EMPLACEMENT OF A COOL, MAGMATIC SLURRY: TUNNEL DIKE, CHIEF JOSEPH DIKE SWARM

Dikes are commonly assumed to be emplaced as low crystallinity, near liquidus magmas, with magmatic flux occurring along the breadth of the intrusion. Textural analyses offer an opportunity to check these assumptions while additionally adding nuance to our understanding of ephemeral dike processes. We apply textural analyses across a relatively dense sampling profile of Tunnel Dike; an impressive ~14-meter-wide dike that stands, in part, in erosional relief. Results from detailed petrography, textural analyses, and thermal modeling are in agreement: the tunnel dike was emplaced as a crystal-rich slurry at relatively cool temperatures. The consequences of this interpretation affect the physics undergirding magma transport and provide a new perspective for interpreting magmatic phenomena. Granular systems are inherently non-Newtonian, resulting in flow instabilities and complex shear-thickening and -thinning cycles. High crystallinity slurries are prone to jamming, which can stall dike propagation, as is commonly observed remotely in volcanic settings, or focus the flux of magmatic slurry into isolated channels. At the same time, a jammed granular system can maintain the aperture of the dike through force chains, allowing for 1) interstitial fluid flux, behaving like proppant-filled cracks generated in the hydrofracking industry, and 2) maintain the strongly perturbed stress-field surrounding the dike, which can impact wall-rock deformation styles and subsequent dike trajectories. If magmas are transported as crystal-rich slurries, then the set of inferences that can be made is modified. Fragmentation of granular load during transport augments size distributions, with the size distribution becoming a function of both crystallization and fragmentation, which skews inferred quantities such as characteristic cooling time. If magma viscosity is non-linear, and dikes can stall locally, then inferring magmatic flux rates or durations from outcrop-scale views of dikes becomes inapplicable without detailed textural analyses. Our findings highlight the need to reconsider traditional assumptions about dike emplacement, recognizing the complexities introduced by crystal-rich slurries and their impact on magma transport dynamics and subsequent geological phenomena.