Geochemical characteristics of a newly discovered Vigarano chondrite fragment (CV3) - Study of redox conditions with unconventional Mössbauer spectroscopy

The Vigarano meteorite fell in 1910 in the province of Ferrara, Italy, and, after the impact, was divided into two blocks, “Cariani” and “Morandi”, respectively, named after the owners of the farms where they were recovered (Trevisani, 2011 and reference therein). A new series of debris was found in a storehouse near the impact site at the beginning of the XXI century and was classified as part of the Cariani stone (Trevisani, 2011). This meteorite has been widely studied and represents the reference type of the reduced CV3 carbonaceous chondrite group (McSween, 1977). Notwithstanding it is worth noticing that the main contribution to the nature and origin of this meteorite is from fragments of the Morandi block, while knowledge on the other block is scarce. This work aimed to characterize one of these newly discovered fragments to confirm its belonging to the Vigarano meteorite, with both bulk (XRF, and ICP-MS), in-situ (EMPA) major and trace element contents, and C-S elemental and isotopic (EA-IRMS) analyses. EMPA and petrographic thin section analyses did not identify phyllosilicates and carbonates, which were determined in the literature dealing with the Morandi block (McSween, 1977). Therefore, additional Mössbauer spectroscopy analyses were conducted to investigate iron speciation and possibly to determine the formation system’s redox conditions and volatile elements circulation in the planetary body. Both XRF and ICP-MS analyses showed a composition in agreement with the literature data (McSween, 1977; Dauphas & Pourmand, 2015), confirming at first, that the recovered unnamed meteorite fragments belonged to the Vigarano meteorite, even though the fragment had no CAIs evidence, and, for major elements, a higher Mg/Si (1.43 versus 0.94) and Fe/Si (1.59 versus 1.46) and lower Ca/Al rate (0.71 versus 1.09). Slightly differences between the two blocks were also observed for trace element contents: the newly found fragment had a lower Eu (0.09 versus 0.11 ppm), Gd (0.29 versus 0.40 ppm), Dy (0.40 versus 0.53 ppm), Er (0.25 versus 0.30 ppm), Lu (0.04 versus 0.05 ppm) contents than what found in the well-studied Morandi block (Dauphas & Pourmand, 2015). Due to a lack of carbonates and less sulfides presence, carbon and sulfur elemental contents (0.92±0.05, 1.38±0.05 wt.%) and isotopic ratios (δ13CPDB: -17.8 PDB, δ34SCDT: +0.3 CDT) were lower compared with literature CV3 values (e.g., McSween, 1977). The iron speciation determined with Mössbauer analyses led to an Iron Oxidation Index (IOI), whose results increase with increasing oxidized iron to the total iron content, with a value of 2.13, consistent with the 2.12 shown in the classification of reduced CV3 group (Garenne et al., 2019). The sample contained 17% Fe3+ , attributable to incipient thermal metamorphism that tends to form magnetite and oxidize Fe in silicates and matrices. The fragment is more oxidized when comparing the IOI with the literature value (Garenne et al., 2019). Dauphas N. & Pourmand A. (2015) - Thulium anomalies and rare earth element patterns in meteorites and Earth: Nebular fractionation and the nugget effect. Geochim. Cosmochim. Ac., 163, 234-261, https://doi.org/10.1016/j. gca.2015.03.037. Garenne A. et al. (2019) - The iron record of asteroidal processes in carbonaceous chondrites. Meteorit. Planet. Sci., 54, 2652-2665, https://doi.org/10.1111/maps.13377. McSween H.Y. JR. (1977) - Petrographic variations among carbonaceous chondrites of the Vigarano type. Geochim. Cosmochim. Ac., 41, 1777-1790, https://doi.org/10.1016/0016-7037(77)90210-1. Trevisani E. (2011) - History of the Vigarano meteorite (Emilia-Romagna, Italy) and recovery of an important part of their mass. Rend. Lincei Sci. Fis., 22, 315-326, https://doi.org/10.1007//s12210-011-0137-1.