Long-wave infrared hyperspectral imagery of weathering trajectories on Hawaiian basaltic rock

Abstract

A long wave infrared (LWIR) hyperspectral imager, the University of Hawaii's Airborne Hyperspectral Imager (AHI), was used to relate systematic changes in LWIR spectral features to weathering trajectories on the surfaces of basaltic rocks. Kahle and others proposed that in relation to the LWIR spectra, that devitrification of chilled glassy margins dominate the first stages of weathering, followed by the accretion of silicate coatings and the oxidation of iron[1-3]. We are using the AHI's higher spectral and special resolution to better constrain this relationship between the LWIR and weathering trajectories. The main study area was along the northern flank of Mauna Loa on the Island of Hawai'i. We collected samples ranging from a few decades to over 8000 years old. Samples a few hours to a few days old were collected from Kilauea. A Nicolet FTIR spectrometer was used to acquire reference spectra in the range of 5 to 15 μm. Three features are readily identifiable: two narrow features (A: ~8.1μm and B: 9.1μm) and one broad feature (C: 9.5 to 13 μm). The most striking change is in the C feature which changes from a large and dominant feature in the fresh Kilauea pahoehoe, to a subtle feature in the 1935 Mauna Loa flow. The only overall age related spectral change observed is the reduction of relative spectral feature intensity with increasing age. We also noted that within samples of the same age, there are some striking differences in the spectral shape.