Effect of graphite on the electrical conductivity of the lithospheric mantle

Some seismic models derived from tomographic studies indicate elevated shear‐wave velocities (≥4.7 km/s) around 120–150 km depth in cratonic lithospheric mantle. These velocities are higher than those of cratonic peridotites, even assuming a cold cratonic geotherm (i.e., 35 mW/m2 surface heat flux) and accounting for compositional heterogeneity in cratonic peridotite xenoliths and the effects of anelasticity. We reviewed various geophysical and petrologic constraints on the nature of cratonic roots (seismic velocities, lithology/mineralogy, electrical conductivity, and gravity) and explored a range of permissible rock and mineral assemblages that can explain the high seismic velocities. These constraints suggest that diamond and eclogite are the most likely high‐Vs candidates to explain the observed velocities, but matching the high shear‐wave velocities requires either a large proportion of eclogite (>50 vol.%) or the presence of up to 3 vol.% diamond, with the exact values depending on peridotite and eclogite compositions and the geotherm. Both of these estimates are higher than predicted by observations made on natural samples from kimberlites. However, a combination of ≤20 vol.% eclogite and ~2 vol.% diamond may account for high shear‐wave velocities, in proportions consistent with multiple geophysical observables, data from natural samples, and within mass balance constraints for global carbon. Our results further show that cratonic thermal structure need not be significantly cooler than determined from xenolith thermobarometry.

Section: 1029/2018gc007534supporting

confidence: 51%

Multidisciplinary Constraints on the Abundance of Diamond and Eclogite in the Cratonic Lithosphere

Garber

Maurya

Hernandez

et al. 2018

“…Because crystalline graphite is remarkably conductive (10 −5 Ω m ), some few percent of it might cause lowered resistivities (<100 Ω m ) as modeled in the ULM, even without perfect interconnection (Figure 11c). Recent studies indicate grain boundary graphite films within olivine can only exist in isolated pockets and therefore cannot form interconnected grain boundary film networks under hydrostatic stress for realistic molar abundances related to carbon content measured in mantle samples (Zhang & Yoshino, 2017). Sulfides and sulfide films are also another possible source of conductive anomalies with similar physical attributes to graphite; however, their stability fields do not extend to the upper mantle depths, and observed conductivities cannot be produced with realistic modal abundances (Selway, 2014; Watson et al., 2010).…”

Section: Discussionmentioning

confidence: 99%

MATE: An Analysis Tool for the Interpretation of Magnetotelluric Models of the Mantle

Özaydın

Selway

2020

Interpretation of electrical conductivity anomalies observed in magnetotelluric models provides an important opportunity to understand the nature of the lithospheric mantle and its dynamics. Over the course of the last two decades, a great number of experimental petrology studies have been carried out which can be utilized to construct electrical conductivity distribution models for a given composition and geotherm. We have developed an open‐source software (MATE, Mantle Analysis Tool for Electromagnetics) with an easy‐to‐use graphical interface that creates such theoretical models. The program is developed in such a way that additional effects and models can be added very easily. To investigate the conductivity distribution of the cratonic mantle, a series of experiments was made. Results indicate that it is of utmost importance to analyze the magnetotelluric models using accurate compositions, water distributions, and geometric models. Hence, using only olivine conductivity models can lead to erroneous interpretations of both conductivity and estimated water content. Analysis of the potential causes for conductive anomalies shows that the upper and lower lithospheric mantle can be interpreted separately with the transition between them at 75–125 km. Conductive anomalies in the upper lithospheric mantle (<1,000 Ωm) are likely to be explained by the existence of well‐connected minor phases associated with metasomatic fluids, whereas in the lower lithospheric mantle, hydration and/or well‐connected minor phases (e.g., phlogopite or amphibole) can explain conductive anomalies.

“…Conductive sulfide minerals have been found along grain boundaries and fractures in peridotitic xenoliths (Ducea & Park, ). Graphite (∼10 −5 Ω m) or solid carbon, if abundant (>100 ppm), stable, and interconnected, could explain high conductivities in the mantle (Duba & Shankland, ), though recent studies have called into question the thermodynamic stability of interconnected graphite on olivine grain boundaries at upper lithospheric mantle conditions (Zhang & Yoshino, ). Regardless of stability, any conductive mineral phase would require a mechanism for alignment in the paleo‐spreading direction to generate the modeled anisotropy.…”

Section: Fluid Pathways and Sheared Olivinementioning

confidence: 99%

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

Chesley

Key

Constable

et al. 2019

Geophysical observations of anisotropy in oceanic lithosphere offer insight into the formation and evolution of tectonic plates. Seismic anisotropy is well studied but electrical anisotropy remains poorly understood, especially in the crust and uppermost mantle. Here we characterize electrical anisotropy in 33 Ma Pacific lithosphere using controlled-source electromagnetic data that are highly sensitive to lithospheric azimuthal anisotropy. Our data reveal that the crust is ∼18-36 times more conductive in the paleo mid-ocean ridge direction than the perpendicular paleo-spreading direction, while in the uppermost mantle conductivity is ∼29 times higher in the paleo-spreading direction. We propose that the crustal anisotropy results from subvertical porosity created by ridge-parallel normal faulting during extension of the young crust and thermal stress-driven cracking from cooling of mature crust. The magnitude of uppermost mantle anisotropy is consistent with recent experimental results showing strong electrical anisotropy in sheared olivine, suggesting its paleo-spreading orientation results from sub-Moho mantle shearing during plate formation.Plain Language Summary A major goal in geoscience is to understand the creation and evolution of oceanic lithosphere. To that end, geophysicists study how properties like electrical conductivity vary with direction and depth in the oceanic lithosphere. We call such directional variation "electrical anisotropy." Since electrical conductivity is particularly sensitive to fluids, certain minerals, and past deformation, the patterns of electrical anisotropy in the crust and mantle provide evidence for how the lithosphere forms and evolves. For the first time, we use an active-source electromagnetic technique to constrain the electrical anisotropy of Pacific oceanic crust and the shallowest portions of the mantle. Our model shows that the oceanic crust and uppermost mantle are highly anisotropic. We interpret the electrical anisotropy in the crust as fluid-filled cracks that parallel the paleo mid-ocean ridge. This suggests that crustal electrical structure begins to form at the mid-ocean ridge and continues to evolve over time through those early weaknesses. If such cracks are also present in other tectonic plates, then the oceanic crust may be a more important reservoir of water than previously thought. Uppermost mantle electrical anisotropy is consistent with strong shear deformation of olivine that freezes into the mantle early in its formation.