Elemental and triple oxygen isotope geochemistry of new Northwest Africa (NWA) ordinary chondrites (L5, L6): Implications for onion-shell to rubble-pile OC parent body structure: Implications for onion-shell to rubble-pile OC parent body structure

The new L-type ordinary chondrites (OCs) are characterized by fayalite (25.7–26.1 mol%) and ferrosillite (21.5–22.2 mol%) contents in olivine and low-Ca pyroxene, respectively. The Fe/Mg and Fe/Mn ratios in olivine, as well as oxygen isotope compositions in bulk samples, compared to those of other impact-related brecciated OCs (e.g., NWA 869, L3-6, Ghubara, L5) and olivine from Itokawa dust particles. The data demonstrate a correlation between Fe/Mg and Fe/Mn ratios in olivine, and comparability in oxygen isotope compositions between the newly analyzed L-type meteorites and various clasts of NWA 869 used as a proxy for rubble-pile asteroid. Furthermore, these ratios from bulk and olivine of equilibrated ordinary chondrites (EOC) derived from the literature data fall close to the co-variation line (1:1). However, the ratios of different fractions in unequilibrated ordinary chondrites (UOC) such as chondrules, clasts, and matrix, do not fall close to the co-variation line. The coupled anti- correlation trend observed between the bulk and olivine ratios in EOC (i.e., H, L, and LL) is primarily due to variations in the total Fe content caused by redox reactions of silicate to metal in the nebula and metal to silicate in the parent body. The proposed scenario for the formation and evolution of larger asteroids in the early solar system suggests that the they accreted and evolved as an onion-shell structure, and over time, they may have undergone internal heating due to radioactive decay. The internal heating may have caused metalsilicate
segregation and thermal metamorphism. Eventually, catastrophic impacts may have caused the fragmentation of the asteroid into smaller pieces. Later, gravitational collapse reassembled the parent asteroid’s fragments into several daughter asteroids that resembled rubble-piles, with the H-chondrites representing the deepest part and the L- and LL-chondrites representing the outer parts of the larger asteroid(s). Additionally, the oxygen isotope compositions of new L-chondrites are similar to those of the impact-related clasts in brecciated chondrites NWA 869, indicating that they have a shared L-type parent asteroid that was once part of a larger asteroid.