Petrology of the opaque assemblages in unequilibrated ordinary chondrites

Samuel P. Alpert, Denton S. Ebel, Michael K. Weisberg, Jeremy R. Neiman

Meteoritics & Planetary Science
Version of Record online: 05 February 2021


“Opaque assemblages (OAs) are small (submillimeter) objects composed primarily of metals, sulfides, and oxides that exist in nearly all chondritic meteorite groups as discrete objects in the matrix or associated with chondrules. The size, morphology, and petrology of OAs vary greatly between different chondrite groups, with petrologic grade within a single group, and by their apparent textural setting. Two hypotheses may explain the formation of matrix OAs: (1) they were separated from chondrules via surface tension during heating events, or (2) they formed as free‐floating objects in the solar nebula; however, this is the first comprehensive study of the petrology of OAs in ordinary chondrites (OCs) as a group, which seeks to determine if one hypothesis is sufficient to explain all such objects. Here, we use a newly developed machine learning algorithm to show that all OAs from the least equilibrated OC, Semarkona (LL 3.01), are composed of kamacite, taenite, troilite, pentlandite, magnetite, and other minor phases. These OAs form two distinct groups based on their modal mineralogy: one group in and associated with chondrules, and the other group free‐floating in the matrix. Chondrule OAs exhibit a bimodal distribution between sulfide‐ and metal‐rich endmembers in agreement with previous findings. Matrix OAs cluster at roughly equal abundances of sulfides and metals and universally exhibit magnetite rims. The two populations of chondrule OAs cannot be combined to form the modal mineralogies observed in matrix OAs and some matrix OAs exhibit mineralogical layering consistent with fractional condensation. Both observations support the hypothesis that matrix OAs were not formed by expulsion from chondrules and instead formed as free‐floating objects in the solar nebula; however, chondrule OAs must have formed with their host chondrules during heating events.”