CHANGING THERMAL REGIMES THROUGH TIME AND IMPLICATIONS FOR MINERAL GENESIS

Although there are many uncertainties concerning the detailed thermal structure of oceanic and continental lithosphere during earlier stages of Earth history, two inferences may be made with confidence: 1) globally the Earth had much more heat to lose because of secular cooling of the Earth and the secular decay of radiogenic heat producing elements in the Earth; and 2) fragments of surviving Archean and Proterozoic lithosphere had higher geotherms when they initially stabilized than they have today because of secular decay of their intrinsic radiogenic heat production. From the second inference, although gem-quality diamonds are restricted to pipes in Archean cratons, the Archean age commonly assigned to these diamonds is inconsistent with simple thermal back calculations for Archean lithosphere.

Modern oceanic or continental hydrothermal mineralizing systems are buffered in their maximum temperatures by the boiling point curve and ultimately by the solidus appropriate to the magmatic source driving the system. Thus, as these systems are buffered in modern systems, they would have been similarly buffered in ancient systems. If they were hotter, the difference in temperature would have been associated with more ultramafic magma sources, as indicated by Archean komatiites, not directly because the Earth was losing more heat. However, we can conclude that hydrothermal systems would have been more numerous or more aerially extensive in the Archean because the Earth was losing more heat.

Different and/or modified thermotectonic regimes have been proposed for the ancient Earth, such as a higher rate of production of oceanic crust, the production of thicker oceanic crust, and the increased importance of hot spots. No oceanic crust remains from these regimes, but Archean continental lithosophere is distinctive in both the bulk chemical signature and mantle properties that accompany its mineralization.