Were Archean continental geothermal gradients much steeper than those of today?

Abstract

ARCHEAN geothermal gradients have often been supposed to be much steeper than those of today^{1 –6}. Because heat generation from the decay of radioactive nuclides in the Earth was then much greater^4,7,8, extra heat, if it escaped by conduction, must have been carried along steeper thermal gradients. Significant measurements of terrestrial conductive thermal gradients are very difficult to obtain, and estimates of thermal gradients in the past are even harder to make. Estimates made from igneous and metamorphic rock occurrences are of limited value both because igneous rocks occur at levels in the earth shallower by unknown amounts than those at which they form and because nearly all igneous and metamorphic rocks are formed at plate boundaries where thermal gradients are much steeper than within plates. In plate margin areas, advective and convective processes dominate so that the normal conductive gradient of the lithosphere can only be measured or estimated in areas away from plate boundaries. We show here that the rocks of the Superior Province (formed roughly in the interval 3,100–2,500 Myr ago) were not subjected to steep regional geothermal gradients after their assembly into a continent by lateral accretion of island arcs⁸, and hence that the extra Archean heat escaped the Earth in some way other than by conduction through the continent. We suggest that this escape was mainly by cooling of the ocean floor as a boundary conduction layer.