Modelling the evolution of the short-lived Hf-W and Sm-Nd isotope systems in mantle convection models

The ¹⁸²Hf-¹⁸²W (half-life = 8.9 Myr) and ¹⁴⁶Sm-¹⁴²Nd (half-life = 103 Myr) isotope systems offer valuable insights into Earth's early differentiation and evolution. Active during the first ~50 and ~500 million years of solar system history, respectively, these systems preserve evidence of primordial fractionation processes, and for Hf-W system, possible imprints from the late accretion and Earth’s core-mantle interaction. Differences in W and Nd isotope ratios between Archean mantle and modern mantle suggest the long-term mixing of early-formed geochemical reservoirs within the silicate Earth over the Hadean and Archean. The absence of a direct correlation between ¹⁸²W and ¹⁴²Nd ratios in Archean rocks implies that silicate differentiation may not be the only significant process influencing the evolution of these isotopic systems.

Our study uses the global geodynamic model StagYY to track the evolution of the ¹⁸²Hf-¹⁸²W and ¹⁴⁶Sm-¹⁴²Nd isotope systems through mantle convection. With models start at 60 Myr after CAI formation, corresponding to an earlier estimated time of the Moon-forming impact, we investigate changes of isotopic ratios in basaltic material over time due to melting, magmatic crust formation, mantle mixing, and possible external inputs such as core-mantle interaction. Our model results demonstrate that (1) if Earth’s mantle was fully homogenized during the magma ocean period, the ¹⁸²Hf-¹⁸²W and ¹⁴⁶Sm-¹⁴²Nd systems would be naturally decoupled due to the low abundance of ¹⁸²Hf in Earth’s mantle at 60 Myr, and (2) the chemical mixing within the mantle is strongly affected by mantle depletion: models indicate that the early-depleted mantle could remain in the lower mantle for billions of years but rarely resurface and be erupted, while early-formed basaltic crust could also stay at the core-mantle boundary for billions of years due to its high intrinsic density and influence the isotopic ratios of newly-formed crust through model time. These findings provide new insights into the processes shaping Earth's early geochemical evolution and highlight the importance of using thermo-chemical models in studying Earth's early history.