A red-corundum-bearing vein in the Rai–Iz ultramafic rocks, Polar Urals, Russia: the product of fluid activity in a subduction zone

The veins in the mantle wedge peridotite can record the activity of slab-derived melt/fluid, which is the reflection of the material circulation between crust and mantle. A red-corundum-bearing vein is present in the Rai–Iz ultramafic rocks in the Polar Urals, Russia. The ultramafic rocks are harzburgite and dunite, and the red corundum-bearing rocks consist of phlogopite, paragonite, oligoclase, red corundum, and chromian spinel. Red corundum occurs as prophyroblasts or fine grains that contain 92–98 wt% Al2O3 and 2–7 wt% Cr2O3. Chromian spinel has Cr# values ([100Cr/ (Cr + Al)] atomic ratio) of 46–78. Oligoclase is characterized by An values of 20–30. Phlogopite is Ba-rich (BaO = 0.8–2.7 wt%) and paragonite is Sr-rich (SrO = 0.8–2.3 wt%). Zircons from the oligoclasite show oscillatory zoning and Th/U values of <0.2, indicating crystallization from fluid. A zircon weighted-mean 206Pb/238U age of 382 ± 2 Ma and a phlogopite 40Ar/39Ar plateau age of 377 ± 3 Ma indicate that the red-corundum-bearing vein formed at 380 Ma. The occurrence and formation age of the vein, structure and composition of vein minerals, and zircon εHf values (−11 to +13) suggest that the vein was the product of interaction between a subduction-zone-derived fluid and mantle wedge peridotite.