Abstract
Summary It is generally accepted that melt extraction from the mantle at mid-ocean ridges (MORs) 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 the 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 MORs 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 subvertical. 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.
Highlights
At mid-ocean ridges (MORs), plate spreading induces upwelling of the mantle, causing decompression melting
Instead we argue that the reaction-infiltration instability is dominant and that shear-driven instability may be insignificant beneath MORs
The theory allows for the perturbations to evolve by both reactive and shear mechanisms simultaneously. We show that this theory can reproduce previous estimates of their growth rate in both the reaction-only and shear-only limits
Summary
At mid-ocean ridges (MORs), plate spreading induces upwelling of the mantle, causing decompression melting. A key line of evidence for channelized transport comes from geological observations of tabular bodies of nearly pure olivine (dunite) in ophiolites, which are otherwise dominantly of olivine+pyroxene lithology (harzburgite). These have been interpreted as the relics of former channels in which focused melt flow has dissolved all pyroxene and replaced it with olivine (Quick 1982; Kelemen 1990; Kelemen et al 1995a, 1997, 2000; Braun & Kelemen 2002).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
