Temperature-dependent heat transfer: Implications for geotherms, metamorphism and magmatism

Abstract: 

The thermal evolution of planetary crust and lithosphere is largely governed by the rate of heat transfer by conduction. The governing physical properties are thermal diffusivity (D) and conductivity (k = DρC), where ρ is density and C is heat capacity. Recent advances in laser-flash analysis (LFA) permit accurate (±2%) measurements of solid materials to geologically relevant temperatures. We have collected LFA data for a wide range of crustal rock types, as well as many minerals, glasses and melts, at temperatures up to 1300˚C. The results have some interesting implications for tectonic and magmatic processes, including the identification of several thermo-rheological feedbacks in metamorphic and magmatic systems. Some major findings include:

(i) For most rocks, D strongly decreases from 1.5-2.5 mm2/s at ambient conditions, approaching 0.5 mm2/s at mid-crustal temperatures. Geotherms within the crust are nearly linear, and the crust and Moho are colder (and stronger) than calculated assuming constant D ≈1 mm2/s. At low (near-surface) temperatures, crustal mineralogy has a huge effect on D, so that near-surface geotherms can vary dramatically even for the same heat flux, e.g. across granite-greenstone terranes.

(ii) The temperture dependence of D and C leads to positive feedback between strain heating in shear zones and more efficient thermal insulation, removing the requirement for unusually high radiogenic heat production to achieve crustal melting temperatures in orogenic belts such as the Himalaya.

(iii)The rheology of lava is highly dependent on temperature T (directly and via changing crystal content), and rheology feeds back to T through viscous heating. This may be a significant ongoing heat source (~100 W/m3) within thick dacitic lava flows at Santiaguito, Guatemala, enabling highly viscous lava to travel long distances (~4 km in ~2 yrs). Short-lived but very powerful (≥1 MW/m3) viscous heating within rheomorphic ignimbrites can explain unusually long run-out distances some large-volume (≥10 km3) silicic lavas.