Constraints on Inner Core Composition From SeismicAnisotropy: The Importance of Light Elements and Nickel

The inner core is seismically anisotropic, with PKIKP waves traversing the inner core parallel to the rotation axis faster than those in the equatorial plane. This anisotropy increases with depth into the inner core and may result from alignment of iron crystals deformed during inner core growth. Using previously calculated elastic properties of iron, we seek to determine the most likely iron-light element alloy (FeC, FeO, FeS, or FeSi). For each FeX alloy, we interpolate elastic tensors across the pressure and temperature range of the inner core and model the anisotropy resulting from flow during core growth. Lastly, we compare predicted PKIKP travel times with observations to determine the best fitting alloy. We find that iron-sulfur fits better than pure iron, but also that no light element alloy matches the observed anisotropy better than iron-nickel alloy. Future studies of core compositions should include nickel to explain seismic observations.