Meteorite zircon constraints on the bulk Lu−Hf isotope composition and early differentiation of the Earth

摘要

Knowledge of planetary differentiation is crucial for understanding the chemical and thermal evolution of terrestrial planets. The ¹⁷⁶Lu−¹⁷⁶Hf radioactive decay system has been widely used to constrain the timescales and mechanisms of silicate differentiation on Earth, but the data interpretation requires accurate estimation of Hf isotope evolution of the bulk Earth. Because both Lu and Hf are refractory lithophile elements, the isotope evolution can be potentially extrapolated from the present-day ¹⁷⁶Hf/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf in undifferentiated chondrite meteorites. However, these ratios in chondrites are highly variable due to the metamorphic redistribution of Lu and Hf, making it difficult to ascertain the correct reference values for the bulk Earth. In addition, it has been proposed that chondrites contain excess ¹⁷⁶Hf due to the accelerated decay of ¹⁷⁶Lu resulting from photoexcitation to a short-lived isomer. If so, the paradigm of a chondritic Earth would be invalid for the Lu−Hf system. Herein we report the first, to our knowledge, high-precision Lu−Hf isotope analysis of meteorite crystalline zircon, a mineral that is resistant to metamorphism and has low Lu/Hf. We use the meteorite zircon data to define the Solar System initial ¹⁷⁶Hf/¹⁷⁷Hf (0.279781 ± 0.000018) and further to identify pristine chondrites that contain no excess ¹⁷⁶Hf and accurately represent the Lu−Hf system of the bulk Earth (¹⁷⁶Hf/¹⁷⁷Hf = 0.282793 ± 0.000011; ¹⁷⁶Lu/¹⁷⁷Hf = 0.0338 ± 0.0001). Our results provide firm evidence that the most primitive Hf in terrestrial zircon reflects the development of a chemically enriched silicate reservoir on Earth as far back as 4.5 billion years ago.