Australia: The Land Where Time Began

A biography of the Australian continent 

Precambrian heat flow

Heat flow is one of the most important physical parameters that varied over geologic time. Most of the heat production of the Earth results form radioactive decay of isotopes dispersed throughout the core, mantle and crust. it is believed the heat flow should have been much greater in the distant past. In a model of heat flow of the Earth based on a K/U ratio derived from measurements taken from crustal rocks, it has been estimated that 4 Ga the rate of heat flow would have been 3 times more than that of the present, and at 2.5 Ga, it would have been about twice the present flow (Mareschal & Jaupart, 2006). For the K/U ratio similar to those found in chondritic meteorites the decrease would have been greater than in the crustal rocks.

The younger Earth is believed to have been hotter than at present, as indicated by the estimated heat flow of the Archaean. Just how hot the mantle was is uncertain, as is how warmer the continental lithosphere would have been  in the past, though the greater heat flow in the Archaean implies that it would have been warmer than at the present. As there is no known way to directly determine the heat loss/heat production ratio in the early Earth the actual situation is uncertain. If heat flow was mainly by conduction, a relatively inefficient mechanism, the lithosphere would have been warmer. If convection beneath the lithosphere of the oceans was the main mechanism, a much more efficient heat-dissipating mechanism, the lithosphere of the continents need not have been much hotter (Lenardic, 1998). To fully understand, or at least understand better, the tectonic processes that operated in the early Earth, and to determine if they were different from those operating at the present, the thermal regime of the Archaean needs to be clarified.

There is an apparent inconsistency between the crust and the mantle of the Archaean lithosphere, according to observations from the crust and mantle parts of the lithosphere from that time. This adds to the problem of determining the thermal regime of the Archaean. Relatively high temperatures are suggested, 500-700 or 800oC, for the crust in the Archaean, from geologic evidence found in many of the cratons, as well as the many metamorphic mineral assemblages that are of high temperature and low pressure, and large volumes of granitoid intrusions. The suggested temperatures are approximately similar to those found in regions of elevated geotherms. Countering this, evidence from mantle geophysical surveys and isotopic studies that have been carried out on mantle nodules indicate the cratonic mantle is strong and cool, the geotherm being relatively low since the Archaean. Thermobarometric studies of silicate inclusions in diamonds from the Archaean has produced some of the most compelling evidence for a cool Archaean mantle lithosphere, suggesting that during the Late Archaean at depths of 150-200 km, temperatures were similar to those of the present (Boyd et al., 1985; Richardson et al., 2001). Important boundary conditions for thermal models of Archaean and Proterozoic tectonic processes are provided by the relationship that has been found, though the apparent inconsistency has not yet been reconciled (see Source 1).

The evidence from mantle xenoliths indicates that in the Archaean the cool mantle roots beneath cratons reached their current thickness, 200 km, over a very short period of time. This evidence also allows estimates of geotherms in the ancient mantle (Pearson et al., 2002; Carlson et al., 2005). The lithosphere of these cratonic roots is thicker than old oceanic lithosphere, but it is believed that if the lithosphere had cooled by conduction since the Archaean, from above, it would have been much thicker. There is no age progression with depth of the mantle roots, ruling out progressive thickening resulting from conductive cooling from above (James & Fouch, 2002; {Pearson et al., 2002). It has been suggested that the cratonic roots were heated by convection in the underlying mantle, resulting in their relatively small thickness and long-term survival (Sleep, 2003). The heat that flows upward to the surface is thought to have been balanced by the heat flowing to the base of the lithosphere from the remainder of the mantle, after the cratonic roots had stabilised. According to this model, a lithosphere layer, that is chemically buoyant, forms as a highly resistant cover above the mantle convection, which is thought to allow it to remain at an almost constant thickness over time. Based on these considerations, the formation and long-term survival of cratonic mantle roots has helped make it possible to constrain the heat transfer mechanisms during the Precambrian.

Differing views of the tectonic styles that may have been operating in the Precambrian have resulted from differences in the inferred heat loss from the Earth's interior (e.g. Hargraves, 1986; Lowman et al., 2001; van Thienen et al., 2005). According to a conventional view, it has been suggested that increasing ocean ridge system length, or an increase of plate production rate, compared to the present, could dissipate the increased heat supply in the Archaean mantle (Bickle, 1978). It has been claimed that the oceanic lithosphere heat loss is proportional to the cube root of the total mid-ocean ridge length (Hargraves, 1986). A similar increased rate of plate subduction is implied by an increase of plate production rate, assuming the Earth is not expanding. Some form of plate tectonics was occurring in the Precambrian at a much greater rate than at present, as suggested by these calculations. The implication of the rapid rates are of a solid surface of the early Earth in which there were many small plates, as opposed to the fewer large plates of the present. Numerical models of mantle convection produce results that are in accord with this interpretation, suggesting that a large number of small plates dissipate more heat from the interior of the Earth than a smaller number of large plates (Lowman et al., 2001).

For the Late Archaean, this conventional view has been disputed. It has been suggested that lithosphere thinning, leading to increased heat flow through the lithosphere, could have been the main cause of the increased heat flux from the Archaean mantle (van Thienen et al., 2005). Plate tectonics has been suggested to be capable of removing the heat required at a plate tectonic rate that was comparable, or even lower, than at the present, for a steadily (exponentially) cooling Earth (van Thienen et al., 2005). As well as being contrary to the idea that faster spreading would be required to remove the extra heat in a hotter Earth (e.g. Bickle, 1978), it also suggested a slower plate tectonic rate than at the present, resulting from reduced forces of slab pull and ridge push in the hotter mantle. It has been shown that all geochemical constraints on heat-producing element abundance in the crust and mantle are satisfied by a slower style of plate tectonics in the Archaean, as well as evidence for a mantle that is gradually cooling over time, in the framework of whole mantle convection (Korenga, 2006). The thermal necessity of having rapid spreading or subduction and/or ocean ridges that are extensive, is removed by this result. All interpretations remain speculative, as the thermal conditions that were present during the Archaean are quite conjectural (see Source 1).

Sources & Further reading

  1. Kearey, Philip, Klepeis, Keith A. & Vine, Frederick J., 2009, Global Tectonics, 3rd Edition, Wiley-Blackwell.
 
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading