Abstract— <p>The results of a mineralogical-petrographic study of a new variety of mantle rocks—zoisite-bearing xenoliths from the Zagadochnaya kimberlite pipe (Yakutia) are presented. The studied rocks are a high-calcium variety of mantle eclogites—grospydites (Ca# in garnet &gt; 50 mol %). The mineral composition of the studied eclogites consists of garnet (15–40 vol %), omphacite (26–58 vol %), kyanite (7–41 vol %), and zoisite (8–18 vol %). Zoisite forms idiomorphic, short-prismatic crystals 50–200 μm in size. Zoisite exhibits uneven distribution, forming clusters mainly around kyanite grains, often intergrown with garnet and kyanite. Crystallization of the rock-forming minerals (garnet, omphacite, kyanite) occurred simultaneously at temperatures of 800–1000°C and pressures of 3–4 GPa. We suggest that the formation of zoisite in the studied grospydites from the Zagadochnaya pipe results from the prograde breakdown of lawsonite in the temperature range of 800–900°C. The exceptional rarity of zoisite eclogites and grospydites in kimberlites is apparently due to the rarity of lawsonite varieties, which in turn is caused by the specific (atypical) protolith composition of the xenoliths from the Zagadochnaya pipe. The calculated bulk H₂O content in different samples corresponds to a range of 1600 to 4250 g/t. The water concentrations in the studied grospydites are an order of magnitude higher than in the surrounding peridotite mantle, which makes it possible to consider them as a local water reservoir in the cratonic mantle. The discovered zoisite-bearing varieties are a new, special type of mantle rock that is critically important for the geochemical cycle of water at great depths.</p>