Zircon halogen geochemistry: Insights into Hadean-Archean fluids

Abstract

Understanding the distribution of halogens in rocks can potentially trace ancient lithosphere, hydrosphere, and atmosphere interactions. Although no sedimentary rocks older than 3.8 Ga are known, insights into sediment-atmosphere exchange on early Earth could be obtained from knowledge of halogen contents in ancient zircons. Here we present the first study of halogen abundances in Jack Hills zircons together with younger zircons of known provenance to provide geologic context. The relatively low (ca. 0.1-0.6 μg/g) chlorine concentrations in most Hadean and Archean Jack Hills zircons are similar to the average concentration in younger igneous zircons. In contrast, significant Cl enrichments are found in a subset of ca. 3.9-3.8 Ga zircons ([Cl]average= 1.43 ± 0.27 μg/g) that appear to record halogen transport under hydrothermal conditions. Such Cl-bearing fluids in early Earth history may reflect extraction of halogens from the interior to near surface environments.