Continental Extension and the Geodynamics of Mantle Melting

In both active and passive continental rifting, mantle partial melting is the consequence of mantle decompression. All other things being equal, the timing between partial melting and continental extension, as well as the volume of basaltic melt, depend on the depth at which the mantle adiabat intersects the solidus. In other terms, it depends on the mantle potential temperature. The mantle potential temperature (TP) is the temperature measured at the Earth’s surface from extrapolation of the mantle adiabat (McKenzie and Bickle, 1988). It is a convenient way to express the temperature of the ambient convective mantle, which on average is about 1330ºC.

Computer models in 2D and 3D show that, under continental lithospheres, TP varies depending on the size and distribution of continents at the Earth’s surface. In particular, during the aggregation of supercontinents, the potential temperature can increase by up to 150ºC in response to the change in the wavelength of mantle convection (Coltice et al., 2007; 2009). This seems to be confirmed by petrological and geochemical data on continental flood basalts from the Central Atlantic Magmatic Province, which marks the early stage of the dispersal of Pangaea. Most CAMP primary magmas point to a TP between 1400 °C and 1500 °C (Hole, 2015). These temperatures - too hot for decompression melting of a normal convective mantle, and too cold for decompression melting in a mantle plume – are compatible with mantle warming under supercontinents.

The presence of a hotter mantle underneath supercontinents has some important implications for the formation and evolution of continental margins formed during the breakup. The slope of the mantle solidus (~2.6 K per km) is such that a convective mantle 150ºC warmer starts to melt ~ 57 km deeper than a normal ambient mantle. Assuming an upwelling velocity of 0.5 cm per year (~minimum half spreading rate), volcanism would start up to ~12 myr earlier compared to decompression melting of a normal ambient mantle. In addition, the volume of melt extracted through decompression melting of a hotter mantle could be 3 to 4 times larger than that extracted from a convective mantle with a cooler potential temperature. Hence, the dispersal of supercontinents above a hotter mantle may result in dominantly volcanic continental margins, in which abundant volcanism occurs early in the extension history.

• Coltice, N., Phillips, B.R., Bertrand, H., Ricard, Y., and Rey, P.F., 2007, Global warming of the mantle at the origin of flood basalts over supercontinents: Geology, v. 35, p. 391–394, doi:10.1130/G23240A.1.
• Coltice, N., Bertrand, H., Rey, P.F., Jourdan, F., Phillips, B.R., and Ricard, Y., 2009, Global warming of the mantle beneath continents back to the Archaean: Gondwana Research, v. 15, p. 254–266, doi:10.1016/j.gr.2008.10.001.
• Hole, M.J., 2015, The generation of continental flood basalts by decompression melting of internally heated mantle: Geology, v. 43, p. 311–314, doi:10.1130/G36442.1.
• McKenzie, D., and Bickle, M.J., 1988, The volume and composition of melt generated by extension of the lithosphere: Journal of Petrology, v. 29, p. 625–679, doi:10.1093/petrology/29.3.625.