Chondrule formation in the outer disk from the primary three-dimensional chemical composition of CM chondrules

Abstract

Chondrules and their associated fine-grained rims record key processes in the early protoplanetary disk, yet the links between chondrule chemistry, morphology, and matrix complementarity remain poorly constrained. We investigate the major, minor, and trace element compositions of 66 chondrules and FGRs from the relatively unaltered CM carbonaceous chondrites Asuka 12236, Paris, and Maribo, together with their 3D morphology, using LA-ICP-MS and X-ray tomography. CM chondrules record systematic 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 assemblages. GEMS-like materials in pristine CM matrices closely mirror chondrule compositions and likely represent complementary condensates derived from evaporated mesostasis. Dust accreted onto chondrules is predominantly CI-like but contains about 14 wt.% complementary condensate material represented by chondritic amorphous silicates, reconciling Mg/Si complementarity between chondrules and matrix with the preservation of primordial organics and presolar grains. Morphological observations show no significant sectioning bias, consistent with CM chondrules being dominated by agglomerates of ~100 um microspherules. Many display grape-bunch textures produced by welding of smaller chondrules with metal-rich or CI-like rims. This structure may explain the chondrule moderately volatile-element plateau at about 0.3xCI. We propose a "micro-chondrule-first" scenario in which localized heating events produced small molten droplets that subsequently accreted CI-like dust and ice, aggregated, and experienced limited aqueous alteration. These observations place new constraints on chondrule formation in the outer disk and highlight the importance of localized melting and aggregation processes.