Temperature-dependent thermal transport properties of major Archean rock types and implications for the thermal evolution of Archean terranes
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] While it is generally agreed that plate tectonics operated on Earth in the Proterozoic, a form of vertical tectonics may have operated during the Archean, leading to formation of dome-and-keel terranes. Early continental crust was comprised of rock associations such granite-greenstone (GG) belts and intrusions of high heat-producing tonalite-trondhjemite-granodiorite (TTG), and the burial of TTG rocks below GG rocks is believed to have triggered dome-and-keel formation. The thermal transport properties of these rocks, thermal diffusivity (D), and thermal conductivity (k), are not well constrained, especially at high temperatures as traditional contact methods for measuring these properties may suffer from a combination of imperfect physical contacts and ballistic radiative transfer. Using the Laser Flash Analysis (LFA) technique, we determined D of a suite of 14 GG and TTG rocks at temperatures up to 1100 K, at atmospheric pressure. Our measurements yielded a range of D at room temperature from ~3.8 mm2/s for banded iron formation to ~0.8 mm2/s for syenite. D for all samples decreases with increasing T, and the range of D for the suite narrows to ~0.45 and 0.70 mm2/s for granodiorite and tholeiite basalt respectively by ~1000K. We applied these results to endmember numerical models of the thermal evolution primitive continental crust, and a model of dome-and-keel formation. The results show that a crustal profile with heat production appropriate to 3.4 Ga that is 40 km thick, with a 16 km TTG pluton below a 14 km GG belt could result in partial melting of the TTG pluton. Our models also show that equilibrium crustal temperatures are highly dependent upon lithospheric thickness, heat production of the TTG pluton, and crustal thickness. Using temperature-dependent thermal transport properties results in significantly different predictions of crustal temperatures and heat flux, compared to models that use constant thermal properties. Our models suggest that the higher radiogenic heat production in the Archean, combined with realistic thermal properties, removes the necessity of unusually high mantle heat fluxes or heat-producing element concentrations, which were essential features of previous models of dome-and-keel formation.
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