Quantitative Thermal Emission Spectroscopy at High Temperatures: A Laboratory Approach for Measurement and Calibration

Abstract

AbstractAcquiring accurate high temperature laboratory‐based infrared emission spectra of geologic samples is important to constrain their radiative and spectral properties. This is important in calculations of lava flow cooling, crust formation, and ultimately lava flow propagation modeling. However, measuring accurate emission at high temperatures remains a challenge. A new micro‐furnace design was created to integrate with a Fourier transform infrared spectrometer, replacing the previous furnace and improving the performance and error metrics. Importantly, this approach accounts for all significant error sources and uses only one spectrometer to acquire sample and calibration emission data over greater temperature (473–1,573 K) and spectral (4,000–500 cm−1, 2.5–20 μm) ranges. Emissivity spectra of forsterite and quartz samples were acquired to test the calibration procedure. Forsterite, with no expected phase transitions over the temperature range, showed spectral change above ∼1140 K, potentially due to amorphization–a process not well described in past studies. The quartz results revealed the expected polymorph transformations at ∼846 and ∼1323 K. A Hawaiian basalt sample served as a representative rock test and showed an increase in emissivity (∼25%) with decreasing temperature. The greatest emissivity increase (∼60%) occurred in the middle infrared region (3,333–2,000 cm−1, 3–5 μm). This is significant for thermal/mass flux calculations using satellite data in this spectral region, which rely on emissivity to derive accurate temperatures. All results are consistent with our previous investigations, but with improved mean accuracy (<2%), uncertainty (<4%), and spectral contrast (<20%). The improved metrics were achieved by constraining the sample measurement geometry, sample temperature stability, and environmental contamination within the experiment.