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Quantitative Thermal Emission Spectroscopy at High Temperatures: A Laboratory...

Thompson, J., D. B. Williams, R. J. Lee, and M. Ramsey (2022), Quantitative Thermal Emission Spectroscopy at High Temperatures: A Laboratory Approach for Measurement and Calibration, J. Geophys. Res..
Abstract: 

Acquiring 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 no expected phase transitions over the temperature range, showed spectral change above ∼1140 K, spectra of forsterite and quartz samples were acquired to test the calibration procedure. Forsterite, with the expected polymorph transformations at ∼846 and ∼1323 K. A Hawaiian basalt sample served as a potentially due to amorphization–a process not well described in past studies. The quartz results revealed 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. Plain Language Summary Acquiring high temperature laboratory-based infrared emission spectra of geologic samples is important to constrain their thermal and spectral properties. However, these measurements are challenging to acquire accurately. These measurements are increasingly important in calculations of lava flow cooling, crust formation rates, and ultimately lava flow propagation modeling. A new micro-furnace was created to integrate with an existing Fourier transform infrared spectrometer, replacing the previous furnace and improving the performance metrics. Importantly, unlike other recent laboratory studies, this approach accounts for all significant error sources and uses only one spectrometer to acquire both sample and calibration emission data over greater temperature and spectral ranges. Critically, the calibration can be directly tied to well-established emissivity measurements at lower temperatures validating the approach. Emissivity spectra of forsterite and quartz samples were acquired to test the calibration procedure and validated the approach. The mineral results revealed the expected polymorph transformations and potential amorphization of the crystal lattice–a process not well described in past studies. A Hawaiian basalt sample served as a representative rock test and showed an increase in emissivity with decreasing temperature. This is significant for satellite data acquired in this spectral region, as they rely on emissivity to derive accurate temperatures.

Research Program: 
Earth Surface & Interior Program (ESI)
Mission: 
Terra-ASTER
Funding Sources: 
80NSSC18K1001 80NSSC17K0445