Stagnant lid convection and carbonate metasomatism of the deep continental lithosphere

Abstract

The current (and past) mantle adiabat beneath continents is hot enough that ascending carbonate‐rich domains partially melt liberating kimberlites. Over the last 2 Ga, most of these kimberlites have been trapped and frozen in the deep continental lithosphere, forming a massive CO2 reservoir. Geochemical studies of kimberlites that reached the surface indicate the pervasiveness of their effects within the deep lithosphere. These magmas extensively reacted with the deep lithosphere on the way up. Conversely, studies of deep mantle xenoliths brought up in kimberlites detect repeated episodes of metasomatism by these fluids. The heat balance between convection and conduction through the lithosphere provides gross constraints on the cumulative global effects of this process. Stagnant lid (including chemical lid) convection supplies heat to the base of continental lithosphere in equilibrium with the conductive heat flow. The thermal gradient beneath continents is constrained by xenolith geotherms and mantle heat flow q is ∼20 mW m−2. The upwelling velocity V at the base of the thermal boundary layer is ∼q/TηρC, where ρC is volume specific heat, 4 MJ m−3 and Tη is the temperature to change viscosity by a factor of e, ∼60 K. The upwelling velocity is thus ∼2 km per million years; the mass of the mantle circulates beneath continents over ∼2 Ga. The rate that plate tectonics recirculates the mantle is comparable and limits the supply carbonate‐rich undepleted material to the melting zone of upwelling stagnant lid convection and thus the production of kimberlites. Overall the amount of CO2 emplaced by kimberlites into continental lithosphere is a significant fraction (∼1/2) of that in the convecting mantle, resulting in a major deep lithospheric reservoir of CO2 of ∼5000 × 1018 moles, an equivalent thickness of ∼2 km carbonatized rock. This process, however, is insufficient to affect the overall density of deep lithosphere. The observed linearity of the xenolith geotherm precludes equivalent thicknesses of several kilometers depending on the radioactive element concentration of actual kimberlites. Kimberlites are partial melts often of subducted carbonatized oceanic crust and hence have CO2 to radioactive element ratios higher than the bulk mantle. Most kimberlites remain within the deep lithosphere because ascending dikes do not usually penetrate the region of horizontal compression in the strong cool part of the lithosphere. Conversely stresses relax quickly within the warm deep lithosphere allowing dikes to repeatedly intrude.