Can Mӧssbauer methods of classifying ordinary chondrites help to identify non-representative samples of these meteorites?

Mössbauer spectra of nonweathered ordinary chondrites consist of four main mineral phases: olivines, pyroxenes, metallic phase and troilite. These minerals represent more than 95% of the whole mass of an ordinary chondrite. Distribution of these mineral phases in micro-scale is not homogeneous. Nevertheless, preparation of representative sample of ordinary chondrite for Mössbauer measurements is possible. To do that a part of 1 g nonweathered material, selected from inside of meteorite without any specific intention is needed. The Warsaw group has been working on investigation of meteorites for 25 years and has analysed about 150 Mössbauer spectra of various meteorites. Among them we found 15 spectra, which could be suspected of being non-representative. These spectra were obtained from Baszkówka, Amber, Bjurböle, Krasnoi-Ugol and Chelyabinsk meteorites. The analysis of how the samples of meteorites were selected for investigation, has shown that the non-representativeness of samples may be due to: intentional choice of sample, preparation of sample from a too small part of material or the use of non-credible source of meteoritic samples. For confirmation of these assumptions, we used a new method of classification of ordinary chondrites – the 4M method. It turned out that this method is a very useful tool for investigation of non-representative samples of equilibrated ordinary chondrites.

New Evidence for Wet Accretion of Inner Solar System Planetesimals from Meteorites Chelyabinsk and Benenitra

We investigated the hydrogen isotopic compositions and water contents of pyroxenes in two recent ordinary chondrite falls, namely, Chelyabinsk (2013 fall) and Benenitra (2018 fall), and compared them to three ordinary chondrite Antarctic finds, namely, Graves Nunataks GRA 06179, Larkman Nunatak LAR 12241, and Dominion Range DOM 10035. The pyroxene minerals in Benenitra and Chelyabinsk are hydrated (∼0.018–0.087 wt.% H2O) and show D-poor isotopic signatures (δDSMOW from −444‰ to −49‰). On the contrary, the ordinary chondrite finds exhibit evidence of terrestrial contamination with elevated water contents (∼0.039–0.174 wt.%) and δDSMOW values (from −199‰ to −14‰). We evaluated several small parent-body processes that are likely to alter the measured compositions in Benenitra and Chelyabinsk and inferred that water loss in S-type planetesimals is minimal during thermal metamorphism. Benenitra and Chelyabinsk hydrogen compositions reflect a mixed component of D-poor nebular hydrogen and water from the D-rich mesostases. A total of 45%–95% of water in the minerals characterized by low δDSMOW values was contributed by nebular hydrogen. S-type asteroids dominantly composed of nominally anhydrous minerals can hold 254–518 ppm of water. Addition of a nebular water component to nominally dry inner solar system bodies during accretion suggests a reduced need of volatile delivery to the terrestrial planets during late accretion.