Halite and stable chlorine isotopes in the Zag H3-6 breccia

Abstract

Zag is an H36 chondrite regolith breccia within which we have studied 14 halite grains less than or equal to 3 micrometers. The purity of the associated NaCl-H2O brine is implied by freezing characteristics of fluid inclusions in the halite and EPMA analyses together with a lack of other evaporite-like phases in the Zag H36 component. This is inconsistent with multi-stage evolution of the fluid involving scavenging of cations in the Zag region of the parent body. We suggest that the halite grains are clastic and did not crystallize in situ. Halite and water-soluble extracts from Zag have light Cl isotopic compositions, delta-37Cl = 1.4 to 2.8 ppm. Previously reported bulk carbonaceous chondrite values are approximately delta-37Cl = +3 to +4 ppm. This difference is too great to be the result of fractionation during evaporation, and instead, we suggest that Cl isotopes in chondrites are fractionated between a light reservoir associated with fluids and a heavier reservoir associated with higher temperature phases such as phosphates and silicates. Extraterrestrial carbon released at 600 degrees C from the H34 matrix has delta-13C = -20 ppm, consistent with poorly graphitized material being introduced into the matrix rather than indigenous carbonate derived from a brine. We have also examined 28 other H chondrite falls to ascertain how widespread halite or evaporite-like mineral assemblages are in ordinary chondrites.