Anisotropy of thermal diffusivity in the upper mantle

Abstract

Heat transfer in the mantle is a key process controlling the Earth's dynamics. Upper-mantle mineral phases, especially olivine, have been shown to display highly anisotropic thermal diffusivity at ambient conditions, and seismic anisotropy data show that preferred orientations of olivine induced by deformation are coherent at large scales (>50 km) in the upper mantle. Thus heat transport in the upper mantle should be anisotropic. But the thermal anisotropy of mantle minerals at high temperature and its relationship with deformation have not been well constrained. Here we present petrophysical modelling and laboratory measurements of thermal diffusivity in deformed mantle rocks between temperatures of 290 and 1,250 K that demonstrate that deformation may induce a significant anisotropy of thermal diffusivity in the uppermost mantle. We found that heat transport parallel to the flow direction is up to 30 per cent faster than that normal to the flow plane. Such a strain-induced thermal anisotropy implies that the upper-mantle temperature distribution, rheology and, consequently, its dynamics, will depend on deformation history. In oceans, resistive drag flow would result in lower vertical diffusivities in both the lithosphere and asthenosphere and hence in less effective heat transfer from the convective mantle. In continents, olivine orientations frozen in the lithosphere may induce anisotropic heating above mantle plumes, favouring the reactivation of pre-existing structures.

Lithosphere thermal structure at the eastern margin of the Bohemian Massif: a case petrological and geophysical study of the Niedźwiedź amphibolite massif (SW Poland)

“…Most of the results (e.g. Clauser and Huenges 1995;Seipold 1998;Tommasi et al 2001;Abdulagatov et al 2006;Katsur 2007;Whittington et al 2009) indicate that thermal conductivity and thermal diffusivity stabilize for the p-T range of upper lithospheric mantle (i.e. for: 1-4 GPa, and 700-1,500°C).…”

Anisotropy of elastic properties and thermal conductivity of the upper mantle -- a case study of xenoliths shape: Evidence from xenoliths in basalts in North Eurasia

“…Values of were thus obtained along each of the axes of the xenoliths of ellipsoidal shape. The thermal conductivity of olivine is controlled by parameters of its crystal [Gibert et al, 2003[Gibert et al, , 2005Tommasi et al, 2001]. Thermal conductivity also depends on the size of the olivine grains, i.e., the contribution of the olivine neoblasts to is insignificant.…”

“…The anisotropy of thermal conductivity also depends on the pyroxene content in the xenoliths, with an increase in this parameter decreasing the anisotropy [Tommasi et al, 2001], but does not principally modify these relations.…”

Methods

“…Most known single-crystal thermal diffusivities are anisotropic (e.g., Kobayashi, 1974;Tommasi et al, 2002). Therefore, thermal conductivity might be expected to be anisotropic if the samples are monominerallic or if the sample is dominated by a fabric.…”