Modelling Mie scattering in pyrolite in the laser-heated diamond anvil cell: Implications for the core-mantle boundary temperature determination

Abstract

The core-mantle boundary (CMB) at ~2900 km depth and ~135 GPa is the interface between the solid silicate mantle and the liquid metallic outer core. The temperature at CMB (TCMB) is a key parameter in assessing the heat flow out of the core and is thus important for our understanding of the geodynamo and mantle convection. The upper bound on TCMB is constrained by the solidus temperature (onset of melting) of pyrolite, a chemical proxy of the mantle rock, at ~135 GPa because the lowermost mantle is predominantly solid. Previous laser-heated diamond anvil cell (LH DAC) studies reported pyrolite solidus temperatures of 3430–4180 K (± <200K) at P ~ 135 GPa (Andrault et al., 2011; Fiquet et al., 2010; Kim et al., 2020; Nomura et al., 2014). These temperatures were determined by pyrometry assuming that the optical properties of the sample are wavelength-independent in the visible to near-IR spectral range. Here we use Mie theory to show that this assumption is grossly inadequate for a polyphase sample with the grain sizes and refractive indices representative of those in a pyrolite compressed in the diamond anvil cell. We compute the wavelength-dependent light extinction coefficient of pyrolite samples and show that the previous melting studies may have underestimated pyrolite solidus temperature by up to 20–30%. This implies that the upper bound on the TCMB is of the order of ~5000 K.