Effect of Magma Degassing on Diking Processes at Mid-Ocean Ridges

Imaging data beneath fast and intermediate spreading mid-ocean ridges indicate that magma chambers take the form of thin lenses. This shape implies that the pressurization of the magma chamber could result in the tensile stress concentration near the tips of the lens while the rest of the host rock would be in the state of compression. Therefore, should an episode of magma replenishment in the magma lens cause diking, the dikes would likely initiate near the lens tips. Our calculations, based on the principles of fracture mechanics, also suggest that the dikes further propagate almost vertically towards the seafloor from the lens tips [Sim et al., 2004]. Alternatively, the diking may result from solidification of the magma lens. Because the density of magma is lower than that for the fresh rock, crystallization leads to the pressure decrease in the lens. Consequently, the stress distribution in the host rock changes and becomes tensile in the middle part of the magma lens and compressive in the tip regions. In this scenario, the dikes are more likely to initiate from the central areas but may not propagate to the seafloor [Sim et al., 2004]. In both cases, the diking process may be enhanced by simultaneous magma degassing, which reduces the rate of pressure decrease resulting from the withdrawal of magma from the magma lens. Depending upon the depth, temperature, and composition, volatiles in the magma may considerably increase its compressibility. As the dike propagates, pressure in the chamber decreases, causing volatiles dissolved in the magma to exsolve and to form gas bubbles that tend to migrate upwards. At the time scale of dike propagation, a gas bubble may be confined within the magma lens. In this scenario, the pressure in the magma lens is higher than in the case when no or little volatile exsolution occurs. Expressing the magma compressibility as a function of the volume fraction of gas, we investigate this scenario through a range of parameters related to the magma lens characteristics. The analysis yields the conditions under which the change in pressure, generated by the exsolution, can facilitate the dike propagation for both central and tip dikes. Under certain circumstances, magma degassing may enable even central dikes to reach the seafloor. Sim, Y., L. N. Germanovich, R. P. Lowell, and P. Ramondenc (2004), Diking, magma lenses, and location of hydrothermal sites at mid-ocean ridges, Eos Trans. AGU, 85(47), Abstract B13A-0206.