Halogens in Chondritic Meteorites

Abstract

We review the abundances, distributions and isotopic compositions of halogens in chondritic meteorites and discuss formation of the mineral and other carriers of these elements. Halogens provide tracers of processes that occurred in circumstellar and interstellar environments, within the solar nebula, and after accretion within asteroidal meteorite parent bodies. Knowledge of the halogen abundances in chondrites is fundamental to understanding the halogen contents of the Earth and other terrestrial planets. However, the full potential of halogens to constrain processes in the solar nebula and the chondrite parent bodies has not yet been realized due to both analytical challenges and uncertainties in halogen condensation temperatures. Analytical challenges make robust determinations of absolute halogen abundances extremely difficult, and some large uncertainties remain. Halogens in chondritic meteorites are present in both soluble (water extractable) and insoluble fractions, and they are therefore highly susceptible to alteration by weathering. Potential contamination from terrestrial sources is a perennial problem, even for meteorite falls. However, although there are significant variations in the analytical data, even within individual chondrite groups, it is clear that there are distinct, but complex differences between the three main classes of chondrites, carbonaceous, ordinary, and enstatite. In most chondrites, the abundances of the halogens are controlled by their cosmochemical volatility, and the halogens are depleted in most of the chondrite groups relative to CI chondrites. Measurements of halogen isotope compositions in chondrites are somewhat limited. In general, bulk δ37Cl values show little deviation from the terrestrial standard (−0.3 ± 0.3‰ relative to SMOW), although there are some notable exceptions with more negative values down to –4‰ that are possibly related to either the initial condensation of ices, or to parent body fluid interactions. In addition, 129Xe and 36S excesses in individual components of chondritic meteorites indicate that the short-lived radioisotopes, 129I and 36Cl, were present in the early solar system. The 129I-129Xe system is well established as an important chronometer for constraining the early chronology of solar system materials, but the chlorine-S system has not yet been used widely as a chronometer because of uncertainties in the initial 36Cl/35Cl ratio. Distinct halogen-bearing phases are generally rare in chondritic meteorites, but include silicates, silicate glasses, aluminates, sulfides, halides, phosphates, and oxides. With the exception of the enstatite chondrites, where chlorine (and possibly other halogens) are present in djerfisherite and chlorine-enriched chondrule glass, the primary (nebular) mineralogical carriers of the halogens in the most pristine (low petrologic type 3) carbonaceous and ordinary chondrites are poorly constrained. In type 1 and 2 carbonaceous chondrites that have been affected by interaction with aqueous fluids at low temperatures, such as the CI, CM, and CR chondrites, the mineralogical carriers of the halogens are also highly uncertain. In contrast, the redistribution and concentration of halogens into discrete minerals, particularly chlorine-rich phases, has occurred in essentially all carbonaceous and ordinary chondrites that have been affected by even moderate degrees of metamorphism, sometimes in the presence of aqueous fluids. Fluorine-bearing chlorapatite occurs typically as the major halogen-bearing phase in type 4–6 ordinary chondrites, as well as CK and other metamorphosed carbonaceous chondrites. Extensive metasomatic effects observed in the Allende subgroup of the oxidized CV chondrites (CVOxA) have resulted in the formation of sodalite, wadalite, and the rare aluminates chlormayenite and adrianite, most typically in CAIs, although sodalite also occurs in chondrules and matrices. Evidence for limited development of sodalite and rare scapolite is also present in chondrules in some petrologic type 3 ordinary chondrites and in CAIs in CO3 chondrites. Rare occurrences of fluor-richerite and fluorphlogopite have been reported as crystallization products of impact melts in enstatite chondrites, and fluorine- and chlorine-bearing biotite also occurs in some high petrologic type R chondrites.