Magmatic Focusing to Mid‐Ocean Ridges: The Role of Grain‐Size Variability and Non‐Newtonian Viscosity

Abstract

Melting beneath mid‐ocean ridges occurs over a region that is much broader than the zone of magmatic emplacement that forms the oceanic crust. Magma is focused into this zone by lateral transport. This focusing has typically been explained by dynamic pressure gradients associated with corner flow, or by a sublithospheric channel sloping upward toward the ridge axis. Here we discuss a novel mechanism for magmatic focusing: lateral transport driven by gradients in compaction pressure within the asthenosphere. These gradients arise from the covariation of melting rate and compaction viscosity. The compaction viscosity, in previous models, was given as a function of melt fraction and temperature. In contrast, we show that the viscosity variations relevant to melt focusing arise from grain‐size variability and non‐Newtonian creep. The asthenospheric distribution of melt fraction predicted by our models provides an improved explanation of the electrical resistivity structure beneath one location on the East Pacific Rise. More generally, we find that although grain‐size and non‐Newtonian viscosity are properties of the solid phase, their effect on melt transport beneath mid‐ocean ridges is more profound than their effect on the mantle corner flow.

Predicting Rates and Distribution of Carbonate Melting in Oceanic Upper Mantle: Implications for Seismic Structure and Global Carbon Cycling

“…The volume of off-axis (carbonate and silicate) melting is equivalent to~1 km of crust for ages greater than 4 Ma. The pooling region corresponds to a melt extraction width of 100 km for a half-spreading rate of 2.5 cm/year, which is consistent with extraction widths (50-100 km) inferred from ridge-scale models of two-phase flow at mid-ocean ridges (e.g., Keller et al, 2017;Turner et al, 2017). Additionally, within the range of typical mantle temperatures (1350-1400°C), we predict similar global melt production rates as the model of Li et al (2016).…”

Crustal Cracks and Frozen Flow in Oceanic Lithosphere Inferred From Electrical Anisotropy

“…Nevertheless, we suspect some minimum strain, and thus spreading, rate must be exceeded in order to generate anisotropy. Because large strain rates should occur within ∼20 km of the mid‐ocean ridge axis (Turner et al, ), we conclude that the observed electrical anisotropy arose from a shear‐induced grain boundary fabric that froze into the shallow mantle early during plate formation.…”

Distinctive Seafloor Fabric Produced Near Western Versus Eastern Ridge‐Transform Intersections of the Northern Mid‐Atlantic Ridge: Possible Influence of Ridge Migration

“…More recent models suggest that buoyant melt rises vertically after segregating from the upwelling mantle until it reaches a sloping low‐permeability barrier near the base of the lithosphere. Such permeability barrier may result from melt crystallization (Gregg et al, ; Hébert & Montési, ; Sparks & Parmentier, , ; Spiegelman, ) or an upward decrease in grain size (Turner et al, ). Melt is then thought to migrate upslope toward the ridge axis by following the underside of that permeability barrier (Figures , c, and d).…”