Mantle evolution in terrestrial planets - Partitioning behavior of heat producing elements

Melting processes inside the mantle of terrestrial planets are highly dependent on the planet’s heat budget and trigger the redistribution of incompatible trace elements from solid mantle rocks into the melt. Due to density contrasts, convective mantle motion, and other physical factors, this melt ascents towards the surface and takes most of these incompatible elements with it. Some of these ascending elements like ⁴⁰K, ²³²Th, and ²³⁸U, are radioactive and are important contributors to the Earth’s or other terrestrial planet’s heat budget. By redistributing these elements, the mantle is left depleted and the surface enriched, leading to a shift in the production of heat from the mantle towards the crust.

To be able to calculate the amount of redistributed heat producing elements, we model partition coefficients after Blundy and Wood 2003 [1] depending on temperature, pressure, and melt composition and trace them in a global convection code (Fig.1). For the incorporation of the elements’ partitioning behavior at higher upper mantle pressures, we took the thermodynamic approach from Blundy et al. 1995 [2] and parameterized it to higher pressures (Fig.2). The redistribution of heat producing elements between the mantle and crust has important implications for the cooling behavior of the interior. In turn, the latter affects the generation of melt in the mantle and the formation of crust.

[1] Blundy, J.D. and Wood, B.J. (2003), RiMG, 52 (1), 59–123.
[2] Blundy, J.D., Falloon, T.J., Wood, B.J., and Dalton, J.A. (1995), J. Geophys. Res., 100, 15501-15515.

 

Fig. 1: Redistributed amount of the heat sources ⁴⁰K, ²³²Th, and ²³⁸U on Mars after 600 Myrs.

 

Fig. 2: The clinopyroxene/melt partition coefficient of Na parameterized to higher pressures. With increasing pressures, partition coefficients increase. At the turning point, the model ceases to work. The applicability of the approach could be increased from 0-4 GPa to ≤12 GPa.