Thermal evolution of cratonic roots

Cratons stabilized in the Archean because their mantle roots remained strong despite temperatures in the convecting mantle and amounts of radiogenic elements in the crust and in the lithospheric mantle that were larger than today. Thermal evolutionary models are developed using constraints from heat flow and heat production data in Archean provinces. The large time-scale of diffusive heat transport implies that the lithospheric mantle can remain thermally decoupled from the crust for as long as 1 Gyr. Heat production in the lithospheric mantle is a key variable in determining thermal conditions that permit stabilization of the crust and the preservation of a thick cratonic mantle root.Archean provinces are currently characterized by low heat flow, with an average of 41 mW m− 2 less than the global continental average (56 mW m− 2). The range of regionally averaged heat flow values in Archean provinces (18–54 mW m− 2) is narrower than in Proterozoic and Paleozoic terranes. However, at the end of the Archean, when crustal heat production was double the present value, surface heat flow would have varied over a range (≈ 45–90 mW m− 2) at least as wide as that presently observed in Paleozoic provinces. The high crustal heat production during the Archean is not sufficient to account for elevated lower crustal temperatures recorded in some metamorphic assemblages without some additional heat input, or without the crust being thicker, or the vertical distribution of radioelements being different from today's.Thermal models and the observed correlation between the degree of upper crustal enrichment and heat production values indicate that crustal melting and differentiation were largely driven by heating due to in-situ radiogenic heat production. Even for values of the surface heat flow higher than average in cratons, the crust can be stabilized before 2.5 Ga if the radioelements are confined to shallow crustal layers. For present surface heat flow of 40–45 mW m− 2, calculations indicate that, prior to differentiation, the lowermost crust was near the solidus.Present heat production in the mantle root is constrained by estimates of the mantle heat flow. Further constraints can be obtained by modeling the past thermal regime of the root when heat production was higher. If heat generation is high and/or if the root is thick, the lower lithosphere must have cooled more rapidly than the convecting mantle, which implies that the temperature gradient was inverted at the base of the lithosphere and a weak mechanical layer formed in the middle of the root, precluding its survival. A low temperature gradient at the base of the root leads to the development of convective instabilities and possible removal of the lowermost part.Following its isolation from the convecting mantle and stabilization, thick continental lithosphere adjusts slowly to in-situ heat production and basal heat supply. We show that, due to in-situ heat production, temperatures in the lithospheric mantle may rise above their initial values inherited from the process of root formation due to in-situ radiogenic heat production.