The internal structure and dynamics of Earth have been shaped by the 660 km boundary between the mantle transition zone and lower mantle. However, due to the paucity of natural samples from this depth, the nature of this boundary—its composition and volatile fluxes across it—remain debated. Here we analyse the mineral inclusions in a rare type IaB gem diamond from the Karowe mine (Botswana). We discovered recovered lower-mantle minerals ringwoodite + ferropericlase + low-Ni enstatite (MgSiO₃) in a polyphase inclusion, together with other principal lower-mantle minerals and hydrous phases, place its origin at ~23.5 GPa and ~1,650 °C, corresponding to the depth at the 660 km discontinuity. The petrological character of the inclusions indicates that ringwoodite (∼Mg_1.84Fe_0.15SiO₄) breaks down into bridgmanite (∼Mg_0.93Fe_0.07SiO₃) and ferropericlase (∼Mg_0.84Fe_0.16O) in a water-saturated environment at the 660 km discontinuity and reveals that the peridotitic composition and hydrous conditions extend at least across the transition zone and into the lower mantle.

地幔过渡带和下地幔之间的 660 公里边界塑造了地球的内部结构和动力学。然而，由于来自这个深度的天然样本缺乏，这个边界的性质——它的组成和穿过它的挥发性通量——仍然存在争议。在这里，我们分析了来自 Karowe 矿（博茨瓦纳）的稀有 IaB 型宝石钻石中的矿物包裹体。我们在多相包裹体中发现了回收的下地幔矿物菱镁矿 + 方镁石 + 低镍顽火石 (MgSiO ₃ )，以及其他主要的下地幔矿物和含水相，其起源于 ~23.5 GPa 和 ~1,650 °C，对应到 660 公里不连续处的深度。包裹体的岩石学特征表明，菱镁矿 ( ∼ Mg _1.84Fe _0.15 SiO ₄ ) 在 660 km 不连续处的水饱和环境中分解为布氏橄榄石 ( ∼ Mg _0.93 Fe _0.07 SiO ₃ ) 和铁方镁石 ( ∼ Mg _0.84 Fe _0.16 O) 并揭示了橄榄岩组成和含水条件在最少穿过过渡带并进入下地幔。