| مستخلص: |
Basaltic fissure eruptions, which are the most common type of eruption on Earth, are fed by dykes which mediate magma transport through the crust. Dyke propagation processes are important because they determine the geometry of the transport pathway and the nature of any geophysical signals associated with magma ascent. Here, we investigate small‐scale (mm–cm wide) banding features at the margins of dykes in the Teno Massif (Tenerife, Spain) and the Columbia River Basalt Province (CRBP) (USA). Similar marginal bands have been reported for dykes in numerous localities around the world. Dyke margins record valuable information about propagation because they are the first material to solidify against the host rock at the propagating dyke tip. We find that the marginal bands are defined by cyclic variations in phenocryst concentration and vesicularity, and we infer that these cyclic variations in texture are a product of cyclic variations in magma flow rates and pressures within the dyke tip. This indicates that dyke emplacement occurs in pulses, with propagation repeatedly hindered by the rapid cooling and solidification of magma in the narrow dyke tip. Using a 1D conduction model, we estimate the time taken for each band to cool and solidify, which provides a timescale of several minutes to tens of minutes for the pulses. The occurrence of similar bands in various volcanic settings suggests that pulsatory propagation is a common, if not ubiquitous, process associated with dyke emplacement. Plain Language Summary: Dykes are cracks in the Earth's crust through which magma rises, sometimes feeding volcanic eruptions. Here, we investigate small‐scale banding features that are commonly found in solidified magma in dykes, using examples from the Teno Massif (Tenerife, Spain) and the Columbia River Basalt Province (USA). The bands form at the margin of the dyke, against the crack wall, which means that they record what happened when the dyke first formed; that is, when magma first flowed into the crack. We find that the bands are defined by repeated variations in the amount of crystals and bubbles, which we infer to have resulted from variations in magma flow rates and pressures in the dyke tip. This shows that the dykes formed in steps, rather than all in one go. By calculating the time taken for the bands to cool and solidify, we find that each propagation step lasts several minutes to tens of minutes. The occurrence of similar bands in various volcanic settings around the world suggests that most or all dykes form this way. This is important because the stepwise formation of the cracks might create small earthquakes that can be detected as the magma rises toward the Earth's surface. Key Points: Banded textures at dyke margins are the result of pulsatory magma flow in the tip of the propagating dykeDyke propagation is not continuous, but occurs in steps via a cycle of cooling, stalling, inflating, rupturing and propagatingCooling and solidification of magma in the narrow dyke tip has a strong influence on dyke propagation [ABSTRACT FROM AUTHOR] |
