GRAIN SIZE EVOLUTION DURING EMPLACEMENT OF PARTICLE-LADEN DIKES, CHIEF JOSEPH DIKE SWARM

The Chief Joseph Dike Swarm (CJDS), located in northeastern Oregon, is the interpreted magmatic plumbing system for the vast Columbia River Flood Basalts, Earth’s youngest flood basalt province and challenges traditional interpretations of dike formation. Contrary to the belief that dikes are emplaced as near liquidus, crystal-poor magmas, evidence suggests that many of these dikes may have been emplaced as crystal-rich slurries. This study explores the possibility of fragmentation influencing grain size distributions within these dikes. Utilizing electron backscatter diffraction (EBSD), we conducted non-subjective analysis of crystal grain boundaries, generating extensive datasets of several thousand grains. These were processed using stereological conversion toolkits including ShapeCalc and CSDcorrections, and the results were plotted in log-log space. The size distributions revealed a mixed-mode pattern, not fitting purely fractal or lognormal models. In areas where a fractal pattern emerged, the fractal dimension (D) ranged from 2-3, which is consistent with granular shear fragmentation. Notably, the highest fractal dimensions were observed near the dike margins where magma undergoes the most significant shear/strain. The lognormal segments of the distribution could result from various processes including: other fragmentation modes, Ostwald ripening, or size-dependent crystallization. Considering the cooling of magma post-emplacement, we propose a preliminary model that simulates grain size distributions across multiple crystallization-fragmentation cycles. These findings highlight the complexity of interpreting CSDs and question prevailing assumptions about magma transport physics through the crust. The CJDS thus offers a unique lens into the intricate processes shaping Earth's youngest flood basalt province.