Constraints on MTZ Water Content From Joint Inversion of Diurnal Variations and Magnetospheric Signals

Electrical conductivity of mantle minerals is highly sensitive to water content, so estimates of this physical parameter based on long-period electromagnetic induction data can help to constrain mantle hydration. Reliable constraints require conductivity model with sufficient resolution at relevant depths, as well as a comprehensive uncertainty analysis. In this global induction study, we combine data from the primarily ionospheric diurnal variation and longer-period magnetospheric ring current bands. Joint inversion of these two data sources allows significantly improved vertical resolution of radial conductivity variations, especially in the mantle transition zone (MTZ). We then evaluate how uncertainties in the resulting conductivity model, in thermal models, and in laboratory results affect water content estimates, focusing on the MTZ. Our analysis suggests that the conductivity model is best explained by a dry MTZ with pyrolitic composition. Considering the uncertainties, an upper bound on average water content in the MTZ is 0.3 wt.% of water.