Basalts record a limited extent of mantle depletion: cause and chemical geodynamic implications

Abstract

Radiogenic isotope ratios in basalts from mid-ocean ridges are commonly thought to represent the time-integrated extent of incompatible element depletion of the sub-ridge, peridotitic mantle. Earth’s peridotitic mantle, however, is variably incompatible element depleted, and inherently heterogeneous as a consequence of prior melting. After aging for several 107–109 years in the mantle before remelting today, the heterogeneous peridotites are characterised by a much larger range of radiogenic isotope ratios than ridge basalts. The simple reason why ridge, but also ocean island basalts, reflect only a limited range of this enormous isotopic spectrum of peridotites is that mixing of melts from Earth’s heterogeneous mantle moderates peridotite heterogeneity. Variable peridotite compositions may nevertheless be responsible for isotopic differences between ridge and ocean island basalts, and contribute significantly to the thermochemical buoyancy of mantle plumes, and density-driven mantle flow in general. Variable peridotite depletion therefore connects geochemical and geophysical observables, and is a critical parameter for advancing our understanding of basalt generation, plume formation, and chemical geodynamic models of mantle convection.

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