Conductive Channels in the Deep Oceanic Lithosphere Could Consist of Garnet Pyroxenites at the Fossilized Lithosphere–Asthenosphere Boundary

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.

Reduced Viscosity of Mg2GeO4 with Minor MgGeO3 between 1000 and 1150 °C Suggests Solid-State Lubrication at the Lithosphere–Asthenosphere Boundary

“…Models considering an anisotropic melt distribution perpendicular to the ridge allow to reproduce the observed conductivity data for young oceanic plates (<5 Ma) [78]. Regarding the 20-25 Ma Cocos Plate offshore Nicaragua, garnet pyroxenites best explain the anisotropic conductive anomaly at 40-60 km depth, but local melt pooling could well explain another anomaly at about ≈100 km depth [90]. In addition, magnetotelluric surveys in the Pacific Plate with ages >50 Ma do not show any electrical anomaly that would correspond to the LAB [177,178].…”

“…The presence of a melt/fluid fraction that would be characteristic for the LAB has become the basis for interpretations in numerous studies (e.g., [16,23,62]). Beneath the East Pacific Rise, indeed, the high conductivity values observed in the vicinity of the LAB (e.g., [78]) can be explained neither by olivine hydration [176] nor by garnet pyroxenite channels [90]. Conductivity values ≥ 0.1 S.m −1 at 50-100 km depth require >300 wt.…”

“…Shear stress gradients [86] can trigger melt redistribution [87][88][89]). As a consequence, conductive melt channels constitute a reasonable explanation for the conductive LAB of very young oceanic plates [78,79,90]. Hence, melt-rich channels have been extrapolated to older lithospheres to account for anisotropic electrical anomalies of the LAB (e.g., [16,82,89]), some studies further arguing that the LAB beneath young plates (<25 Ma) would consist of a thin, partially molten layer of low viscosity that would decouple the overlying brittle lithosphere from the deeper convecting mantle [16].However, other parameters are known to significantly increase the electrical conductivity, such as small grain size, which provides a high density of grain boundaries that transport charges [91], and the presence of volatiles that increase this GB diffusion (e.g., [92]).…”

“…In addition, phase transformations induce significant grain size reduction (e.g., [39,94]), which suggests that such structural modification may impact the electrical conductivity as much as melting does [90]. Furthermore, the link between high-conductivity anomalies and the potential presence of melt-rich channels has been seriously questioned by recent experimental results [95] and could be explained by the presence of garnet pyroxenite, i.e., fossilized LAB [90].…”

“…Such H 2 O saturation is well known at convergent margins, where dehydrating slabs transfer H 2 O to the upper plate and cause subduction volcanism (e.g., [97,98]), but it is not clear how H 2 O could be trapped at the LAB, which is a global feature. If H 2 O is trapped, it would likely remain within hydrous pyroxenites constituting the fossilized early-stage LAB that emplaced near the spreading ridge [90], and the LAB of older plates (e.g., 23 Ma, offshore Nicaragua) likely contains much less H 2 O [90]. With lower amounts of H 2 O (e.g., 50 ppm) the hydrous solidus is almost equal to the dry solidus, inducing partial melting at temperatures around 1400 • C [96].…”