Macrocrystic corundum and Fe–Ti oxide minerals entrained in alkali basalts from the Eger (Ohře) Rift: Mg−Fe 3+ -rich ilmenite as tracer of an oxidized upper mantle

摘要: Macrocrysts of corundum, ilmenite, and spinel-group minerals from alluvial deposits the Eger Rift were studied for composition, texture, mineral inclusions. All macrocrysts show usually magmatic corrosion textures indicating disequilibrium with transporting alkali-basalt magma. Corundum grains, exclusively sapphires, classified by trace-element signatures as metamorphic types. Some sapphire grains erratic compositions that may have resulted a metasomatic overprint. The inclusion inventory corundum suggests crystallization differentiated alkaline silicate melt. itself was never observed an mineral. Magnesium- Fe3+-rich described typical mantle-derived species, is dominant heavy in almost all Rift. Most discrete are similar appearance composition to kimberlite- basanite-related ilmenite. Ilmenite included zircon differ bulk ilmenite larger concentrations Nb Mn. mantle origin Mg–Fe3+-rich confirmed compositional thermo-barometric comparison clinoproxenitic hornblenditic xenoliths, which probably originated Moho region. Fe–Ti two-oxide geothermometry oxygen-barometry coexisting ilmenite–magnetite pairs yield equilibrium temperatures between 900 1,080 °C oxygen fugacities log10fO2 −0.1 1.1 (relative NNO buffer), indicate upper well mantle/crust transition zone below rift at least partially oxidized. transported source region surface explosive magmas, implied presence basaltic-pipe breccias close vicinity some placer deposits.