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The Influence of a Basaltic Crust on the Thermochemical Evolution of a Stagnant Lid Planet

Presentation #217.02 in the session Mars’ and Martian Moons’ Surface Properties and Composition (Oral Presentation)

Published onOct 23, 2023
The Influence of a Basaltic Crust on the Thermochemical Evolution of a Stagnant Lid Planet

The thermochemical evolution of terrestrial planets is strongly influenced by the formation and presence of a basaltic crust. On the one hand, crust formation contributes to the efficient cooling of the interior: 1) the mantle melt that rises to the surface and forms the crust effectively transports heat out of the interior. 2) Radioactive heat sources are strongly enriched in the melt and redistributed towards the surface. On the other hand, crust formation can also cause the planetary interior to cool more slowly: 1) Volatiles such as CO2 and H2O are enriched in the melt and transported to the surface, from where they partially escape into the atmosphere. The decrease of volatiles in the mantle in turn leads to an increase in viscosity and thus to a slowing of convection - cooling becomes less efficient. 2) The basaltic crust has a lower thermal conductivity than the mantle and therefore thermally insulates the convecting interior. The thermal conductivity of compact basaltic material is 1.5 - 3 W/mK, while typical mantle material has an average value of about 4 W/mK. The thermal conductivity of the crust material is further reduced by an increase in porosity/cracks, e.g. through impacts. The extent of the decrease in thermal conductivity depends on atmospheric pressure, but also on whether the pores/cracks are filled with water/ice. Whether a crust tends to increase or decrease the cooling of the interior also influences the volcanic history and associated atmospheric evolution.

Using parameterized thermal evolution models, we test the influence of the crust on thermal evolution for a Mars-like stagnant-lid planet. We assume that the bulk crust formed early and neglect later crust formation. Free parameters are the mean thermal conductivity of the crust (kc), the enrichment of heat sources in the crust relative to mantle (λ), the crust thickness (Dc) and the reference mantle viscosity (ηr). We find parameter variations of kc and λ where the cooling history is similar to that with a model without crust. In general, there is the trend that with decreasing crustal conductivity, the enrichment of radioactive sources in the crust must be larger to compensate for the crustal influence. Deviations from these specific parameter combinations can then lead to either slower (smaller kc or λ) or faster (larger kc or λ) cooling. For instance, the variation in present-day lithosphere thickness and mantle temperature for the parameter range studied in kc and λ at ηr = 1021 Pas are 200 km and 70 K (Dc=40 km) and 300 km and 140 K (Dc =80 km). The parameter study also shows under which conditions present-day volcanism is more likely on Mars.

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