The chemistry of granites has changed significantly over the past 3.85 billion years. We can track this evolution by examining a trace elements. I specifically focus on uranium, thorium and potassium because they produce heat as a result of radiogenic decay. Whether they are concentrated in the crust or mantle, they have a significant influence on the thermal evolution of Earth's interior.
The heat produced by U, Th and K has decreased with time along with their concentration. A rock that is 3.85 billion years old today generates approximately one-quarter of the heat it would have produced at its inception. This decrease has made the ancient rocks cooler, and as a result, more resistant to deformation forces. However, this is only because rocks in the more recent past have higher heat production at present than these ancient rocks.
If the processes that generate melts in the interior have been the same throughout Earth's history, then we might actually suspect that the difference in heat production would be independent of when the rock was created. The percentage of trace elements extracted during melting is independent of when the melt is generated. Also the rate of decay of heat producing elements is the same in the source as it is in the product. What does this mean? A granite that was generated ca. 4 billion years ago would have half the initial radioactivity at 2 billion years. A granite produced at ca. 2 billion years ago would be drawn from a source that has half as much radioactivity as the 4 billion year old sample, but if the percentage of heat producing elements is the same for both the 4 and 2 billion year old granites, then the heat production should be the same for both.
However, ancient granites have far lower heat production than 2 billion year old granites. Granites generated at ca. 2 billion years have about the same heat production as granites produced at present. Either the processes that form granites changed from 4 to 2 billion years ago, or the granites with high heat production were preferentially destroyed. The first is certainly true because we can see this in bulk composition of granites, but we also see it in granites with a fairly uniform composition. In fact we see it in most igneous rock types. Therefore, we suggest both processes could explain the low heat production of ancient granites.
What process could preferentially destroy rocks with high heat production? It may not have been one specific process, but any process that results in the deformation of the crust. Crust with higher than average heat production would have been very hot and weak whereas crust with low heat production would have been cold and strong. The latter would have difficulty resisting deformation and may be preferentially destroyed leading to a bias towards lower heat producing crust.
Hasterok, D., Gard, M., Cox, G., Hand, M., 2019. A 4 Ga record of granitic heat production: Implications for geodynamic evolution and crustal composition of the early Earth. Precambrian Research, Precambrian Research 331, 105375. https://doi.org/10.1016/j.precamres.2019.105375
Gard, M., Hasterok, D., Hand, M., Cox, G., 2019. Variations in continental heat production from 4 Ga to the present: Evidence from geochemical data. Lithos, Lithos 342-343, 391–406. https://doi.org/10.1016/j.lithos.2019.05.034
Comments