OIBODIEs: advancement on mineralogical and geochemical analysis on ungrouped achondrites

Meteorites, together with return mission samples, represent the unique material to investigate Solar System bodies. Among meteorites, ungrouped achondrites represent natural sampling of ancient, from fairly to highly, differentiated bodies and provide fundamental information about their genesis and the processes associated with the origin of our Solar System. These extra-terrestrial bodies (i.e., asteroids and comets) are usually investigated through indirect analytical methods allowing the characterisation of many of them but without achieving the high-precision level provided by direct analysis. Within the scope of the “Olivine-bearing ungrouped achondrites and their parent bodies in the Solar System” (OlBODIEs, ASI-INAF) Project, we selected several ungrouped achondrites aiming to highlight the most prominent aspects related to their classification, combining multidisciplinary mineralogical-petrographic- geochemical-isotopic-spectroscopical approaches. We present preliminary results based on mineralogical, petrographic, geochemical and isotopic characterisation of achondrite meteorites to provide a detailed characterisation of the samples. We selected a set of ungrouped achondrites with brachinites affinity, bearing variable amount of olivine and other mineralogical phases (e.g., pyroxene, graphite, spinel). Eight samples (Al Huwaysah 010, the paired MIL 090206-090405, the paired NWA 5363-5400-6077-6292, and NWA 6112) were investigated through Scanning Electron Microscope and Electron Microprobe (EMPA) to determine the mineral chemistry of major (olivine, pyroxene and plagioclase) and minor phases (Fe-Ni alloys, oxides, sulphides, phosphates), while EBSD analyses are planned to investigate olivine petrofabric. The presence of secondary phosphates in all NWA 5400 paired samples (Cl-apatite replaced by Fe-phosphates) points to Fe-rich fluid circulation on their parent body, while the Fe-sulphide + Fe-Ni + orthopyroxene fine-grained assemblage in olivine rims resembling ureilite reduction rims in the other 4 samples (and absent in the 5400 group) is probably due to reduction of iron in olivine according to the reaction Mg2SiO4 + Fe2SiO4 + C = 2(MgSiO3) + 2Fe + CO2. These two different paragenesis mark a strong dichotomy within this brachinite-like meteorite group. We combine mineralogical and petrographic determinations along with geochemical (by ICP-MS) and isotopic analyses (by TIMS) performed on bulk samples and, as a future application, to specific mineralogical phases. In this light, we are also developing a high-precision measurement procedure for Cr isotope compositions (i.e., e53Cr and e54Cr). These will be coupled with other methods (e.g., reflectance spectroscopy, oxygen isotopes) to infer about the parent body nature (if a single compositionally heterogeneous source or multiple sources) and to reinforce the confidence of association of the studied samples with their parent body family.