Chondritic Lu/Hf in the early crust-mantle system as recorded by zircon populations from the oldest Eoarchean rocks of Yilgarn Craton, West Australia and Enderby Land, Antarctica

Abstract

A persistent question in early Earth investigations is the depletion history of the mantle as recorded in the isotopic compositions of the oldest rocks and minerals. Here we present new, in situ, sensitive high resolution ion microprobe (SHRIMP) U–Th–Pb geochronology, and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) Lu–Hf tracing data for individual zircons extracted from the oldest rocks of the Antarctic and Australian continents. These are 3.88 and 3.85 Ga orthogneisses from the Gage Ridge and Mt. Sones in Enderby Land and a 3.73 Ga Meeberrie gneiss from the Yilgarn Craton. The zircons from each sample show complex U–Th–Pb systematics due to the mobile behaviour of Pb, resulting in several age populations with discordant to reversely discordant arrays on concordia plots. In contrast, measured 176Hf/177Hf compositions from the same analysis sites show narrowly defined compositional ranges, with 3 or fewer groups for each sample, reflecting the resistance of 176Hf/177Hf in zircons to disturbance even under high-temperature, granulite conditions. The initial 176Hf/177Hf calculated for the most primitive Hf population for each sample using the best age estimate yields near-chondritic weighted mean compositions of − 1.6 ± 0.5, − 1.6 ± 0.4 and − 0.6 ± 0.7 epsilon units (deviations in parts per 10,000 from a chondritic uniform reservoir (CHUR) reference) at 3877 ± 62, 3850 ± 50, and 3731 ± 4 Ma respectively. These results provide no evidence for significant Lu–Hf fractionation on the early Earth expected to result from processes including accretion, intra-mantle differentiation, or massive Hadean crustal growth. Show more