Glass, crystallinity, and VNIR reflectance from vent to margin at Jordan Craters, OR USA

Abstract

We present an in-situ analysis of the reflectance of Visible Near Infrared (VNIR) and the petrologic texture of the surface of a relatively young lava flow at Jordan Craters, OR. Jordan Craters is a small young basaltic lava field in Oregon, USA which terminates in a lake, providing a wide variety of petrographic textures related to cooling rates and prolonged seasonal exposure to water, post-emplacement. Field VNIR measurements of 191 in-situ spots were used to calculate the overall reflectance between the bands 500–1000 nm (R500–1000) which serves as a proxy for glass abundance. Analyses were conducted on the surfaces of lava, spatter, and scoria from the vent, mid-portion, and margin of the flow. We assigned the 15 collected hand samples to five cooling environments based on their location and modal mineralogy. Crystal content was measured microscopically and compared with the spectra classification for 15 hand samples. Slower cooling environments had more crystals (30–60%) of plagioclase, olivine, Fe-oxides, and devitrified glass (15–30%) whereas faster cooling environments have higher proportions of vitreous glass (∼70%). In order from slowest cooling to fastest cooling the groups were lava interior, oxidized vent, flow margin, glassy vent, and mid-flow surface. Mid-flow indicates surfaces where inflation or breakouts of the top flows occurred at least five meters from the margin. Additionally, locations that were exposed to prolonged heat or environmental water had higher proportions of devitrification and Fe-oxide crystals. The values of R500–1000 mirrored the cooling histories of the hand samples with interior analyses having the highest values and vent samples having the lowest, though there was much scatter in the vent category due to lumping of oxidized and glassy together in the in-situ dataset. Our findings indicate that VNIR may be used to identify variations in petrologic texture across basaltic lava flows due to eruptive and emplacement conditions such as vent vs mid-flow surface or quickly cooled vs slowly cooled with careful sampling and sufficient number of analyses. This has implications for understanding the cooling dynamics of lava flow surfaces as well as furthering the study of lava flows as paleoenvironmental indicators.