Long-term preservation of geochemical heterogeneities in early Earth: tracking short-lived isotopes in geodynamic models

The short-lived isotope systems, including ¹⁴⁶Sm-¹⁴²Nd (half-life = 103 Ma) and ¹⁸²Hf-¹⁸²W (half-life = 8.9 Ma), provide evidence for mantle differentiation events in early Earth, as both the daughter nuclides are more incompatible than the parent nuclides. For the ¹⁴⁶Sm-¹⁴²Nd system, both positive and negative μ¹⁴²Nd measurements are observed in Hadean-Archean mantle-derived rocks, which possibly indicates a major differentiation event of the silicate Earth before the extinction of ¹⁴⁶Sm (e.g., Boyet and Carlson, 2005, Science). The diminishing trend of μ¹⁴²Nd between Hadean and Archean, on the other hand, suggests continuous mantle mixing during this period. However, for the ¹⁸²Hf-¹⁸²W system, Hadean-Archean mantle-derived rocks often show positive μ¹⁸²W anomalies followed by a decline in at 2.5~3.0 Ga ago without a mixing trend (e.g., Carlson et al., 2019, Chem. Geol.). Also, μ¹⁴²Nd and μ¹⁸²W often show no or negative correlation in Hadean-Archean mantle derived rocks (e.g., Rizo et al. 2016, Geochim. Cosmochim. Acta), which requires a mechanism to decouple these two isotopic systems.

In this study, we implement both ¹⁸²Hf-¹⁸²W and ¹⁴⁶Sm-¹⁴²Nd system in a global thermochemical geodynamic model, StagYY (Tackley, 2008, PEPI), to track the evolution of these isotope systems through Earth’s mantle evolution. Based on the particle-in-cell method, the geodynamic model incorporates melting and magmatic crust production that allow us to track both fractionation (by melting and crustal production) and mixing (through mantle convection) of trace elements through time. We discuss in detail how (1) the ‘basalt barrier’ at the base of the mantle transition zone (Davies, 2008 EPSL), (2) crustal delamination from intrusive magmatism, or plutonic-squishy-lid tectonics (Lourenco et al., Nat. Geo. 2018; GCubed 2020), and (3) late accretion could affect the tectonics of early Earth, and the preservation of geochemical heterogeneities and decoupling of two isotopic systems in the mantle through time.