REDEFINING VOLCANIC CONDUIT DYNAMICS: INELASTIC DEFORMATION OF GRANULAR MEDIA DURING DIKE EMPLACEMENT IN THE CHIEF JOSEPH DIKE SWARM

Encompassing over 210,000 km2 of land in the Pacific NW, the Columbia River Basalt Group (CRBG) is the youngest Large Igneous Province (LIP) and has an intricate and exhumed plumbing system known as the Chief Joseph Dike Swarm (CJDS). This study focuses on two dikes, hereafter PR1 and PR2, which have great exposures in the Grande Ronde River Canyon near Anatone, WA. PR1 (~2m width) and PR2 (~18m width) were emplaced into main-phase CRBG flows. Oriented core samples were collected at regular spacing across the dikes. Current views of magma dynamics assume near liquidus temperatures and high flux rates through fissures. The data collected was used to evaluate the magmatic conditions present during emplacement and challenged the previous assumption by revealing a large crystal proportion and near solidus behavior in the magmatic system. In these dikes, a high-strain environment during magma migration resulted in an anastomosing shear zone that controlled the spatial distribution of crystal deformation. Samples show evidence of force chains involving discrete bands of deformed crystals (e.g., mechanical twinning, subgrain formation, bending, and kinking) separating packets of smaller, undeformed crystals. The studied dikes display a large proportion of deformed crystal grains, but there are some differences. PR2, the widest dike, shows a higher proportion of deformed crystals than PR1, perhaps due to greater heat content. Interestingly, despite being narrower, PR1 is coarser than PR2, suggesting the presence of crystal cargo during emplacement. We interpret the large proportion and spatial correlation of deformed crystals due to in situ deformation during emplacement. Moreover, undeformed crystals were smaller than deformed crystals, suggesting that smaller crystals either grew in low-strain environments or did not participate in force chains. These findings question the commonly assumed near liquidus conditions in volcanic conduits. During the emplacement of magmas in the CJDS, crystalline mushes mobilized and deformed plastically. This is a paradigm-shifting emplacement mechanism for LIPs and challenges the assumption that magmatic systems are primarily fed by crystal-free magmas.