CHANGING THERMAL REGIMES THROUGH TIME

The interior of the Earth is cooling. This cooling is a response to loss of its heat of its formation and an exponential decrease in internal heat production from the unstable, long half-life isotopes that provide most of the energy for recent tectonics and volcanism, primarily ²³²Th, ^{235, 238}U, and ⁴⁰K. This cooling has been accompanied by a decrease in the near-surface heat flow and associated thermal and tectonic processes associated with ore genesis. There is not a simple relationship among heat loss and ore genesis processes because of complicating factors such as early impact processes, oxygenation of the atmosphere, and perhaps changes in continental lithospheric buoyancy and tectonism through time. However, there is a signal in nickel, gold, base metal, and other ore deposits that is a response to changing thermal regimes through time.

Heat generation from uranium, thorium and potassium at the beginning of the Archean was about 3 times higher than at present and if secular cooling of the Earth is included, heat loss may have been 4 to 5 times higher. The thermal regime has been cooling to the present. Today about 71% of the global heat loss is through the oceans, primarily in the formation and cooling of oceanic lithosphere. This percentage could have been significantly higher earlier in Earth history by an increased rate of seafloor spreading, length of ocean ridges, or thickness of oceanic crust formed, or some combination of these factors. The percentage of heat that can be lost through the ocean floor is limited, however, by the leat lost before subduction which is limited by the area of the oceans at any time in the past. Hotter and/or thicker average ancient oceanic crust would limit the development of back-arc basins and associated ore deposits. Even if most of the excess heat loss in the past were lost through the oceans, average continental geotherms would be higher than at present. More rapid heat loss through oceanic crust requires a higher temperature drop across the system to drive the tectono-volcanic engine, requiring higher asthenosphere temperatures that would also occur under continental lithosphere. In addition secular decay of radiogenic heat production implies that this heat generation was higher in ancient continental crust that it is in the same crust at present, yielding higher temperatures for ore genesis.