Detailed study on ungrouped achondrites combining trace elements and Cr isotopes as geochemical tools

The direct investigation of Solar System bodies has always represented one of the most challenging topics for many scientific disciplines due to the relative difficulty to have access to materials directly fallen on the Earth’s surface and even more to the sample return missions. Among meteorites, ungrouped achondrites represent fundamental natural sampling of different parent bodies and provide fundamental information about their genesis, as well as the processes associated with the origin and the evolution of the Solar System. Indirect methods of analysis (e.g., using radar or spectroscopy), although allowing the characterisation of a large number of samples, usually are not able to achieve the high-precision level provided by direct analytical methods, fundamental for classification purposes. We present preliminary results of geochemical and isotopic composition on a significant set of samples of ungrouped achondrites, which have been investigated though a combined approach of direct (i.e., mineral chemistry) and indirect (i.e., reflectance spectroscopy) methods, yielding a partial attribution to specific meteorite groups and potential parental bodies. In order to further corroborate the evidence derived from these analyses, we performed high-precision analyses of bulk major and trace element abundances (by ICP-MS) and we set up the method for Cr isotope (53Cr/52Cr, 54Cr/52Cr) determination. Chromium isotopes systematic represents an important tool to discriminate different meteorite types by the presence of the heterogeneously distributed 54Cr nucleosynthetic anomalies and similar 53Cr excess. Few of the investigated samples, whose geochemical and isotopic composition were already available from previous studies, were used as reference to test the reproducibility of the methods. The same was done on other types of meteorites (such as Allende and some ordinary chondrites), for which we obtained promising correspondence that assessed the validity of the analytical methods. Along with meteorite samples we also test the reproducibility of the isotopic measurements in particular on the isotopic standard NIST979 and NIST3112a yielding good results. Therefore, the obtained results on ungrouped achondrites were compared to literature values to recognise the respective classification groups and thus further constrain the association of the studied samples with potential parent body families. Major and trace element compositions substantially confirmed the attribution provided by the preliminary analyses with few exceptions that are likely due to sample alteration. The Cr isotope analysis, which is still in progress, will provide the final attribution to the preliminary classification.