WALL-ROCK THERMAL MODELING INFORMED BY TEXTURAL ANALYSES IN THE TUNNEL DIKE, CHIEF JOSEPH DIKE SWARM

The Chief Joseph Dike Swarm (CJDS) in Northwestern United States was the primary feeder to the Columbia River Flood Basalts (CRFB), Earth’s youngest flood basalt province. This dike system’s textural diversity gives insight into the magma migration mechanisms that could have contributed to climate change during the emplacement of the CRFB. The Tunnel Dike, situated in the Grande Ronde river canyon in Anatone, Washington, is a part of the CJDS and provides fundamental microtextural constraints on the magma transport system, including the magmatic flux rates. Previous studies have used the wall-rock adjacent to this dike to determine the duration of magmatic flux, with the foundational assumption that magma remained at the liquidus temperature during the lava flow feeding stage. However, our findings are consistent with at least two magmatic pulses indicated by two distinct crystal size distributions in dike orthogonal transects. Specifically, an outer fine-grained texture and an interior, relatively uniform porphyritic texture, both documented using Electron Backscatter Diffraction (EBSD) and traditional texture analysis methods. This complex textural relationship cannot be explained via a simple cooling model, and instead supports the notion that textures would have had to form over a range of temperatures to account for the two groups. We use here, a transient one-dimensional thermal model to explore a variety of scenarios, including those proposed by other researchers, but casting a wider net, including a multi-pulse scenario as informed by our textural analyses. We have produced time and temperature space diagrams for a range of contact temperatures, rather than keeping the liquidus temperature constant. Our results highlight the fundamentality of conducting textural analysis in magma transport, and thermal modeling studies. Taking into consideration the textural analysis of the Tunnel Dike, the temperature of the wall-rock at the contact must have been lower than previously assumed, which necessarily lengthens the estimated duration of magmatic flow through the dike. Contact temperature is critical to inferring magmatic flux durations and rates across dike systems, and not accounting for textural diversity will result in inaccurate and unrealistic outcomes.