BAYESIAN MARKOV-CHAIN MONTE CARLO INVERSION OF LOW-TEMPERATURE THERMOCHRONOLOGY AROUND TWO 8-10 M WIDE COLUMBIA RIVER FLOOD BASALT DIKES

Flood basalt volcansim involves large volumes of magma emplaced into the crust and surface environment on geologically short timescales. The mechanics of flood basalt emplacement, including dynamics of the crustal magma transport system and the tempo of individual eruptions, are not well constrained. Here we study two exhumed dikes from the Columbia River Flood Basalt province in northeast Oregon, USA, using apatite and zircon (U-Th)/He thermochronology to constrain dike emplacement histories. Sample transects perpendicular to the dike margins document transient heating of granitic host rocks by dike emplacement. We develop a thermal model with distinct melt-fraction temperature relationships for basaltic magma and granitic wallrock and unsteady flow within the dike. We model partial resetting of thermochronometers due to He diffusion in spherical grains as a response to dike heating. A Bayesian Markov-Chain Monte Carlo framework is used to jointly invert for six parameters related to dike emplacement and grain-scale He diffusion. We find that the two dikes, despite similar dimensions on an outcrop scale, exhibit different spatial patterns of partial resetting away from the dike. These patterns result in distinct predicted emplacement histories. We extend previous modeling of a presumed feeder dike at Maxwell Lake in the Wallowa Mountains of northeastern Oregon, finding a posterior probability distribution function that predicts steady heating from sustained magma flow over 1-6 years and elevated farfield host rock temperatures. This suggests regional-scale heating in the vicinity of Maxwell Lake, which might arise from nearby intrusions. The other dike, within the Cornucopia subswarm, is predicted to have a 1-4 year thermally active lifespan and an unsteady heating rate suggestive of a low flow rate compared to Maxwell Lake, in a cool near-surface thermal environment. In both cases, misfit of predicted near-dike partial resetting of thermochronometers suggests either heat transfer via fluid advection in host rocks or pulsed magma flow in the dikes. Our results highlight the diversity of dike emplacement histories within the Columbia River Flood Basalt province and the power of Bayesian inversion methods for quantifying parameter trade-offs and uncertainty in models for magma transport.