Dynamical models of mantle volatile evolution and the role of phase transitions and temperature‐dependent rheology

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Abstract
Helium isotopic variations demand the preservation of a primitive volatile source for ocean island basalts (OIB) in conjunction with a well‐mixed and more radiogenic source for mid‐oceanic ridge basalts (MORB). Dynamical models of the Earth's evolution should be able to predict this basic geochemical observation. We have developed a number of increasingly more realistic models of mantle convection that satisfy present‐day heat loss and plate velocities to study the effects of convective mixing, radiogenic ingrowth, and degassing on the mantle3He/4He evolution. We have included mechanisms that have been proposed to enhance the isolation of individual reservoirs in the mantle such as high lower mantle viscosity, strongly temperature‐ and pressure‐dependent rheology, and an endothermic phase transition at 670 km depth. Although the combination of these mechanisms can produce regions of lower mixing efficiency, our models cannot satisfactorily explain the existence of two distinct OIB and MORB3He/4He sources. If further improvements to the model, such as simulated plates and continents, still fail to explain the geochemical constraints, it may be prudent to consider sources of primitive helium beyond the current paradigm requiring them to be stored within the terrestrial mantle since early in the Earth's history.