Magmatic channelization by reactive and shear-driven instabilities at mid-ocean ridges: a combined analysis

Abstract

Summary It is generally accepted that melt extraction from the mantle at mid-ocean ridges is concentrated in narrow regions of elevated melt fraction called channels. Two feedback mechanisms have been proposed to explain why these channels grow by linear instability: shear flow of partially molten mantle and reactive flow of the ascending magma. These two mechanisms have been studied extensively, in isolation from each other, through theory and laboratory experiments as well as field and geophysical observations. Here, we develop a consistent theory that accounts for both proposed mechanisms and allows us to weigh their relative contributions. We show that interaction of the two feedback mechanisms is insignificant and that the total linear growth rate of channels is well-approximated by summing their independent growth rates. Furthermore, we explain how their competition is governed by the orientation of channels with respect to gravity and mantle shear. By itself, analysis of the reaction-infiltration instability predicts the formation of tube-shaped channels. We show that with the addition of even a small amount of extension in the horizontal, the combined instability favours tabular channels, consistent with the observed morphology of dunite bodies in ophiolites. We apply the new theory to mid-ocean ridges by calculating the accumulated growth and rotation of channels along streamlines of the solid flow. We show that reactive flow is the dominant instability mechanism deep beneath the ridge axis, where the most unstable orientation of high-porosity channels is sub-vertical. Channels are then rotated by the solid flow away from the vertical. The contribution of the shear-driven instability is confined to the margins of the melting region. Within the limitations of our study, the shear-driven feedback does not appear to be responsible for significant melt focusing or for the shallowly dipping seismic anisotropy that has been obtained by seismic inversions.

Effects of Stress‐Driven Melt Segregation on Melt Orientation, Melt Connectivity and Anisotropic Permeability

“…It is quite possible that MVO has different properties at small and large strains, perhaps as a result of the development of melt-rich bands. Melt bands may form under conditions for which melt volumes are elongated toward the maximum compression direction (Butler, 2012;Katz et al, 2006;Rees Jones et al, 2020), but their later evolution may be influenced by the kind of uniplanar flow and MVO anisotropy we describe here. Flow perpendicular to the plane of melt-rich bands appears limited at high strain.…”

“…Another form of melt localization, the reaction infiltration instability (Aharonov et al., 1995; Pec et al., 2017, 2020; Spiegelman et al., 2001) is expected to dominate over stress‐driven melt localization throughout most of the mid‐ocean ridge melting regime, though both operate together in many regions (Rees Jones et al., 2020). While King et al.…”

“…This observation could have important implications on melt focusing toward mid-ocean ridges. For example, a recent model by Rees Jones et al (2020) suggests that melt-rich bands are orientated sub-vertically toward the base of the lithosphere. In this case, we expect melt to ascend and pool along the base of the lithosphere, with very little melt flow parallel to it, because of the lack of large-scale melt connectivity perpendicular to the melt-rich bands.…”

“…Another form of melt localization, the reaction infiltration instability (Aharonov et al, 1995;Pec et al, 2017Pec et al, , 2020Spiegelman et al, 2001) is expected to dominate over stress-driven melt localization throughout most of the mid-ocean ridge melting regime, though both operate together in many regions (Rees Jones et al, 2020). While King et al (2011) suggested that melt-rich regions produced by a combination of stress and reaction have a similar morphology to those produced by shear alone, the properties we measure here have yet to be examined for melt channels produced by RIIs or by a combination of stress and reaction.…”