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.

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不平衡普通球粒陨石中不透明集合体的岩石学

不透明组合体（OAs）是小的（亚毫米）物体，主要由金属，硫化物和氧化物组成，它们几乎以所有离散的陨石形式存在于基质中或与球粒相关联。在不同的球粒陨石组之间，OA的大小，形态和岩石学差异很大，岩石学等级在一个组内，并且其表观构造也不同。两种假设可以解释基质OA的形成：（1）在加热过程中它们通过表面张力与软骨分离，或（2）在太阳星云中以自由漂浮的形式形成；但是，这是对普通球粒陨石（OC）中的OA岩石学的首次综合研究，旨在确定一种假设是否足以解释所有这些对象。这里，我们使用一种新开发的机器学习算法来证明，平衡度最低的OC（Semarkona，LL 3.01）中的所有OA均由滑石，钙钛矿，三菱沸石，膨润土，磁铁矿和其他次要相组成。这些OA基于其模态矿物学形成两个不同的组：一组位于软骨中并与软骨相关联，另一组在基质中自由漂浮。与以前的研究结果一致，线粒骨OA在硫化物和金属富集的最终成员之间表现出双峰分布。基质OAs的硫化物和金属丰度大致相等，并且普遍具有磁铁矿边缘。两种软骨素OAs不能组合形成在基质OAs中观察到的模态矿物学，并且某些基质OAs表现出与分级凝结一致的矿物学分层。两种观察都支持这样的假说：矩阵OA不是由球状星团排出而形成的，而是在太阳星云中作为自由漂浮的物体形成的。但是，在加热事件中，软骨球OA必须与它们的宿主软骨一起形成。