Radiometric Temperature Measurements in Nongray Ferropericlase With Pressure- Spin- and Temperature-Dependent Optical Properties

Abstract

Accurate temperature determination is central to measurements of physical and chemical properties in laser-heated (LH) diamond anvil cells (DACs). Because the optical properties of samples at high pressure-temperature (P-T) conditions are generally unknown, virtually all LH DAC studies employ the graybody assumption (i.e., wavelength-independent emissivity and absorptivity). Here we test the adequacy of this assumption for ferropericlase (13 mol.% Fe), the second most abundant mineral in the Earth's lower mantle. We model the wavelength-dependent emission and absorption of thermal radiation in samples of variable geometry and with absorption coefficients experimentally constrained at lower mantle P and P-T. The graybody assumption in LH DAC experiments on nongray ferropericlase contributes moderate systematic errors within ±200 K at 40, 75, and 135 GPa and T < 2300 K for all plausible sample geometries. However, at core-mantle boundary P-T conditions (135 GPa, 4000 K) the graybody assumption may underestimate the peak temperature in the DAC by up to 600 K in self-insulated samples due to selective light attenuation in highly opaque ferropericlase. Our results allow insights into the apparent discrepancy between available ferropericlase melting studies and offer practical guidance for accurate measurements of its solidus in LH DACs. More generally, the results of this work demonstrate that reliable temperature measurements in LH DACs require that the optical and geometrical properties of the samples are established.