RETROGRADE MINERAL REACTIONS - A HEAT-SOURCE IN THE CONTINENTAL-CRUST

Chemical reactions of water with silicates are strongly exothermic. Th erefeore, retrograde metamorphism is also a strongly exothermic proces s and thus a potentially powerful heat source. Depending on the time s cale its contribution to the terrestrial heat flow can be equivalent t o, or even exceed, that of radioactive decay. During hydration mafic r ocks and gneisses liberate 60 J/cm(3) for each weight percent H2O they bind. It is argued that retrograde reactions constitute relatively sh ort-lived, single or repeated episodes during crustal cooling, and tha t they can influence, or even strongly perturb, the geotherm during th ousands to a million years. The thermal effects involved are nontrivia l; they should be considered in interpretations of heat flow measureme nts. They could constitute the main heat source for hydrothermal miner alisations not connected with magmatic intrusions and possibly contrib ute to the thermal development of sedimentary basins. These effects ar e large enough to be one of the possible explanations for the differen ce between the predicted and measured heat flows at the KTB site.