A multiple-system study of the geochemical evolution of the mantle with force-balanced plates and thermochemical effects

Abstract

Here, multiple isotope systems are tracked simultaneously in models of mantle convection and it is show that this can provide powerful constraints on the role of oceanic crust recycling in the development of isotopic end-member compositions. The dynamical models are based on high-resolution cylindrical calculations with force-balanced plates and variable chemical density. The dynamic results span a parameter space of variable realistic excess crustal density compared to experimental estimates and convective vigor measured by plate velocities and surface heat flow. Isotope geochemistry is then modeled for the U–Th–Pb, Sm–Nd, Rb–Sr, and Re–Os isotope systems. The role of a dense crustal layer in development of a HIMU-isotope signature is confirmed. The extraction of continental crust is found to be essential for the formation of all isotope compositional end-members, including HIMU. This extraction is implemented as an ad-hoc process secondary to partial melting at mid-ocean ridges and constrained by estimated isotopic abundances in the present-day crust. Whereas previous studies generated mantle isotopic arrays that spanned DMM–HIMU, the additional isotope systems in this analysis indicate that enrichment purely from ancient oceanic crust may also generate an EM-I component without invoking the subduction of sediment. In this case, the EM-I signature may be indicative of mantle enriched by oceanic crust produced before 2.25 Byr, while the HIMU signature indicates enrichment by oceanic crust extracted more recently. However, it is found to be difficult to maintain a true DMM isotopic end member in Sr–Nd isotope space when significantly enriched end-members are present. This may highlight the sensitivity of the Rb–Sr system to mass exchange between the upper and lower mantle.