Reaction Enthalpies of Subsolidus Equilibria in Pelitic Rocks. Magnitude and Influence on Cretaceous Metamorphism of the Koralm Complex, Eastern Alps.

Summary: The unusual high metamorphic grade of Eoalpine metamorphism in the Koralm Complex, Eastern Alps may, in principle, have contributions from three different heat sources. These include (i) conduction of heat from deeper crustal levels; (ii) advection of heat by fluids or magmas and (iii) internal heat production. We have shown elsewhere that heat conduction alone is unlikely to be the only important mechanism of heat transfer into the complex. The other potential heat sources need therefore to be evaluated. It is equally important to evaluate heat sinks and an integrated thermal model needs a careful comparison of the importance of heat sources and sinks during metamorphism. Here, we assess the importance of enthalpy production by metamorphic reaction to retrograde metamorphism of the Koralm complex and we speculate on alternative heat sources for the prograde path. In particular, we calculate the change of enthalpy content of assemblages with temperature for two pelitic bulk compositions representative of the Koralm complex. Our calculations confirm that the influence of enthalpies of reaction on the overall energy budget of the crust is small and that enthalpy changes across reactions may be well-approximated by an appropriate average heat capacity. For the two bulk compositions from the Koralm complex considered here, these are cp = 1380 J kg"'K" ' and cp =1550 J kg~'K~'. Our results have two implications with particular relevance for the Koralm