Investigation of O-isotopes in Na-Al-rich chondrules and Ca,Al-rich inclusions from ordinary chondrites
Final Report Abstract
Zusammenfassung Allgemeinverständliche Darstellung der wichtigsten wissenschaftlichen Fortschritte und ggf. ihrer Anwendungsaspekte. It is assumed that the first particles generated in our solar system were formed in a short period of time close to the young Sun and were then distributed throughout the whole early solar disk. The so-called refractory inclusions were accumulated in every group of chondritic meteorites, which sampled material from the first million years of our solar system. Each chondritic meteorite group represents a distinct parent body which sampled a distinct region in the early solar disc. Therefore, their refractory inclusions consist of various different mineral phases which can be investigated to decipher their formation history and therefore the composition, the transportation processes, and evolution of the early solar disc. Previous investigations revealed different oxygen isotopic compositions in different mineral phases within a single refractory inclusion. One explanation was that during their condensation from the solar gas, the refractory inclusions had been transported through regions with different isotopic compositions. However, some of the mineral phases which show oxygen isotopic differences were formed nearly contemporaneously, which implies a very fast transportation through the solar nebula during their formation, which can hardly be explained. My new work reveals that oxygen isotopic differences within a single refractory object can be caused by later alteration on their parent bodies even at low temperatures. An O-isotopically heterogeneous formation region is not necessary to explain their O- isotopic differences. Neither is a fast and hardly explainable transportation between regions of different oxygen isotopic compositions during their formation. This finding strongly contributes to the modelling of the formation and evolution of our solar system. In the second part of this project, I investigated the connection between these refractory inclusions and (Na),Al-rich chondrules. Chondrules are defined as Si-rich spherical droplets within chondritic meteorites which were formed by the (so far unknown) chondrule formation process at very high temperatures up to 1700-1800 °K. Chondrules are one of the main constituents of chondritic meteorites. As refractory inclusions were the first material formed in our solar system and the majority of chondrules were formed 1-3 million years later, the question arose to what extent they are connected, and whether refractory inclusions were molten during the chondrule formation process and formed some of the chondrules. To find a possible link between these two major components, I investigated the rare Al-rich chondrules which are a subgroup of the so-called “normal” chondrules. Al-rich chondrules are enriched in Al, Ca, and Ti and so are refractory inclusions. Because of that, it was suggested that refractory material (mixed with Si- and Fe-rich material) had been reworked within these chondrules. To gain more information about the connection and the transportation history of refractory inclusions, I investigated Al-rich chondrules from two different groups of chondrites, the ordinary chondrites and the CO chondrites. If these chondrules are also enriched in sodium, they are called Na-Al-rich chondrules. As mentioned before, each chondrite group represents a parent body that sampled its own distinct region in the solar disc. It is assumed that ordinary chondrites sampled material from a region in the inner solar disc (between Sun and Jupiter) and the CO chondrites a region beyond Jupiter. Combined with my previous work about Ti-isotopes in (Na),Al-rich chondrules (Ebert et al., 2017a, 2018a), the oxygen isotope investigation revealed that the refractory inclusions were reworked to (Na),Al-rich chondrules in CO chondrites. This implies that the refractory inclusions were present during the chondrule formation process in the outer part of the solar disc. However, the (Na),Al-rich chondrules from ordinary chondrites show many isotopic and mineralogical similarities with the investigated refractory material and (Na)Al-rich chondrules from CO chondrites, but also isotopic differences. This finding is the most interesting: The refractory material for the (Na),Al-rich chondrules in ordinary chondrites (inner part of the disc) was slightly different from the refractory material in (Na),Al-rich chondrules from CO chondrites (outer part of the disc). A high amount of refractory material from various chondrites from the outer region has been investigated, and none was found which would fit the isotopic fingerprint of the (Na),Al-rich chondrules from the inner part. On the other hand, refractory material which isotopically fits the (Na),Al-rich chondrules from the CO chondrites is present in the ordinary chondrites. A consequence of these results is that a (so far unknown) refractory material group was exclusively present in the inner solar disc and not in the outer parts. Furthermore, in order to explain the isotopic differences, refractory material close to the young Sun had to be generated over a longer period of time (maybe more than 500k years) and not only in a short time interval (10-20k years). This project successfully provided new and highly relevant results for the understanding of the evolution and transportation processes in our early solar disk. Previous models about the transportation of refractory material (e.g. the X-wind model) described a process where the refractory material (from the area around Sun) was transported outside of the disc and then “fell” back into the disc, where it was accreted by different parent bodies. However, this model was not able to explain the observed distribution of refractory material within different chondrite groups and it was therefore challenged by a new model about a viscous expanding disc were the refractory material was transported within an expanding disc. My new results strongly support the new model about an expanding disc, because transportation of material beyond the disc cannot explain the exclusive presence of distinct refractory material groups in the inner solar disc. On the other hand, the expanding model even predicts the presence of different refractory material groups in distinct regions of the early solar disc.
Publications
- (2019) (Na-)Al-rich chondrules and Ca,Al-rich inclusions from unequilibrated ordinary chondrites. Meteoritics & Planetary Science 54, S2, #6250 (abstract)
Ebert S., Bischoff A., and Krot A. N.
- (2020) Oxygen-isotope heterogeneity in the Northwest Africa 3358 (H31.) refractory inclusions – fluidassisted isotopic exchange on the H-chondritic parent body. Geochimica et Cosmochimica Acta 282, 98-112
Ebert S., Nagashima K., Krot A. N., and Bischoff A.
(See online at https://doi.org/10.1016/j.gca.2020.05.012)
