Heat flow constraints on the mafic character of Archean continental crust
Résumé
The bulk composition of the continental crust can be constrained from its surface heat flow. Published values of heat flow and heat production from 17 areas ranging in age from 3.3 to 0.4 Ga are compared. In the Superior province, the ca. 2.7 Ga accreted belts have a heat flow of 41mWm-2, significantly higher than that in the older, ca. 3.0 Ga, craton core (32mWm-2) (Jaupart et al., 2014). The mean surface heat flow values are 40 ± 1.2mWm-2 for the Archaean areas, and 53 ± 1.9mWm-2 for the post-Archaean. Heat flow values into the base of the crust are constrained from P-T estimates in lithospheric mantle xenoliths, and subtracted from the surface heat flow to calculate the bulk crustal heat production, whereupon there is a strong correlation between the surface heat flow data and the calculated average bulk crustal heat production values. Average crustal heat production values are described in terms of two component mixtures of felsic crust with > 57% SiO2, and mafic crust with < 57% SiO2, which is suggested to be more robust than relying on an average seismic model, given the complexity of the crustal structure and how it varies between provinces, and sometimes within a single province. The proportions of the mafic crustal component required to account for the regional bulk heat production values in 5 areas older than 2.8 Ga range from 60-78% consistent with a fixed mafic proportion of ∼ 70%. This is higher than the proportion of lower, mafic crust in the present bulk crust of Rudnick and Gao (2014), which is 43%, and the estimated bulk crust MgO values change from 10-6.6% in areas older than 2.8 Ga, to ∼ 5% MgO for the younger areas. The bulk crust in the late Archaean was on average more mafic than the bulk crust at the present day, and it had a bulk crust Rb/Sr value of 0.12 ± 0.04.