K-Ar in the context of whole mantle convection

Abstract:

The K-Ar system and the budget of atmospheric ⁴⁰Ar are fundamental constraints on mantle structure and dynamic history. In particular, the observation that ~50% of ⁴⁰Ar produced by ⁴⁰K decay resides in the mantle has been used to argue that the lower mantle is relatively convectively isolated from the upper mantle (Allegre et al., 1996). To test this hypothesis, we model K-Ar evolution within updated versions of the Brandenburg et al. (2008) geodynamic models. The convective vigor of these whole mantle thermochemical convection models is calibrated to reproduce present-day plate velocities. We use Markov chain Monte Carlo sampling to find optimal values of various chemical parameters that produce matches to the mantle and atmospheric ⁴⁰Ar/³⁶Ar ratios, continental crust K content, and atmospheric ⁴⁰Ar content. We vary as free parameters the extraction to and recycling history of K in the continental crust, the subduction efficiency of Ar, and the initial distribution Ar between the mantle and atmosphere.

We find that when using appropriate chemical parameters, the models can reproduce the K-Ar observational constraints. Despite ~50% of the ⁴⁰Ar budget remaining in the mantle, only ~25% of the mantle remains unprocessed by partial melting. Thus, the K-Ar constraints do not require large regions of convectively isolated mantle. Furthermore, the unmelted material in the models is distributed throughout the mantle rather than concentrated in the lower mantle.

We also find that the K-Ar constraints are best fit when K is preferentially extracted to the continental crust dominantly after the Archean. This contrasts a recent study arguing for very early K extraction (Guo and Korenaga, 2020). We note that late K extraction to the continental crust in our models may reflect the continental crust becoming more felsic with time, rather than forming late.

The models can further be used to explore the spatial distribution of mantle ⁴⁰Ar/³⁶Ar ratios. The model upper mantle is characterized by ⁴⁰Ar/³⁶Ar ~30,000, similar to MORBs, whereas the lower mantle has ⁴⁰Ar/³⁶Ar ratios as low as 10,000, as observed in some OIBs. These low values primarily derive from subducted atmospheric Ar rather than primordial mantle Ar, suggesting that low OIB values are due to the presence of subducted atmospheric Ar rather than primordial mantle Ar.