Salty Bennu: Halite and Sylvite in Fines Returned by OSIRIS-REx

Salt phases such as halite (NaCl) and sylvite (KCl) are important hosts of halogens (e.g., Br and I) and volatile alkali metals (i.e., Na, K) and are very rare in extra-terrestrial materials. Halite and sylvite have been discovered in carbonaceous chondrite clasts in ordinary chondrite meteorite falls Zag H3-6 regolith breccia (Zag), Monahans (1998) breccia (Monahans), and Sidi El Habib H5 chondrite, as well as on the surface of particles from asteroid Itokawa. Yet, it is uncertain whether the evaporites are indigenous or exogenous to their respective parent bodies, and their origin is debated. Analysis of the extinct 129I/129Xe decay system and 40Ar/39Ar show that halite in Zag may have formed within the first 8 Myr of the solar system, possibly from aqueous fluids during local and episodic low-temperature alteration on the parent body. Halite and sylvite in Zag are characterized by light δ37Cl values of −1.7 ‰. Halite in Zag could have been transported from the outer solar system, perhaps a P- or D-class asteroid, as indicated by O, H, N, and C isotopes. An alternative is a cometary or interstellar ice origin due to their deuterium-rich and 16O-poor composition.