Discovery of a NaPx–En inclusion in diamond: possible transition zone origin

Abstract

A mineral inclusion, which has a composition of 16 mole % NaPx (Na₂MgSi₅O₁₂) and 84 mole % enstatite (Mg₄Si₄O₁₂), was found in a diamond specimen from the “No. 50” kimberlite pipe in the Province of Liaoning, China. This inclusion contains a significant amount of Na₂O, and part of the silicon may be six-coordinated by oxygen. It appears to have formed at a pressure of at least 16.5 GPa, on the basis of existing high-pressure experimental results, and suggests a new phase derived from the transition zone (410–670 km). Combined with previous reports on rock samples from the transition zone, our discovery of the NaPx–En inclusion supports the proposal that the transition zone is mineralogically and geochemically heterogeneous.