Thermal transport properties of major Archean rock types to high temperature and implications for cratonic geotherms

The thermal structure of Archean terranes depends in part on the thermal transport properties of their distinctive rock types. We determined thermal diffusivity (D) of a suite of 14 greenstone, mafic granulite and tonalite-trondhjemite-granodiorite (TTG) rocks at temperatures up to 1000°C at atmospheric pressure using laser flash analysis, which lacks systematic errors associated with conventional contact techniques. At room temperature, D ranges from ~3.8mm²s^-1 for banded iron formation to ~0.8mm²s^-1 for a quartz-poor trondjhemite. For all samples thermal diffusivity decreases with increasing temperature, such that D for the suite converges around ~0.7±0.1mm²s^-1 at ~700°C. Combining these results with measured density and heat capacity calculated from modal mineralogy provides thermal conductivity as a function of temperature for each rock suite. Whereas near-surface thermal conductivity varies from 2.7 to 4.2Wm^-1K^-1 depending on rock type, thermal conductivity of the lower crust is ~2Wm^-1K^-1 and only weakly dependent on lithology, which is more thermally resistive than the uppermost mantle (~3.5Wm^-1K^-1).Use of temperature-dependent thermal transport properties in numerical models reveals that surface geotherms can be relatively insensitive to crustal heat production, because high heat-producing TTG rocks typically have high thermal diffusivity and conductivity at low temperatures and, consequently, are more efficient conductors of the heat they produce. This finding implies that calculations of crustal heat production from surface borehole measurements may entail significant uncertainties.