Vesiculation in Columbia River basalts

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Abstract
Thick (>30 m) flows of Columbia River basalt contain internal vesicular zones within otherwise dense, sparsely vesicular basalt. These zones are continuous over distances ranging from 0.5 km to 30 km; most are characterized by an abrupt transition from vesicle-rich to vesicle-poor rock above and by a gradational lower margin. The zones were formed by post-emplacement migration, coalescence, and entrapment of aqueous vapor bubbles. Under appropriate physicochemical conditions, bubbles nucleate at the lower solidification front and rise buoyantly until retarded by the higher viscosities below the upper solidification front. In some cases, ponding of bubbles against this ceiling occurs before freezing-in of the vesicles by the downward passage of the upper front. We have developed a one-dimensional dynamic model of the process of vesicular zone emplacement by starting with the measured vesicle distribution in a lava flow and calculating movement of vesicles according to Stokes Law as the flow is melted, or “uncrystallized.” Data for the solidification history of the Cohassett flow are based on a previously developed cooling model. Melting of the flow was accomplished by reversing movement of the upper and lower solidification fronts; when a solidification front passes a vesicle, the vesicle is free to move in the reverse direction since time is moving in reverse. The model clearly illustrates the ponding effect of the upper solidification front and shows that nucleation of vesicles on the lower solidification front is required in trials where the model approximates a reasonable distribution of bubbles at T = 0. Internal vesicular zones consist of layers representing variations in number and size of vesicles. The regular layer spacing suggests a cyclic enrichment and depletion of bubble nuclei. If the production of nuclei is controlled by the state of the system at the lower solidification front, then the layers can be explained by oscillations between periods of crystallization, during which the system moves toward higher oversaturation pressure and periods of vapor exsolution tending to reestablish equilibrium. The layered distribution of vesicles in Columbia River basalt flows, for example, the McCoy Canyon and Rocky Coulee flows, can be explained by such a mechanism.

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