Destruction and Regrowth of Lithospheric Mantle by Emplacement of Large Igneous Provinces

The growth of the lithospheric mantle following tectonic thinning of the plate is well understood and is a primary consequence of the conductive cooling that is a key driver of mantle convection. However, it is less well understood how the lithosphere interacts with and responds to sub-plate temperature anomalies within the the underlying convecting mantle. Here, we investigate the evolution of the lithospheric mantle during and after intraplate magmatism. First, we examine the thickness of lithosphere beneath oceanic intraplate magmatic provinces that are < 10 Ma in age. Modelling of major oxide and rare earth element concentrations, alongside seismic tomography, indicate that these provinces lie upon lithosphere that is significantly thinner than expected given the age of underlying lithosphere. For example, Hawaii overlies lithosphere that is 50–80 km thick, despite the age of the plate suggesting a thickness of > 100 km. Next, we explore the lithospheric thickness beneath ancient intraplate magmatic provinces that have a record extending back to Jurassic times. Geochemical modelling demonstrates that, like recent magmatism, these provinces were also erupted atop thinner than expected lithosphere. Seismic tomography provides a further constraint on the thickness of the lithosphere by constraining the thermal structure of the upper mantle. By exploiting these tomographic images we show that the lithospheric mantle beneath ancient seamounts gets progressively thicker as a function of their eruption age. Importantly however, the lithosphere is consistently thinner by up to 40 km than would be expected if the plate cooled and thickened from a mid ocean ridge without perturbation. Finally, we extend our analysis to ancient continental large igneous provinces (LIPs). LIPs are massive accumulations (>100,000 km³) of magmatic material that are emplaced within a short period of geological time (1-2 Ma). We show that the lithospheric thickness beneath ancient continental LIPs increases as a function of time since eruption, following a similar relationship to oceanic LIPs. Our results suggest that the emplacement of LIPs causes ubiquitous thinning of the lithospheric mantle to thicknesses of 40–80 km, followed by systematic, progressive re-thickening via conductive cooling. Furthermore, they suggest that continental lithospheric mantle re-thickens to depths of > 200 km, supporting the idea that cratons can be destroyed by LIP emplacement and reformed following the end of eruption. Thinning and re-thickening of the lithospheric mantle during and after intraplate magmatism demonstrates that the lithosphere-asthenosphere boundary is routinely perturbed by sub-plate mantle convection. An understanding of LIP formation and its effect on the lithospheric mantle is necessary to reveal causal links with mass extinctions, continental break-up, and regional epeirogenic events.