ハフニウムおよびタングステン同位体から読み解く初期地殻進化

抄録

Knowledge of the nature and petrogenesis of Earth's early crust is central to understanding the early Earth. Here I discuss Lu–Hf and Hf–W isotope systematics of Archean samples that extended our knowledge of early crustal evolution. Recent developments in laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) allow us to obtain precise and accurate Lu–Hf isotopic data for zircon with high spatial resolution. The application of LA-ICPMS zircon Lu–Hf isotopic analysis to 4.0–3.6 Ga Acasta gneisses from northwestern Canada revealed that some of the early Archean samples including the Earth's oldest known rocks have sub-chondritic initial ¹⁷⁶Hf/¹⁷⁷Hf, indicating reworking of pre-existing crust. The finding is consistent with the presence of zircon xenocrysts with U–Pb ages up to 4.2 Ga in the Acasta gneisses. The implications are that Hadean granitoid crust was more widespread than previously thought, and that it had significantly contributed to the genesis of the early Archean crust. Recent high-precision W isotopic studies revealed positive ¹⁸²W anomalies of up to 0.15ε in ～3.8 Ga Itsaq rocks from West Greenland and 2.8 Ga Kostomuksha komatiites from Russia. I explored the geologic significance of the ¹⁸²W anomalies by combining with trace element and other isotopic data. In this context, the W isotopic data are interpreted to reflect early silicate differentiation events on Earth. Under the assumption that the bulk silicate Earth has a ～5% higher Sm/Nd than the chondrite average, the ¹⁸²W–¹⁴²Nd–¹⁴³Nd chronometry constrains the age of the source mantle differentiation for the Itsaq samples to 4.53–4.49 Ga. The age may reflect the timing of silicate differentiation during a sequence of magma ocean solidification.