Downwelling dense mantle residues and hotspot magmatism

The geodynamic origin of melting anomalies found at the surface, often referred to as hotspots, is classically attributed to mantle plume processes. The coincidence of hotspots and regions of relatively thin lithosphere, however, questions the necessity for mantle plumes in driving hotspot magmatism, especially as the ability of mantle plumes to thin strong mantle lithosphere is disputed. Here, we propose a new mechanism for the self-sustained generation of magmatism at hotspots where the lithosphere-asthenosphere boundary occurs at < ~100 km. By considering the effects of both chemical and thermal density changes during partial melting of the mantle (using appropriate latent heat and depth-dependent thermal expansivity parameters), we find that mantle residues experience an overall instantaneous increase in density when melting occurs at < ~3 GPa. This controversial finding is due to thermal contraction of material during melting, which outweighs chemical buoyancy effects when melting at shallow pressures (where thermal expansivity is high, at ~4.91 x 10^-5 K^-1). These dense mantle residues have a tendency to sink beneath melting regions, driving the return flow of fertile mantle into the melting region and locally increasing magmatic production. This mechanism presents an alternative to the upwelling of hot mantle plumes for the generation of excess melt at hotspots and the genesis of large igneous provinces during continental breakup. We model the development of magma-rich margins using geodynamic numerical models and find a close match between modelled volcanic crustal thicknesses and real-world observations. “Hot”-spots and large igneous provinces, therefore, may not require the elevated temperatures commonly invoked to account for excess melting.