Halogen and K-Cl isotope records in non-carbonaceous chondrites: evolution of the volatile element inventory of asteroidal accretionary feedstock
Anthony Gargano, Zoltan Vaci, Justin I. Simon, Wayne Buckley, Zachary Sharp, Piers Koefoed, Aaron Bell, Kun Wang
Icarus, In Press, Journal Pre-proof, Available online 14 November 2026
“Highlights
- Halogen contents of ordinary chondrites decrease with metamorphic grade.
- Cl and K become isotopically heavier with grade, requiring vapor transport processes.
- Enstatite chondrites show muted halogen depletion and isotope fractionation.
- Unequilibrated chondrites may not represent bulk parent-body halogen budgets.
- Fluorine favors enstatite chondrite-like bodies as Earth’s main halogen source.”
“Primitive chondritic meteorites are routinely treated as the building blocks for terrestrial planets, yet their volatile inventories can be modified during parent-body metamorphism, such that the most primitive samples may not be representative of what was incorporated into a growing planet. Here we report F, Cl, Br, and I abundances together with chlorine and potassium isotope compositions for a metamorphic sequence of non‑carbonaceous ordinary (H, L, LL; types 3.1–6) and enstatite (EH, EL; types 3–6) chondrites, along with several achondrites. Halogen and K concentrations decrease with increasing metamorphic grade, whereas δ37Cl and δ41K values generally increase from −4.5 to +8.6‰ and − 1.41 to −0.33‰, respectively. These trends are consistent with open-system volatile loss and/or transfer during sub-solidus metamorphism. Ordinary chondrites display the strongest volatile depletion and largest isotopic range, consistent with enhanced mobility and escape of halogen-bearing vapors in comparatively permeable. In contrast, reduced enstatite chondrites show more muted fractionation, consistent with volatile retention in less permeable systems. These observations support the concept of asteroidal distillation as a mechanism that can overprint near-surface ‘unequilibrated’ materials through condensation of vapors sourced from hotter interiors. Incorporating metamorphic gradients into a simple onion-shell mass balance demonstrates that bulk parent-body halogen budgets can be substantially lower than inferred from the most ‘primitive’ chondrites, providing a direct pathway to reconcile chondrite-based accretion models with the reduced volatile inventory of the bulk silicate Earth.”
