEA-UDEC, in Concepción. First Results: Ten ordinary chondrites have been analyzed so far with Qemscan®, from where semi- quantitative chemical compositions for most phases have been estimated. Five samples have already been classified by microprobe data (Paposo LL6; San Juan 086 L6 breccia; Pampa (a) L6; Los Vientos 357 H6; Taltal 001 H3, clasification taken from [4]), so they have helped to calibrate this new method. From the modal abundances of the minerals identified (olivine, pyroxenes, plagioclase, Fe-Ni, troilite, chromite, apatite, zircon, carbonates and other), each of them represented in false color in a map (Fig. 1), it was possible to obtain the chemical values of Fe0, total Fe (Fe) and Si calculated from the % of these elements in the modal mineralogy, enable to calculate the ratios Fe0/Fe and Fe/SiO2. All of them gave similar results for the chemical group, as well as the quantification of the different components, including the general weathering species, not only present as Fe oxi-hydroxides, but also as alteration minerals in silicates. Other five new ordinary chondrites were analyzed using the same methodology [5], once the comparison was successful, and many new discoveries are ready to be classified by this technique. References:[1] Dunai et al (2005) Oligocene–Miocene age of aridity in the Atacama Desert revealed by exposure dating of erosion-sensitive landforms. Geology 33, 321-324.; [2] Hutzler A. et al. 2016. Description of a very dense meteorite collection area in western Atacama: Insight into the long‐term composition of the meteorite flux to Earth. Meteoritics & Planetary Science 51, 468-482.. [3] Drouard A. et al. (2019) The meteorite flux of the past 2 my recorded in the Atacama Desert. Geology 47, 673-676.; [4] https://www.lpi.usra.edu/meteor/metbull.php [5] Salazar N. et al., 2021 Abstract for the 84th Metsoc Meeting, Chicago, USA. Acknowledgments: FONDECYT 11171090, Maini/UCN and GEA/UDEC Teams, private collectors R. Martinez, D. Ruiz.