Multi-pulse and crystal-laden emplacement of the Tunnel Dike: implications for flood basalt volcanism

Flood basalt provinces, like the Columbia River Basalts (CRB), signify vast volcanic activity capable of inducing paleoclimatic shifts and extinction events via extensive volatile emissions. Two competing models exist for their rates of emplacement: one argues for slow, prolonged eruption based on lava flow morphology, while the other argues for rapid emplacement based on thermally constrained modeling of dikes, assuming crystal-poor, low-viscosity magmas fed through wide conduits. We test these assumptions by investigating the Tunnel Dike, a Chief Joseph Dike Swarm dike thought to be the feeder for several significant CRB flows. Thermal modeling indicates the dike was emplaced as relatively cool, crystal-laden magma, assembling incrementally to its final thickness. Crystal size distributions across the dike exhibit aphanitic margins which transition sharply into a porphyritic interior, where plagioclase commonly displays high-temperature deformation, with spatial distributions suggesting granular flow. These lines of evidence point to a multi-pulse, crystal-rich mush emplacement of the Tunnel Dike. Under these conditions, magmatic flux would have been orders of magnitude slower than previous estimates, aligning with flux rates based upon lava flow morphology of flood basalts and with peak effusion rates of the 1783 Laki fissure eruption. Although based on a single dike, the textures observed in the Tunnel Dike appear to be widespread across the CJDS, suggesting that crystal-rich, incremental dike emplacement may be more broadly characteristic of Columbia River Basalt magmatism. These findings carry implications for estimates of volatile emissions, and, consequently, paleoclimate modeling.