Chemical heterogeneity, convection and asymmetry beneath mid-ocean ridges

Abstract

Summary Geophysical observations at some mid-ocean ridges document an across-axis asymmetry in indicators of magma production. Other observations are interpreted as showing non-monotonic variations in the depth of the lithosphere–asthenosphere boundary. These patterns are inconsistent with the classical models of mantle corner flow and half-space cooling. To investigate this discrepancy, we use models of coupled magma/mantle dynamics beneath mid-ocean ridges in which phase densities are determined by melt–residue partitioning of iron and magnesium, and bulk density is affected by residual porosity. Our models predict that emergent gradients in density drive ridge-local convection. In particular, we show that convective upwelling is enhanced by porous buoyancy and suppressed by compositional buoyancy. Despite this suppression, models that include both compositional and porous buoyancy are more sensitive to long-wavelength mantle heterogeneity than models with porous buoyancy alone. This sensitivity enables models to readily form across-axis asymmetry of upwelling. In some cases, it leads to lithospheric delamination and time-dependent, small-scale convection. We conclude that melting-induced buoyancy effects may explain the magmatic asymmetry and variations in lithospheric thickness that are inferred from observations.

Coupled Geodynamical‐Geochemical Perspectives on the Generation and Composition of Mid‐Ocean Ridge Basalts

“…We considered the simplified scenario of a single‐phase, incompressible, homogeneous upper mantle that rises passively due to the overlying lithosphere's prescribed extension. Such choices ignore the two‐phase‐flow nature of a mid‐ocean ridge system where compositional and porous buoyancy stemming from density differences within the solid and between material phases, respectively, as well as subtleties in the rheology, dictate the flow regime and lead to asymmetrical geometries (e.g., Katz, 2010; Pusok et al., 2022). Nevertheless, symmetrical steady‐state geometries, such as those examined in the present study (e.g., Figure 3), should still provide a reasonable first‐order approximation of MOR dynamics.…”

“…Such a decrease, however, could be counteracted by a slight increase in mantle potential temperature, allowing for the accommodation of a distinct peridotite solidus. Given current uncertainties in the upper mantle potential temperature, a value of up to 1375°C would still be acceptable (Katsura, 2022; Pusok et al., 2022) and could yield the expected crustal thickness.…”

Spatial Variations in the Degree of Upper‐Mantle Depletion in a Mid‐Ocean Ridge–Transform Fault System

“…It is therefore expected that the degree of depletion varies widely between ridge segments. Second, mantle flow and melt migration beneath mid‐ocean ridges have been extensively investigated, including the flow asymmetry caused by ridge migration, buoyancy effects due to temperature, melt, and depletion, and mechanisms of melt focusing toward the ridge axis (e.g., Bai & Montési, 2015; Braun et al., 2000; Conder et al., 2002; Katz & Weatherley, 2012; Keller et al., 2017; Pusok et al., 2022; Sim et al., 2020). Each factor will produce different distributions of the degree of depletion.…”

“…Previous studies have mainly considered the formation of the depleted layer in a two‐dimensional (2‐D) mid‐ocean ridge system (e.g., Braun et al., 2000; Plank & Langmuir, 1992; Pusok et al., 2022). However, the formation of this layer in the uppermost mantle is likely to be more complicated due to the presence of transform faults, which connect neighboring mid‐ocean ridges.…”

Modeling liquid transport in the Earth’s mantle as two-phase flow: Effect of an enforced positive porosity on liquid flow and mass conservation

“…The liquid flow follows the Darcy's law but experiences resistance due to the compaction of the solid matrix. Using this theory, liquid flow has been evaluated in various geodynamic settings including mid-ocean ridges (Katz, 2008;Keller and Katz, 2016;Cerpa et al, 2018;Sim et al, 2020;Pusok et al, 2022), subduction zones (Dymkova and Gerya, 2013;Wilson et al, 2014;Cerpa et al, 2017Cerpa et al, , 2018Wang et al, 2019;Rees Jones et al, 2018), continental rifts (Schmeling, 2010;Li et al, 2023), and intraplate context (Keller et al, 2013;Dannberg and Heister, 2016).…”