Abstract
Magnetotelluric (MT) surveys have identified anisotropic conductive anomalies in the mantle of the Cocos and Nazca oceanic plates, respectively, offshore Nicaragua and in the eastern neighborhood of the East Pacific Rise (EPR). Both the origin and nature of these anomalies are controversial as well as their role in plate tectonics. The high electrical conductivity has been hypothesized to originate from partial melting and melt pooling at the lithosphere–asthenosphere boundary (LAB). The anisotropic nature of the anomaly likely highlights high-conductivity channels in the spreading direction, which could be further interpreted as the persistence of a stable liquid silicate throughout the whole oceanic cycle, on which the lithospheric plates would slide by shearing. However, considering minor hydration, some mantle minerals can be as conductive as silicate melts. Here I show that the observed electrical anomaly offshore Nicaragua does not correlate with the LAB but instead with the top of the garnet stability field and that garnet networks suffice to explain the reported conductivity values. I further propose that this anomaly actually corresponds to the fossilized trace of the early-stage LAB that formed near the EPR about 23 million years ago. Melt-bearing channels and/or pyroxenite underplating at the bottom of the young Cocos plate would transform into garnet-rich pyroxenites with decreasing temperature, forming solid-state high-conductivity channels between 40 and 65 km depth (1.25–1.9 GPa, 1000–1100 °C), consistently with experimental petrology.
Highlights
A MT experiment identified an off-axis conductive asthenosphere at 60–120 km depth beneath a resistive lithosphere at the southern East Pacific Rise (EPR) spreading ridge [1,2]
I further propose that this anomaly corresponds to the fossilized trace of the early-stage lithosphere–asthenosphere boundary (LAB) that formed near the EPR about 23 million years ago
The electrical anomalies would be due to melt pooling at the LAB [8,16], which is relatively well established for the anomaly detected near the EPR [1]
Summary
A MT experiment identified an off-axis conductive asthenosphere at 60–120 km depth beneath a resistive lithosphere at the southern EPR spreading ridge [1,2]. The conductive anomaly has been interpreted as due to partial melting in the asthenosphere (Figure 1a) [1]. Based on similar activity and the spreading rate of the southern and northern EPR, it is reasonable to assume that equivalent ridge processes are occurring for the Cocos and Nazca plates [3]. Both anomalies are highly anisotropic (Figure 1) [1,3], exhibiting significantly larger conductivity values reported either in the spreading direction [1] or in the subduction direction [3], which exhibit a ≈15◦ angle offshore Nicaragua. The observed anisotropy has been interpreted as due to melt channels forming in the shearing direction at the base of the oceanic plate [4], likely parallel to the volcanoes’ alignments, indicated on the geological map (Figure 2)
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