Heat Flow on Hotspot Swells Reflect Fluid Flow Masking Potential Variations in Mantle Heat Flux

The origin of hotspot swells is poorly understood. Heat flow data collected on hotspot swells have been used to argue for and against sublithospheric thermal anomalies. The presence of sublithospheric thermal anomalies has been inferred from interpretations of anomalously high heat flow determinations, whereas the contention that hotspot swells result from normal melting processes within the lithosphere is based on `normal' heat flow values. These arguments depend in part on the choice of a thermal reference model, but more importantly assume conductive heat transfer through the lithosphere. We provide evidence that heat flow measurements collected on hotspot swells likely reflect shallow fluid flow rather than deep thermal variations within or at the base of the lithosphere. Discriminating between environments where heat is transferred conductively or convectively requires closely spaced heat flow determinations (1-2 km) collocated with seismic reflection profiles. Only Hawaii and Reunion have surveys meeting these requirements. The Hawaiian survey consists of two profiles, one north of Oahu and one north of Maro Reef. The Reunion survey also consists of two profiles, both north of Mauritius. These heat flow profiles reveal greater scatter than anticipated with spectral peaks on the order of 10 km consistent with fluid flow. Root mean square variations along the Oahu and Maro Reef profiles are 15 and 5 mW m^-2, respectively, and along both Reunion profiles are about13 mW m^-2. Coupled heat and fluid flow models demonstrate that thermal buoyancy due to bathymetric relief is capable of driving significant fluid flow that may suppress the background thermal field. These models are consistent with heat flow patterns observed at individual seamounts and oceanic basement highs that are more easily sampled and characterized than large hotspot swells. We caution that the ability of fluid flow to mask variations in sublithospheric heat flux make surface heat-flow values a poor discriminator between geodynamic models for hotspot swells.