Icarus pre-proof: heating in asteroids depleted halogens in ordinary and enstatite chondrites

Anthony Gargano and seven co-authors report in an Icarus pre-proof that heating inside asteroids drove off and redistributed fluorine, chlorine, bromine and iodine, according to techtimes.com. techtimes.comtechtimes.comtechtimes.comOpen source ↗ As a result, the bulk halogen budgets of the parent bodies could be much lower than their least-altered meteorites imply. A pre-proof is an accepted manuscript not yet assigned to a journal issue, so details can still change.

Halogens and isotopes in chondrites

The team analyzed ordinary chondrites (H, L and LL groups, petrologic types 3.1 to 6), enstatite chondrites (EH and EL groups, types 3 to 6) and several achondrites. Type 3 rocks are the least metamorphosed, while types 4 to 6 record higher temperatures. For ordinary chondrites these reach roughly 1,200 to 1,300 kelvin (about 1,700 to 1,880°F).

According to the abstract, halogen and potassium concentrations fall as metamorphic grade rises, while the heavier isotopes of chlorine and potassium become more abundant. Values generally increase from −4.5 to +8.6 per mil for δ³⁷Cl and from −1.41 to −0.33 per mil for δ⁴¹K. The authors read this as open-system loss or transfer of volatiles during sub-solidus metamorphism, that is, heating well below the melting point of the rock.

Ordinary chondrites show the strongest depletion and the widest isotopic spread, which the authors tie to vapors escaping through comparatively permeable material. techtimes.comtechtimes.comtechtimes.comOpen source ↗ Enstatite chondrites, which are highly chemically reduced, show much more muted changes, attributed to better volatile retention. The authors also say fluorine favors enstatite chondrite-like bodies as Earth's main halogen source.

Asteroidal distillation and the model

The authors call their central idea asteroidal distillation. Vapors released from the hot interior travel outward and condense in cooler, near-surface material, which can overprint the chemistry of "unequilibrated" samples traditionally used as proxies for the starting composition.

To test this, the team built an onion-shell mass-balance model that treats the asteroid as concentric layers exchanging volatiles. It suggests that bulk parent-body halogen budgets could be considerably lower than the most primitive chondrites indicate. Model parameters, data tables and achondrite results are in the full text and are not described in the abstract.

Earlier work gives context. A 2017 Nature paper led by Patricia Clay found chlorine, bromine and iodine in several chondrite classes to be about 6, 9 and 15 to 37 times lower, respectively, than earlier accepted values, regardless of petrologic type. techtimes.comtechtimes.comtechtimes.comOpen source ↗ Hannah Bloom and colleagues measured potassium isotopes in more than 40 carbonaceous and ordinary chondrites and found no clear correlation with petrologic type. The abstract does not say how its grade-dependent trends relate to Clay's result.