Various Size-sorting Processes for Millimeter-sized Particles in the Sun's Protoplanetary Disk? Evidence from Chondrules in Ordinary Chondrites

Chondrules are approximately millimeter-sized beads of crystallized silicate melt. They formed mainly in the first similar to 3 Ma of the Sun's protoplanetary disk and are the main constituents of chondritic asteroids. Here we report on the size-frequency distributions (2D and 3D) of chondrules in the brecciated ordinary chondrite (OC) Northwest Africa (NWA) 5205. We investigated three large (centimeter-to decimeter-sized) chondritic lithic clasts of a particular textural type (cluster chondrite) with eye-catching different chondrule sizes. One clast shows the largest mean chondrule size (similar to 1.5 mm) ever measured in a chondrite. As in the other OCs, we find a positive correlation between the minimum and mean chondrule size, which we consider as an argument for chondrule size sorting. Chondrule size-frequency distributions in the clasts are distinctly more symmetric than the about lognormal distributions in other OCs. Furthermore, we find a co-enrichment of chondrule types with a priori small mean sizes (type I, porphyritic) in clasts with overall small mean chondrule sizes. We consider this as the fingerprint of an additional/second size-sorting process, which acted later on these chondrule populations. This process possibly subdivided a typical LL-type chondrule population into several subpopulations with different mean chondrule sizes. We speculate that this second sorting occurred in a unidirectional gas stream or headwind, e.g., by settling of chondrules through an asteroidal atmosphere or interaction with an expanding impact plume. Possibly, fine-grained matrix was almost completely removed by this, and the size-sorted chondrule subpopulations accreted in a hot state separately in different regions of the asteroid.