Chondrule formation in the outer disk from the primary three-dimensional chemical composition of CM chondrulesOPEN ACCESS
Poula Eyðbjørnsdóttir, Anders Johansen, Elishevah van Kooten
Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 2 August 2026
“Chondrules and their associated fine-grained rims (FGRs) record fundamental processes operating in the early protoplanetary disk, yet the relationship between chondrule chemistry, morphology, and matrix complementarity remains incompletely understood. Here we investigate the major, minor, and trace element compositions of 66 chondrules and associated FGRs from relatively unaltered CM carbonaceous chondrites Asuka 12236, Paris and Maribo in relation to their three-dimensional morphology, using a multi-analytical approach including femtosecond LA-ICP-MS and X-ray tomography. Our results show that CM chondrules record a systematic process of metal loss and evaporation of Si-rich mesostasis, driving initially CI-like precursor compositions toward more Mg- and Si-rich bulk compositions along the CI ratio line and toward increasingly Si-poor forsteritic mineral assemblages. GEMS-like materials in pristine CM matrices appear to mirror chondrule compositions and likely represent complementary condensates derived from evaporated Si-rich mesostasis. The dust accreted to chondrules is dominantly CI-like but incorporates ∼ 14 wt% complementary condensate material represented by chondritic amorphous silicates, reconciling the observed Mg/Si complementarity between chondrules and matrix with the preservation of primordial organics and presolar grains.
Morphological observations further reveal no significant sectioning bias in chondrule size or plane, consistent with CM chondrule populations being dominated by agglomerates of ∼ 100 μm sized microspherules rather than larger primary melt droplets. Many chondrules display grape-bunch textures formed by welding of smaller primary chondrules with metal-rich or CI-like rims. This structure may explain the moderate volatile element plateau at ∼ 0.3 × CI observed for average CM chondrule compositions, reflecting incorporation of primary fine-grained rim material into these aggregates. We propose a “micro-chondrule-first” formation scenario in which localized heating events produced small molten droplets that subsequently accreted CI-like dust and ice, aggregated, and experienced limited in situ aqueous alteration. These observations place new constraints on chondrule formation mechanisms in the outer disk and highlight the importance of localized melting and aggregation processes.”
