08 Jun 2022
08 Jun 2022

Influence of heterogeneous thermal conductivity on the long-term evolution of the lower mantle thermochemical structure: implications for primordial reservoirs

Joshua Martin Guerrero1, Frédéric Deschamps1, Yang Li2, Wen-Pin Hsieh1, and Paul James Tackley3 Joshua Martin Guerrero et al.
  • 1Institute of Earth Sciences, Academia Sinica, Taipei
  • 2State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Institutions of Earth Science, Chinese Academy of Sciences, Beijing
  • 3Department of Earth Sciences, ETH Zürich, Zürich

Abstract. The long-term evolution of the mantle is simulated using 2D spherical annulus geometry to examine the effect of heterogeneous thermal conductivity on the stability of reservoirs of primordial material. In numerical models, mantle conductivity is often emulated using purely depth-dependent profiles (taking on values between 3 and 9 Wm-1 K-1). This approach is meant to synthesize the mean conductivities of mantle materials at their respective conditions in-situ. However, because conductivity depends also on temperature and composition, the effects of these dependencies in mantle conductivity is masked. This issue is significant because dynamically evolving temperature and composition introduce lateral variations in conductivity, especially in the deep-mantle. Minimum and maximum variations in conductivity are due to the temperatures of plumes and slabs, respectively, and depth-dependence directly controls the amplitude of the conductivity (and its variations) across the mantle depth. Our simulations allow assessing the consequences of these variations on mantle dynamics, in combination with the reduction of thermochemical pile conductivity with iron composition, which has so far not been well examined. We find that the temperature- and depth- variations combined characterize the mean conductivity ratio from top-to-bottom. For the mean conductivity profile to be comparable to the conductivity often assumed in numerical models, the depth- dependent ratio must be at least 9 times the surface conductivity. When the conductivity profile is underestimated, the imparted thermal buoyancy (from heat-producing element (HPE) enrichment) destabilizes the reservoirs and influences core-mantle boundary (CMB) coverage configuration and the onset of entrainment. The compositional correction for conductivity only plays a minor role that behaves similarly to conductivity reduction due to temperature. Nevertheless, this effect may be amplified when depth- dependence is increased. For the cases we examine, when the lowermost mantle's mean conductivity is greater than the surface conductivity, reservoirs can remain stable for periods exceeding the age of the solar system.

Joshua Martin Guerrero et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-418', Anonymous Referee #1, 30 Jul 2022
    • AC1: 'Reply on RC1', Joshua Guerrero, 22 Aug 2022
  • RC2: 'Comment on egusphere-2022-418', Anonymous Referee #2, 02 Aug 2022
    • CEC1: 'Reply on RC2', Susanne Buiter, 12 Aug 2022
    • AC2: 'Reply on RC2', Joshua Guerrero, 22 Aug 2022

Joshua Martin Guerrero et al.

Joshua Martin Guerrero et al.


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Short summary
Mantle thermal conductivity's dependencies on temperature, pressure, and composition are often suppressed in numerical models. In this study, we examine the effect of these dependencies on the long-term evolution of lower mantle thermochemical structure. We propose that depth-dependent conductivities derived from mantle minerals, along with moderate temperature and compositional correction, emulate the Earth's mean lowermost mantle conductivity values and produce a stable 2-pile configuration.