Electrical image of passive mantle upwelling beneath the northern East Pacific Rise

Abstract

Melt generated by mantle upwelling is fundamental to the production of new oceanic crust at mid-ocean ridges, yet the forces controlling this process are debated. Passive-flow models predict symmetric upwelling due to viscous drag from the diverging tectonic plates, but have been challenged by geophysical observations of asymmetric upwelling that suggest anomalous mantle pressure and temperature gradients, and by observations of concentrated upwelling centres consistent with active models where buoyancy forces give rise to focused convective flow. Here we use sea-floor magnetotelluric soundings at the fast-spreading northern East Pacific Rise to image mantle electrical structure to a depth of about 160 kilometres. Our data reveal a symmetric, high-conductivity region at depths of 20-90 kilometres that is consistent with partial melting of passively upwelling mantle. The triangular region of conductive partial melt matches passive-flow predictions, suggesting that melt focusing to the ridge occurs in the porous melting region rather than along the shallower base of the thermal lithosphere. A deeper conductor observed east of the ridge at a depth of more than 100 kilometres is explained by asymmetric upwelling due to viscous coupling across two nearby transform faults. Significant electrical anisotropy occurs only in the shallowest mantle east of the ridge axis, where high vertical conductivity at depths of 10-20 kilometres indicates localized porous conduits. This suggests that a coincident seismic-velocity anomaly is evidence of shallow magma transport channels rather than deeper off-axis upwelling. We interpret the mantle electrical structure as evidence that plate-driven passive upwelling dominates this ridge segment, with dynamic forces being negligible.

High seismic attenuation at a mid-ocean ridge reveals the distribution of deep melt

“…Low seismic wave speed ( 9 , 11 ) and high electrical conductivities ( 10 ) are strong evidence that melt plays an important role in controlling the physical properties of MOR mantle down to depths of >200 km. We find that a model with melt in a rectangular region down to a depth of 120 km requires 2% melt throughout to produce the required 1.7 s of $\mathrm{normal\Delta }{\mathit{t}}_{\mathit{S}}^{*}$.…”

On the Coupling of Geodynamic and Resistivity Models: A Progress Report and the Way Forward

“…In the upper mantle, conductivity anomalies have been found associated with subduction zones and spreading centres and have been interpreted as partial melt and upwelling asthenosphere (e.g., Naif et al 2013;Key et al 2013;Rippe et al 2013). The long-period 3-D MT model obtained from array data in the north-western United States includes interpretations for upper mantle conductivity heterogeneities in a variety of different tectonic settings (Meqbel et al 2014).…”

“…A broadband magnetotelluric survey was carried out across the northern East Pacific Rise by Key et al (2013). This study revealed a more detailed picture of the upper mantle and mantle upwelling beneath the ridge, providing constraints on melting processes and the volatile content of the mantle.…”

A new miniaturized magnetometer system for long-term distributed observation on the seafloor

“…Performances required for the geomagnetic field measurements are derived from the strength of Earth's main field as background, and fluctuation range and frequency of the local geomagnetic variation (e.g., Jacobs 1987). These are roughly defined as dynamic range of ± 60,000 nT, accuracy of ~ 0.1 nT, and sampling frequency of > 2 Hz.…”