The coupled Hf-Nd isotope record of the early Earth in the Pilbara Craton

Abstract

Initial Hf and Nd isotope compositions of Earth's oldest rocks provide essential information on the differentiation of the Earth into enriched crustal and depleted mantle reservoirs in its early history. The majority of Eo-Paleoarchean rocks worldwide, however, have isotope compositions that appear to be decoupled: initial Hf isotope compositions, determined on zircon, are broadly chondritic with little variation; initial Nd isotopes on bulk rocks, in contrast are highly variable with both supra- and sub-chondritic compositions. Most of these studies are from polymetamorphic terranes where the potential for disturbance of the isotope system is high. This is particularly true for the Sm-Nd system where more easily altered REE-rich accessory phases are the major repositories for these elements.In order to better understand crust-mantle evolution during the Archean—and to address the issue of Hf and Nd isotope decoupling—we examine a suite of well-preserved Paleoarchean granites from the Pilbara Craton. Our approach integrates the initial Hf isotope composition and U-Pb ages of zircon, the initial Nd isotope compositions of titanite and apatite, and U-Pb ages of titanite by laser ablation split stream (LASS) analysis. The zircon and titanite U-Pb data yield crystallization ages of 3.47 to 3.28 Ga, in good agreement with the combined apatite-titanite-WR Sm-Nd isochron ages of each sample, demonstrating that both the U-Pb and Sm-Nd systems have not been modified since igneous crystallization. The initial Hf isotope compositions of zircon from all samples are broadly chondritic with εHf(i) values of −0.3 to +0.8, in agreement with the bulk-rock Hf. The initial Nd isotope compositions of the titanite and apatite are also broadly chondritic (εNd(i) titanite, −1.0 – +2.0; apatite, −0.6 – +0.9) and agree with the Nd isotope composition of the bulk-rock (εNd(i) = +0.2 to +1.2) and the initial 143Nd/144Nd ratios determined from the titanite-apatite-WR isochrons (εNd(i) −0.9 to +1.3).From these data, we make two fundamental observations. First, the granites in this study were derived from a source that was chondritic with respect to both Hf and Nd isotopes from 3.47 to 3.28 Ga; neither system supports the presence of either a strongly depleted mantle or enriched crustal source. Second, the Lu-Hf and Sm-Nd isotope systems in the Pilbara samples are in full agreement. This stands in stark contrast to the record of rocks from Eo-Paleoarchean terranes of higher metamorphic grade, where the Hf and Nd isotope compositions have been “decoupled”. This further underscores the importance of recognizing potential effects of high-grade metamorphism on the Sm-Nd bulk-rock record. The integrated age-isotope approach taken here illustrates a way to assess the integrity of bulk-rock Nd isotope data through examination of the Sm-Nd isotope systematics of the LREE-rich accessory minerals in rocks.