CRYSTAL MUSH EMPLACEMENT AND RAPID COOLING IN THE TUNNEL DIKE OF THE CHIEF JOSEPH DIKE SWARM

This research reinterprets the formation of the Tunnel Dike within the Chief Joseph Dike Swarm, part of the Columbia River flood basalts, by analyzing core drill samples. Textural analysis of plagioclase crystals within these samples suggest the bulk of crystals were not formed in situ but were transported from deeper regions. These crystals align within bands, separated by regions with fewer plagioclase crystals, indicating shearing in a dilatant mush. The occurrence of numerous bent and broken crystals reinforce this interpretation. Notable features such as interstitial dacitic glass, acicular apatite crystals with epitaxial pyroxene, and film-like crystallization of feldspar, all suggest significant undercooling, perhaps due to rapid exsolution of H2O in the shallow crust. Taken in sum, these petrographic observations are consistent with an interpretation that the dike was emplaced as a mobile crystal mush, possessing a high effective viscosity and likely residing near the jammed state. These magmatic features would result in hindered and staccato-like propagation, ultimately leading to rheological arrest and rapid cooling. Conversely, these observations do not support the commonly held assumption that dikes are emplaced as near-liquidus magma. This study offers new perspectives on dynamics of magma transport, especially pertaining to thermal modeling in dike systems and magmatic flux rates. The recognition of a mobile crystal mushes in dikes contributes to a deeper understanding of the complex processes underlying the emplacement of thick dikes in continental flood basalt settings.