Constraints on the thermal state of the continental lithosphere

Abstract

The thermal state of the lithosphere is an important driver of many physical and chemical processes within the Earth. Understanding the distributions of heat flow and radiogenic heat production provides an important constraint on lithospheric thermal models. By some estimates nearly 40% of continental heat flow is produced by radioactive decay in the crust, however the distribution of heat producing elements is poorly constrained. Creating robust models of radiogenic heat production requires an understanding of its natural variability. The creation of a global whole-rock geochemical database provides a framework for discussing global distributions of thermal parameters. I have collated over one million digital rock entries with a range of sample data including major and trace element concentrations, isotopic ratios, and other metadata. Associated naming schema and physical parameter estimates are also computed in a standardised manner, including radiogenic heat production. I then present a new model for continental igneous heat production from ~4 Ga to the present using a novel silica-normalised igneous data set and compare to previous discussions of granitic and sedimentary trends in the literature. Crude normalization for composition indicates lithological control is the dominant factor on heat production after the influence of decay is removed. I find that heat production at formation for different rock types has been relatively constant through time except for the early Archean to ~2.7 Ga. I suggest the heat production–age pattern does not significantly reflect the influences of erosion, secular cooling, depletion, or the supercontinent cycle as suggested by some previous studies, but instead either reflects a shift in the bulk composition of the crust or evidence for bias in the rock record due to thermal stability. Geophysical proxies provide additional constraints on the crustal thermal state. I have developed a global Curie Depth model from the latest satellite-derived lithospheric magnetic model using the equivalent source magnetic dipole method. Forward modelling was conducted to simulate the observed lithospheric magnetic field. Our updated methodology involves additional vector components utilised in the forward modelling calculations, a differing long-wavelength model, and inclusion of a spatially variable magnetic susceptibility estimate. Resultant continental Curie depth estimates show good agreement with observed heat flow observations and provide further evidence that Curie depth estimates can assist in estimates of the thermal state of the lithosphere. Finally, I assess various heat flow models for Antarctica derived from geophysical proxies. Extrapolation from isotherm estimates at depth require models of heat production and thermal conductivity to model surface heat flow. Differences in models can have non-trivial influences on the results produced. Quantifying the uncertainty associated with these thermal parameters is also critical for understanding and interpreting the heat flow solutions. I propose a set of models derived from whole-rock geochemical data, and guided by compositional studies of the crust. Uncertainties associated with this model are estimated via the Monte Carlo method. I show that applying models guided by global insights provides a reasonable fit to the Antarctica continent, and a method of estimating uncertainty in thermal parameters for regions lacking basement geology constraints.