One of the most important physical parameters to have varied throughout geologic time is heat flow. The majority of terrestrial heat production comes from the decay of radioactive isotopes dispersed throughout the core, mantle, and continental crust (Section 2.13). Heat flow in the past must have been considerably greater than at present due to the exponential decay rates of these isotopes (Fig. 11.1). For an Earth model with a K/U ratio derived from measurements of crustal rocks, the heat flow in the crust at 4.0 Ga would have been three times greater than at the present day and at 2.5 Ga about two times the present value (Mareschal & Jaupart, 2006). For K/U ratios similar to those in chondritic meteorites, which are higher than those in crustal rocks, the magnitude of the decrease would have been greater.
The increased heat flow in Archean times implies that the mantle was hotter in the younger Earth than it is today. However, how much hotter and whether a hotter mantle caused young continental lithosphere to be much warmer than at present is uncertain. This uncertainty arises because there is no direct way to determine the ratio of heat loss to heat produc-
tion in the early Earth. If the heat loss mostly occurred by the relatively inefficient mechanism of conduction then the lithosphere would have been warmer. However, if the main mechanism of heat loss was convection beneath oceanic lithosphere, which is very effective at dissipating heat, then the continental lithosphere need not have been much hotter (Lenardic, 1998). Clarifying these aspects of the Archean thermal regime is essential in order to reconstruct tectonic processes in the ancient Earth and to assess whether they were different than they are today.
Another part of the challenge of determining the Precambrian thermal regime is to resolve an apparent inconsistency that comes from observations in the crust and mantle parts of Archean lithosphere. Geologic evidence from many of the cratons, including an abundance of high temperature/low pressure met-amorphic mineral assemblages and the intrusion of large volumes of granitoids (Section 11.3.2), suggest relatively high (500-700 or 800 °C) temperatures in the crust during Archean times, roughly similar to those which occur presently in regions of elevated geotherms. By contrast, geophysical surveys and iso-topic studies of mantle nodules suggest that the cra-tonic mantle is strong and cool and that the geotherm has been relatively low since the Archean (Section 11.3.1). Some of the most compelling evidence of cool mantle lithosphere comes from thermobaromet-ric studies of silicate inclusions in Archean diamonds, which suggest that temperatures at depths of 150200 km during the Late Archean were similar to the present-day temperatures at those depths (Boyd et al., 1985; Richardson et al., 2001). Although geoscientists have not yet reconciled this apparent inconsistency, the relationship provides important boundary conditions for thermal models of Archean and Proterozoic tectonic processes.
In addition to allowing estimates of ancient mantle geotherms, the evidence from mantle xenoliths indicates that the cool mantle roots beneath the cratons quickly reached their current thickness of >200 km during Archean times (Pearson et al., 2002; Carlson et al., 2005). This thickness is greater than that of old oceanic lithosphere but much thinner than it would be if the lithosphere simply had cooled from above by conduction since the Archean (Sleep, 2005). Progressive thickening by conductive cooling also can be ruled out because the mantle roots do not display an age progression with depth (James & Fouch, 2002; Pearson et al., 2002). Instead, the relatively small thickness and long-term preservation of the cratonic roots indicate that they must have been kept thin by convective heat transfer from the underlying mantle (Sleep, 2003). Once the cratonic roots stabilized, the heat supplied to the base of the lithosphere from the rest of the mantle must have been balanced by the heat that flows upward to the surface. In this model, a chemically buoyant layer of lithosphere forms a highly resistant lid above the con-vecting mantle, allowing it to maintain nearly constant thickness over time. These considerations illustrate how the formation and long-term survival of the cool mantle roots beneath the cratons has helped geoscientists constrain the mechanisms of heat transfer during Precambrian times.
Differences in the inferred mechanism of heat loss from the Earth's interior have resulted in contrasting views about the style of tectonics that may have operated during Precambrian times (e.g. Hargraves, 1986; Lowman et al., 2001; van Thienen et al., 2005). A conventional view suggests that an increased heat supply in the Archean mantle could be dissipated by increasing the length of ocean ridge systems or by increasing the rate of plate production with respect to the present (Bickle, 1978). Hargraves (1986) concluded that heat loss through the oceanic lithosphere is proportional to the cube root of the total length of the mid-ocean ridge. Assuming a nonexpanding Earth (Section 12.3), the increased rate of plate production implies a similar increase in plate subduction rate. These computations suggest that some form of plate tectonics was taking place during the Precambrian at a much greater rate than today. The fast rates suggest an image of the solid surface of the early Earth where the lithosphere was broken up into many small plates that contrasts with the relatively few large plates that exist presently. This interpretation is consistent with the results of numerical models of mantle convection, which show that small plates are capable of releasing more heat from the Earth's interior than large plates (Lowman et al., 2001).
More recent calculations have disputed this conventional view, at least for the Late Archean. Van Thienen et al. (2005) suggested that the increased heat flux from the Archean mantle could have been dissipated by thinning the lithosphere and thereby increasing the heat flow through the lithosphere. These authors concluded that for a steadily (exponentially) cooling Earth, plate tectonics is capable of removing all the required heat at a plate tectonic rate comparable to or lower than the current rate of operation. This result is contrary to the notion that faster spreading would be required in a hotter Earth to be able to remove the extra heat (e.g. Bickle, 1978). It also suggests that reduced slab pull and ridge push forces in a hotter mantle would result in a slower rate of plate tectonics compared to the modern Earth. Korenaga (2006) showed that a more sluggish style of plate tectonics during Archean times satisfies all the geochemical constraints on the abundance of heat-producing elements in the crust and mantle and the evidence for a gradual cooling of the mantle with time in the framework of whole mantle convection. This result removes the thermal necessity of having extensive ocean ridges and/or rapid spreading and subduction. It must be appreciated, however, that thermal conditions during Archean times are quite conjectural, so that these and other alternative interpretations remain speculative.
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