The Evolution of Isotope Heterogeneities in Early Earth: A Geodynamic Perspective

The short-lived isotope systems, including 182Hf-182W (half-life = 8.9 Ma) and 146Sm-142Nd (half-life =103 Ma), provide evidence for geochemical heterogeneities in the early Earth evolution. For the 182Hf-182W system, mantle-derived rocks from the Hadean to the Archean often show positive μ182W anomalies, whereas modern OIBs often show negative μ182W (e.g. Rizo et al., 2019, Geochem. Perspect. Lett.). However, the survival of these positive μ182W anomalies until Archean (Tusch et al., 2021, PNAS) requires a mechanism to preserve these anomalies under mantle convection, such as a transition from plume tectonics to modern plate tectonics in Archean (Mei et al., 2019, Geochim. Cosmochim. Acta.), or late input with negative μ182W like core-mantle interaction (e.g. Mundl-Petermeier et al., 2020, Geochim. Cosmochim. Acta.). For the 146Sm-142Nd system, both positive and negative μ142Nd are observed in Hadean rocks (> 4 Ga), which possibly indicates a major differentiation of the silicate Earth before the extinction of 146Sm. Measurements of 142Nd/144Nd in post-Archean rocks indicate a 'fast' homogenisation of these isotopic anomalies with a mixing rate of ~400 Ma (Hyung and Jacobsen, 2020, PNAS), which can hardly fit the stagnant-lid tectonics proposed for early Earth.

In this study, we track both 182Hf-182W and 146Sm-142Nd system in a global thermochemical geodynamic model, StagYY, to trace the evolution of these isotope systems. The geodynamic model incorporates a composite rheology (diffusion creep, dislocation creep, and plastic yielding), a composition- and phase-dependent density, and magmatic crust production by both intrusion and eruption to track both fractionation (by melting and crustal production) and mixing (through mantle convection) through time. We discuss in detail how the basalt barrier (Davies, 2008 EPSL) at the base of the mantle transition zone and crustal delamination from intrusive magmatism (plutonic-squishy-lid tectonics (Lourenço et al., Nat. Geo. 2018; GCubed 2020)) could affect the formation and mixing of these geochemical heterogeneities, and compare them with geochemical measurements and model results with stagnant-lid tectonics.