Are hotspots hotter than ridges?

Abstract:

To understand the origin of hotspots and mid-ocean ridges, it is crucial to investigate the differences in temperature and composition of their mantle sources. Olivine geothermometry suggests 100-300 °C of excess potential temperature at sub-hotspot mantle compared to sub-ridge mantle (Courtier et al. 2007, Putirka 2008). Inspired by a previous work inferring temperature for ridges (Dalton et al. 2014), we re-examine the differences in temperature between hotspots and ridges by obtaining temperature directly from seismic tomography, using a thermodynamically derived self-consistent velocity to temperature conversion (Stixrude and Lithgow-Bertelloni, 2005, 2011). Assuming the upper mantle and plume source are compositionally homogenous and consist of depleted MORB mantle (DMM, Workman and Hart, 2005), we find that: 1) Half or more of oceanic hotspots are not resolvably hotter than ridges; 2) Hotspots with high ³He/⁴He and high buoyancy flux are significantly hotter than ridges; 3) Globally averaged there is no appreciable temperature difference for either hotspots or ridges as a function of their proximity to each other. These conclusions are robust even if there are some compositional differences (e.g. recycled crust) between plume sources and DMM, although the inferred excess temperatures are higher.