Whole-mantle convection with tectonic plates preserves long-term global patterns of upper mantle geochemistry

Abstract

The evolution of the planetary interior during plate tectonics is controlled by slow convection within the mantle. Global-scale geochemical differences across the upper mantle are known, but how they are preserved during convection has not been adequately explained. We demonstrate that the geographic patterns of chemical variations around the Earth’s mantle endure as a direct result of whole-mantle convection within largely isolated cells defined by subducting plates. New 3D spherical numerical models embedded with the latest geological paleo-tectonic reconstructions and ground-truthed with new Hf-Nd isotope data, suggest that uppermost mantle at one location (e.g. under Indian Ocean) circulates down to the core-mantle boundary (CMB), but returns within ≥100 Myrs via large-scale convection to its approximate starting location. Modelled tracers pool at the CMB but do not disperse ubiquitously around it. Similarly, mantle beneath the Pacific does not spread to surrounding regions of the planet. The models fit global patterns of isotope data and may explain features such as the DUPAL anomaly and long-standing differences between Indian and Pacific Ocean crust. Indeed, the geochemical data suggests this mode of convection could have influenced the evolution of mantle composition since 550 Ma and potentially since the onset of plate tectonics.

The composition and mineralogy of rocky exoplanets: A survey of >4000 stars from the Hypatia Catalog

“…Especially compelling are seismic images of subducted slabs that reach the base of the mantle (Jordan 1977;Grand 1997;Fukao and Obayashi 2013); these require a return flow, and appear to end discussion of an isolated lower mantle. Numerical models also appear to resolve supposed conflicts between whole-mantle convection and a mantle with geochemical and seismic heterogeneity (Jordan et al 1993;Schuberth et al 2009;Barry et al 2018). So while controversy still exists over seismic images of deep-seated plumes (e.g., Montelli et al 2004), we agree with recent Agrusta et al (2018), that Earth's mantle has long been well stirred.…”

Origin of elevated PGE contents and Pt/Pd in Bushveld magmas: constraints from W and Ru isotopes

“…until ~3 Ga) based on PGE contents in komatiites, whereas Puchtel et al (2020) suggested that portions of the mantle remained poorly equilibrated with LV until ~2 Ga, based on W and Pt isotopes as well as PGE systematics in komatiites of the Lapland Greenstone belt. Heterogenous inmixing of late veneer is consistent with models of modern mantle dynamics; For example, Barry et al (2017) showed that whole-mantle convection may preserve long-term global patterns of upper mantle geochemistry. Based on these data one could suggest that the Pt-rich Bushveld magmas formed from a mantle that was relatively enriched in late veneer.…”

Subducted oceanic crust as the origin of seismically slow lower-mantle structures

“…This will allow for an improved and more consistent comparison with the seismological observations and models. As for ii), while we expect eventually to be able to perform full 3-D simulations such as in Nakagawa et al (2010) and Barry et al (2017), the required high resolution and time evolution over the age of the Earth makes the use of 2-D geometries still highly attractive. In BB08 a cylindrical 'shrunken core' approach was used to better represent the heat loss properties of the Earth.…”