Constraints of Upper Mantle Thermodynamics with the Very Broadband Rheology Calculator and Magnetotelluric Observations
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Ofori, Samuel
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Abstract
The electrical resistivity of the Earth’s mantle varies by more than five orders of magnitude (100 to 105 ohm-m). Parameters that impact the bulk mantle resistivity include mineral composition, temperature, pressure, melt fraction, and volatile content. Furthermore, the bulk resistivity depends on the interconnectivity of the conductive phases. The thermodynamic state parameters that influence the electrical resistivity also have an impact on seismic properties such as shear wave velocity and attenuation. In 2020, the “Very Broadband Rheology calculator” (VBRc) open-source tool was released for calculating rheological properties of an olivine mantle given an array of thermodynamic state variables using laboratory-derived constitutive models for seismic observations, specifically [1]. To further the scope of the VBRc, we implement methods for calculating electrical resistivity for various thermodynamic states. The objective is to yield a constraint on the possible ranges of each state variable (e.g., temperature, volatile content, and melt fraction) that explain geophysical observables (e.g., electrical resistivity and shear wave velocity). We demonstrate the utility of joint seismic-electric analysis by applying VBRc to existing datasets and synthetic examples, including the SERPENT (Serpentinite Extension and Regional Porosity Experiment across the Nicaraguan Trench) electromagnetic survey. This research proves pertinent to seismic and electromagnetic observations related to geophysical anomalies (e.g., high conductivity and low-velocity regions) observed in the upper mantle.
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2025-04-30
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