Interpretation of conductivity variations in magnetotelluric models of cratonic lithospheric mantle with the new open-source software MATE.

Abstract:

Accurate interpretation of the magnetotelluric models has great potential in resolving the compositional variations and dynamics of the lithospheric mantle. Over the course of the last two decades, a great number of experimental petrology studies have been carried out which can be utilised to construct electrical conductivity distribution models for a given composition and geotherm. We developed an open-source software (MATE, Mantle Analysis Tool for Electormagnetics) with an easy-to-use graphical interface that can be used to produce such theoretical models. The user is able to construct a geotherm, choose a water distribution model and phase-mixing relationships to interpret the imported MT models. With the new software, we analysed a series of scenarios for the cratonic lithospheric mantle. Results point to the importance of analysing the magnetotelluric models using accurate compositions, water distribution and geometric models, and show that using only olivine conductivity models could cause inaccurate calculations of both conductivity and estimated water content. In light of the potential causes for conductive anomalies, cratonic mantle can be interpreted separately as upper and lower lithospheric mantle. Conductive anomalies in the upper lithospheric mantle (<1000 Ωm) are most likely to be caused by the existence of well-connected minor phases associated with metasomatic fluids. In the lower lithospheric mantle, on the other hand, hydration and/or well-connected minor phases (e.g., phlogopite or amphibole) can be interpreted to be the cause of the anomalies.