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 (T_CMB) 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 T_CMB 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 T_CMB is of the order of ~5000 K.

约 2900 公里深度和约 135 GPa 的核-地幔边界 (CMB) 是固体硅酸盐地幔和液态金属外核之间的界面。CMB 处的温度 ( T _CMB ) 是评估从地核流出的热流的关键参数，因此对于我们了解地球动力学和地幔对流很重要。T _CMB的上限受到热石（地幔岩石的化学代表）的固相线温度（熔化开始）的限制，约为 135 GPa，因为最下方的地幔主要是固体。先前的激光加热金刚石砧座 (LH DAC) 研究报告了P处的热石固相线温度为 3430–4180 K (± <200 K ) ~ 135 GPa（Andrault 等，2011；Fiquet 等，2010；Kim 等，2020；Nomura 等，2014）。这些温度是通过高温计确定的，假设样品的光学特性在可见光到近红外光谱范围内与波长无关。在这里，我们使用 Mie 理论来表明，这种假设对于多相样品来说是严重不足的，该样品的晶粒尺寸和折射率代表了在金刚石砧座中压缩的热石中的颗粒尺寸和折射率。我们计算了热解石样品的波长相关消光系数，并表明之前的熔融研究可能低估了热解石固相线温度高达 20-30%。这意味着T _CMB的上限约为 5000 K。