Resolving the conflicting Hf and Nd isotope records of early Earth crust-mantle evolution
Primary author: Ross Salerno
Faculty sponsor: Jeff Vervoort
Primary college/unit: Agricultural, Human and Natural Resource Sciences
The timing and nature of the formation and growth of Earth’s continents has sparked debate for decades. Fundamentally, two end-member models have been proposed. One favors rapid and early di¬fferentiation of continental crust and a complimentary depleted mantle, while the other proposes and early basaltic proto-crust with true continental crust and depleted mantle forming many hundreds of millions of years later. Radiogenic isotope systems (e.g., Nd and Hf) are crucial in this debate, as they provide constraints on the timing of the Bulk Silicate Earth differentiation. Our attention focuses on the isotopic record – particularly from the Earth’s oldest rocks – to help understand the timing of crustal growth. Two fundamentally different stories appear between the Hf and Nd records of these oldest samples. Hf isotope data, now based primarily on zircon Hf and corresponding U-Pb ages, have chondritic Hf compositions through ~ 3.8 Ga – no evidence for crust or depleted mantle. The Nd isotope data, in contrast, appears to show a characteristic depleted mantle signature for these same rocks. The Hf and Nd compositions we report from the Pilbara Craton (3.5-3.3 Ga) agree – both are chondritic. These rocks preserve no evidence of a depleted mantle or significant crustal reservoir on Earth at that time. In the examples where both systems are closed [Pilbara], the Hf and Nd isotope records agree and there is no Hf-Nd paradox.