The Allende multicompound chondrule (<scp>ACC</scp>)—Chondrule formation in a local super‐dense region of the early solar system

<jats:title>Abstract</jats:title><jats:p>In Allende, a very complex compound chondrule (Allende compound chondrule; <jats:styled-content style="fixed-case">ACC</jats:styled-content>) was found consisting of at least 16 subchondrules (14 siblings and 2 independents). Its overall texture can roughly be described as a barred olivine object (<jats:styled-content style="fixed-case">BO</jats:styled-content>). The <jats:styled-content style="fixed-case">BO</jats:styled-content> texture is similar in all siblings, but does not exist in the two independents, which appear as relatively compact olivine‐rich units. Because of secondary alteration of pristine Allende components and the <jats:styled-content style="fixed-case">ACC</jats:styled-content> in particular, only limited predictions can be made concerning the original compositions of the colliding melt droplets. Based on textural and mineralogical characteristics, the siblings must have been formed on a very short time scale in a dense, local environment. This is also supported by oxygen isotope systematics showing similar compositions for all 16 subchondrules. Furthermore, the <jats:styled-content style="fixed-case">ACC</jats:styled-content> subchondrules are isotopically distinct from typical Allende chondrules, indicating formation in or reaction with a more <jats:sup>16</jats:sup>O‐poor reservoir. We modeled constraints on the particle density required at the <jats:styled-content style="fixed-case">ACC</jats:styled-content> formation location, using textural, mineral‐chemical, and isotopic observations on this multicompound chondrule to define melt droplet collision conditions. In this context, we discuss the possible relationship between the formation of complex chondrules and the formation of macrochondrules and cluster chondrites. While macrochondrules may have formed under similar or related conditions as complex chondrules, cluster chondrites certainly require different formation conditions. Cluster chondrites represent a mixture of viscously deformed, seemingly young chondrules of different chemical and textural types and a population of older chondrules. Concerning the formation of <jats:styled-content style="fixed-case">ACC</jats:styled-content> calculations suggest the existence of very local, kilometer‐sized, and super‐dense chondrule‐forming regions with extremely high solid‐to‐gas mass ratios of 1000 or more.</jats:p>