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.

Reflective imaging, on-axis laser heating and radiospectrometry of samples in diamond anvil cells with a parabolic mirror

“…Laser focusing and image quality demonstrate that its performance is comparable the one of 10X apochromats commonly used in laser heating. Whenever theoretically predicted systematic deviations of the emittance of a sample from grey body emittance [33,13] become subject of experimental investigation, the chromatic aberration of the system has to be substantially suppressed, otherwise there is a risk to be misled by spectral distortion.…”

Evolution of Chemical Bonding and Spin-Pairing Energy in Ferropericlase across Its Spin Transition

“…The sample studied here is a synthetic single crystal of ferropericlase ((Mg 0.87 Fe 0.13 )­O) produced by Fe diffusion into an MgO crystal in a gas-mixing furnace at ambient pressure. Electron energy loss spectroscopy (EELS) yielded an Fe 3+ /∑Fe total ratio of ∼10% . The ferropericlase single crystal was then ground and double-side polished to a thickness of ∼16 μm to achieve a transparent, high-optical quality platelet.…”

“…Electron energy loss spectroscopy (EELS) yielded an Fe 3+ /∑Fe total ratio of ∼10%. 41 The ferropericlase single crystal was then ground and double-side polished to a thickness of ∼16 μm to achieve a transparent, high-optical quality platelet. 2.2.…”

Contrasting opacity of bridgmanite and ferropericlase in the lowermost mantle: Implications to radiative and electrical conductivity

“…1A). Synthesis procedures for these samples have been reported elsewhere (Lobanov et al, 2020;Lobanov and Speziale, 2019;Mao et al, 2017). Finally, the cells were brought to a desired pressure as gauged either by the position of the diamond Raman edge (Akahama and Kawamura, 2006) or ruby fluorescence (Syassen, 2008).…”