4.12 - Physics and Chemistry of Deep Continental Crust Recycling

Abstract

The continental crust is the end product of multistage magmatic differentiation, wherein felsic liquids and mafic rocks in the form of cumulates and restites are generated. The felsic liquids segregate toward the surface and leave behind mafic rocks at depth. At high pressures, these mafic rocks stabilize garnet and pyroxene, imparting negative compositional buoyancies that can often exceed thermal buoyancies. If these buoyancy forces exceed viscous resisting forces, the mafic rocks should, in theory, founder into the mantle at rates limited by their viscosities, which, in turn, depend largely on temperature. Foundering results in the net removal of mafic rocks into the mantle so that the remaining crust becomes more felsic. Lower crustal foundering, once initiated, is likely to be fast compared to thermal diffusion timescales, leading to the replacement of the cold mafic lower crust by the hot upwelling asthenospheric mantle. Topographic uplift, high surface heat flux, and enhanced low-melt-fraction basaltic magmatism are predicted consequences of asthenospheric upwelling in response to deep crustal foundering, but geologic context and additional constraints should be considered because these phenomena can also be driven by active upwelling and/or incipient extension. Additional constraints come from seismic studies of the deep lithosphere and asthenosphere, but the most diagnostic are xenolith studies, which constrain temporal changes in the composition and thermal state of the lower crust and mantle at depth. These concepts are highlighted in a review of four regions where lower crustal or lithospheric mantle recycling has been proposed on the basis of a combination of petrologic, geochemical, geologic, and geophysical observations. Finally, the petrogenetic origin of these dense mafic rocks is reviewed, with particular emphasis on their origins during magmatic differentiation in subduction zones. On the basis of mass balance, the proportions of mafic cumulate formation in arc environments are estimated. When scaled up to the global rate of magmatic production in subduction zones, global recycling rates of cumulates in subduction zones are shown to be up to 20% of the rate of oceanic crust production. These cumulates are low in Si and enriched in Mg and most of the first-row transition metals. Recycling of such materials back into the mantle leaves behind a felsic continental crust. These cumulates may also have significant quantities of Nb, Ta, Ti, and Pb. The latter, in particular, suggests that foundered mafic cumulates could represent the low U/Pb reservoir needed to balance the high, time-integrated U/Pb signature of the crust and the uppermost mantle. These recycled lower crustal materials may populate much of the upper mantle and, because of their fertile major-element compositions, can preferentially remelt if they become entrained in plumes or small-scale convective instabilities.