IMPLICATIONS OF TEMPERATURE-DEPENDENT THERMAL DIFFUSIVITY FOR HEAT FLOW IN THE METAMORPHIC REALM

Metamorphism within the lithosphere is driven by heat flow from the mantle, addition of magmas, and internal heat generation. Our recent measurements of thermal diffusivity, D, in various crustal lithologies demonstrate its strong dependence on temperature (D=f(T)). At 25°C, D ranges from 2.3 mm²s^-1 for quartz rich granitoids to 1.3 mm²s^-1 for schists and granulites. At temperatures >500°C, D converges on ~0.5 mm²s^-1 for most lithologies. Melts have D's as low as 0.25 mm²s^-1. In the lithosphere heat flow is controlled by thermal conductivity, k, which is the product of D, density, and heat capacity (C_P). Although C_P increases with T, for crystalline materials it does not fully compensate for decrease in D; hence k also decreases with T.

The strong temperature-dependence of D (and k) influences the shape of the lithospheric geotherm, retention of heat in hot and partially molten rocks, duration of contact metamorphic events, and other heat-flow dependent processes. Rather than having an often-published convex shape of the steady-state lithospheric geotherm, that results from radiogenic heat production and assumption of constant D and C_P, with D=f(T) the geotherm can be nearly straight, in spite of internal heat production. The reason is that for a given heat flux across the lithosphere, high D near the surface is compensated by lower dT/dz, and at the bottom of the lithosphere, low D is compensated by higher dT/dz. Temperatures in the deep crust are predicted to be lower by >150°C compared to previous models, suggesting that high-grade regional metamorphic conditions are not described by the average geotherm. Therefore, additional heat sources, such as strain heating or magma intrusion, are needed to achieve elevated temperatures in the crust.

With D=f(T) of rocks and low D of melts, the predicted duration of pluton crystallization can more than double in comparison with models that utilize constant D=1 mm²s^-1. Consequently, the duration of contact metamorphic events is much longer than previous models would suggest. Maximum temperatures throughout a contact aureole are somewhat lower with D=f(T); however, temperatures in inner aureoles stay elevated significantly longer because of decreased D and longer cooling of a causative pluton.