Fifty-one Years of Hawaiian Hotspot Debate

Abstract:

Tuzo Wilson’s ground breaking realization that the progressively increasing ages of volcanism along linear chains of Pacific islands are best explained by movement of the Pacific plate over a stationary heat source was followed by Jason Morgan’s physically more plausible plume theory. Hawaii, the “mother” of all plumes/hotspots, has become the focus of a continuing burden-of-proof battle about the existence of mantle plumes. The monotonic age progression of this island chain has been amply confirmed, but the hotspot locus is not perfectly fixed, and this has been taken to be a failed test of the plume model. Numerous numerical experiments in bottom-heated settings have demonstrated that plumes are a physically natural mode of convection, but antiplumers focus on the fact that in internally heated media, plumes are unlikely. Early proposals of propagating lithospheric fractures with magma tapped from the underlying asthenosphere have never been sufficiently developed to seriously compete with the convection models. Geochemists eagerly adopted the plume model and developed highly model-dependent criteria for identifying mantle plumes, but these are easily knocked down, because most geochemical tracers carry no information about the specific mantle depth being sampled. Conversely, petrological-geochemical models invoking exclusively asthenospheric magma sources have not explained how, in the absence of a Bunsen burner, high melt production can be localized in one geographic spot for 10⁸ years. Our work has focussed on using dynamic modelling to trace the path of geochemical heterogeneities from the base of the mantle to the volcanoes. We find that large-scale heterogeneities existing in the lower thermal boundary layer are drawn into close proximity in the plume conduit and in the erupting volcanoes. Geochemistry cannot prove mantle plumes, but plumes offer a promising path to understanding the geochemistry of the deep mantle.