Unraveling the complexity of zircons from the 4.0–2.9 Ga Acasta Gneiss Complex

Abstract

The Acasta Gneiss Complex (AGC) of the Northwest Territories, Canada, contains some of the oldest evolved terrestrial crust and has a sustained record of more than a billion years of magmatism (>4.0–2.9 Ga). These rocks provide an opportunity to investigate the nature of continental crust formation on the early Earth. Because complexities in zircons and bulk rocks are characteristic of the early Earth record—including the AGC—strategies are needed to extract accurate and meaningful age and isotopic information. In order to evaluate the accuracy of the early Earth Hf isotope record, we examined AGC zircons from a range of lithologies using paired chemical abrasion isotope dilution U-Pb age and solution Lu-Hf isotope analysis and compared these with previous results obtained using laser ablation split-stream (LASS) analysis. We describe an approach whereby LASS is used to identify rocks with the least complex zircons, and, when appropriate, solution methods are then used to refine the age and Hf isotopic composition to the highest precision. This two-pronged analytical approach results in a more robust determination of the age and Hf isotopic record of complex rocks and zircons and allows identification of complexity in the Hf isotopic record that would not be apparent by solution analysis alone, thereby refining the record of magmatic evolution on the early Earth. Despite the better precision, solution techniques are unsuitable for rocks with complexly zoned zircons. In particular, zircons from the two oldest AGC rocks in this study have positive initial εHf values when determined by solution analysis (+2.8 and +4.9 at 4.0 and 3.9 Ga, respectively) but have negative εHf values when determined by LASS (−3.4 and −4.7, respectively). This is attributable to the presence of later radiogenic overgrowths on the zircon grains which are incorporated in solution analyses but can be avoided using LASS. This provides important clarity to the AGC Hf isotope record. In total, all but one of the AGC rocks we analyzed, including the oldest samples, have negative εHf values and indicate derivation from an enriched reservoir; none of these samples—in contrast to whole rock Nd isotope compositions—have sufficiently positive εHf values to indicate their derivation from a depleted mantle reservoir. Despite the presence of ancient AGC crust, there is no record of corresponding mantle depletion. This implies that extraction of Hadean crust in this region did not happen in sufficient volume to result in widespread mantle depletion in the AGC source by the Eoarchean.Our results underscore the importance of identifying different components in ancient zircons—and the rocks that contain them—and accurately determining the age and isotopic composition of those components. This is critically important for clarifying the record of the formation of enriched crust and development of the depleted mantle in Earth’s early history.