Continental crust heat flow

Table 7 Estimates of bulk continental crust heat production from heat flow data.

McLennan S. M. and Taylor S. R. (1996) Heat flow and the chemical composition of continental crust. J. Geol. 104 396-377. 

Sclater J. G., Jaupart C. J., and Galson D. (1980) The heat flow through oceanic and continental crust and the heat loss of the earth. Rev. Geophys. Space Phys. 18, 269—311. 

The area weighted heat flow for all provinces older than 200 Ma is 51 mW m (Stein, 1995). Estimates of the mantle heat flow by various authors vary between 11 mW m andl8 mW m (Jaupart and Mareschal, 1999). After removing the mantle heat flow, the average contribution of the crust to the surface heat flow in stable continental regions is between 33 mW m and 40 mW m. This implies that the bulk crustal heat production is 0.9 0.1 p,W m. ... 

Jaupart C., Sclater J. G., and Simmons G. (1981) Heat flow smdies constraints on the distribution of uranium, thorium and potassium in the continental crust. Earth Planet. Sci. Lett. 52, 328-344. 

Deming D. (1994) Fluid flow and heat transport in the upper continental crust. In Geofluids Origin, Migration, and Evolution of Fluids in Sedimentary Basins (ed. J. Parnell). Geological Society of London, Special Pubhcation, London, vol. 78, pp. 27-42. 

Fig. 14. Fluxes of CO2 are shown as functions of time for the high heat-flow models in Figures 12 and 13. Impact ejecta are of minor importance compared with the rapid churning of the oceanic crust. In Archaean time, CO2 is mostly controlled by processes involving the creation and subduction of oceanic crust. Continents become increasingly important through Proterozoic time, with the transition from mantle to continental control occurring at c. 1.4 Ga.

Heat flow data provide important constraints on mantle models. For example, combined with the heat producing element content of crust and mantle rocks, and the physical properties of mantle minerals, they can be used to constrain the nature of thermal convection in the mantle. In addition, variations in the mantle contribution to crustal heat flow between the continents and oceans have been used to make inferences about the nature of the different types of mantle underlying continental crust and oceanic crust (Section 3.1.2 and Chapter 4, Section 4.3.1.2). Furthermore, heat flow data, combined with bathymetric measurements, rates of sea-floor subsidence, and the depth of seismic discontinuities are all a function of mantle temperature and can be used to estimate relative, lateral variations in mantle temperature (Anderson, 2000). 

A related debate focused on heat flow data from different regions of the continental crust. Nyblade and Pollack (1993) showed that average heat flow measurements in Archaean cratons are lower than those for Proterozoic cratons. This observation has, however, been interpreted in two quite different ways. On the one hand, it has been argued that cratons of different age have different bulk compositions, and so have different concentrations of heat producing elements (U, Th and K), hence different levels of heat production. Alternatively, the observed differences in heat flow do not derive from the crust but reflect different lithospheric thicknesses between Proterozoic and Archaean cratons reflecting different mantle heat flow contributions (Rudnick et al., 1998 Nyblade, 1999). 

McLennan, S.M., Taylor, S.R., and Hemming, S.R., 2006. Composition, differentiation and evolution of continental crust constraints from sedimentary rocks and heat flow. In Brown, M. and Rushmer, T. (eds) Evolution and Differentiation of the Continental Crust. Cambridge University Press, Cambridge, pp. 92-134. 

Geological, petrological, geochemical, and geophysical observations each provides important constraints on the composition and evolution of the continental crust and combine to allow some broad generalizations. Thus the heat-flow data indicate that the composition of the readily observable upper crust cannot persist below about 10 km, so that the lower crust is in many ways a distinct geochemical entity. 

In comparison with the well-established composition of the upper crust, there is less of a consensus about bulk crustal composition as arriving at the average composition of the bulk, rather than the exposed crust is complicated (Fig. 8). The most important constraint on models of bulk crustal composition relies on the interpretation of continental heat-flow data (Fig. 9). 

The sedimentary rock data provide information only on that portion of the crust exposed to weathering and erosion, but the upper crust is not representative of the entire 41-km thickness of the continental crust. Mass balance calculations show that a 41-km thick crust with K, Th, and U abundances equal to that of the present upper continental crust would require about 80-90% of the entire Earth s complement of these elements to be present in the continental crust. The heat-flow data show that the upper crust (about 10 km thick) is strongly enriched in the heat-producing elements (K, U, and Th). 

FIGURE 9 Plot of K20 versus crustal heat flow, comparing various estimates of bulk continental crust composition. Heat-flow constraints are shown in the vertical dashed lines. The lower limit assumes that the lower crust contributes no heat and all heat-producing elements are contained within the upper crust. The absolute upper limit is given by the total average heat flow from stabilized continental crust and thus assumes no mantle contribution to heat flow. A more realistic upper limit is model-dependent and adopts modest mantle heat-flow contributions suggested by detailed geochemical studies of deeply exposed crustal cross sections. 

Most of the crust is generated in the Late Archean, with lesser additions from later island-arc volcanism, to make up the present crust. The overall crustal bulk composition was calculated from a 60/40 mixture of the Archean bimodal and the Post-Archean andesitic compositions. These result in the following concentrations for the heat-producing elements in the bulk continental crust 1.1 % K, 4.2 ppm Th, and 1.1 ppm U, which give the crustal component of the heat flow of 29 mWm-2, or slightly over half of the total heat flow measured in the continental crust. Thus there may be little difference in bulk composition between the Archean and Post-Archean bulk crust despite a significant difference in their upper crustal compositions. The bulk crustal compositions in fact are quite similar (Post-Archean values of 1.1% K 4.2 ppm Th, and... 

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