Pyroxene–Cr-spinel exsolution in mantle lherzolites of the Syum-Keu ophiolite massif (Arctic Urals)

We consider the composition and microstructures of pyroxene–Cr-spinel exsolution products from lherzolites of the deep mantle section of the Syum-Keu ophiolite massif in the Arctic Urals. Enstatite and diopside from the lherzolites contain lamellae and diverse micron segregations of Cr-spinel. We have first determined the composition of Cr-spinel lamellae and the composition trend of pyroxenes undergoing exsolution. The zonal inhomogeneity of their composition is expressed as a decrease in the contents of Cr2O3, Al2O3, and Na2O from the core of coarse grains free of exsolution structures to the sites with Cr-spinel segregations. The Cr/(Cr + Al) value in the Cr-spinel lamellae varies from 0.23 to 0.33, and the Mg/(Mg + Fe) value, from 0.61 to 0.67. The degree of iron oxidation in the formed Cr-spinel is low (0.10–0.19) and virtually does not depend on the Cr content in this mineral. We calculated the temperature and pressure of the beginning and completion of pyroxene–Cr-spinel exsolution for the equilibrium enstatite–diopside (of different generations) pairs and established the participation of H2O fluid in the exsolution. During the exsolution, the temperature decreases from 970 to 650–700 °C and the pressure grows from 0.9 to 1.3 GPa. The exsolution of pyroxene and migration of elements proceeded under high-temperature solid-plastic flow of mantle material, probably in the local zones where peridotites were subjected to strong shearing strains caused by the above high-velocity flow. The zone with these processes was located in the lithospheric mantle (mantle wedge) above the subduction zone. Migration of ore components from silicates and formation of new Cr-spinel grains contributed to the formation of chromite segregations.