83-4 Igneous chondrule rims as tracers of the nebular dust cycle

Session: Asteroid Observations, Return Missions, and Meteoritics: Interweaving Perspectives and Data

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Chondrules are once-molten igneous inclusions that make up the bulk of chondrites, planetesimal remnants that did not undergo melting and differentiation. Chondrule rims, both dusty and igneous, have been observed in a variety of chondrite classes. This study focuses on the composition of moderately volatile elements (MVEs) in chondrules and their igneous rims as a tracer of the sources of nebular dust and thermal processing of this material.  

Igneous rims on CO3 chondrules have not been extensively analyzed, partly due to their small thicknesses (<30 microns). High-resolution elemental mapping of CO3 thin sections revealed some igneous chondrule rims enriched in manganese, a MVE. EPMA analyses of these rims show that Mn-rich pyroxene in CO3 chondrule rims are also enriched in other MVEs, such as Na and K, compared to pyroxene phenocrysts (large crystals) in the host chondrules. These enrichments are the result of nebular processing of chondrules as opposed to parent body processing, as enrichments do not vary systematically with petrologic subtypes. Previous work show that igneous chondrule rims in CR and CV chondrites are also enriched in MVEs.

Occurrence of MVE enriched igneous chondrule rims for COs and CRs indicate that these rims formed across different locations and times in the nebula. Previous studies have suggested that these rims formed during interaction of chondrules with MVE enriched nebular gas; however, this would require unusually high pressures or dust-to-gas ratios. A plausible alternative scenario for the formation of these chondrule rims involves accretion of MVE enriched dust onto solidified chondrules, which subsequently experienced at least one additional heating event that melted and crystallized the rim.

What could be the source of MVE-enriched nebular dust for chondrule rims? Previous work suggests evaporation of chondrules could produce MVE-rich dust. Instead, we propose outgassing planetesimals and planetary embryos as an overlooked source of MVE enriched nebular dust. This is consistent with results from meteorite outgassing studies and the many studies that have documented MVE depletion of differentiated meteorites and terrestrial planets. If all of the planetesimals that went into building the Earth (6×10²⁴ kg) experienced volatile depletion of 2% (1×10²³ kg), that is two orders of magnitude greater than the mass of the Asteroid Belt. These volatiles could then condense onto nebular dust and chondrules in the local surroundings.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11300

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