Al 2 O 3 in Anorthositic Slabs: A New Perspective on LLSVPs and Seismic Velocity Variations

Abstract Aluminum is a key component of crustal and mantle minerals, influencing melting and geochemical evolution within the Earth. Al 2 O 3 exists as corundum at the surface and in upper mantle conditions, but transforms into high‐pressure phases like Rh 2 O 3 (II) at lower mantle depths, enabling stability over a wide pressure range. It is also a dominant phase in anorthositic lithologies, which may represent remnants of Earth's early crust and contribute to deep mantle heterogeneities. We calculate the thermoelastic properties of these Al 2 O 3 phases and model their behavior in mantle lithologies such as anorthosite, mid ocean ridge basalt, and pyrolite along relevant geotherms. Results suggest that Al 2 O 3 could persist as Rh 2 O 3 (II) within Large Low Shear Velocity Provinces and the Dʺ layer, while corundum could remain stable in high‐velocity zones. Integrating these phases into multiphase assemblage provides new constraints on deep mantle composition and supports the idea of preserved primordial crustal materials in the lower mantle.