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

0432618 - GLÚ 2015 RIV AT eng J - Článek v odborném periodiku
Seifert, W. - Rhede, D. - Förster, H.-J. - Naumann, R. - Thomas, R. - Ulrych, Jaromír
Macrocrystic corundum and Fe–Ti oxide minerals entrained in alkali basalts from the Eger (Ohře) Rift: Mg−Fe³⁺-rich ilmenite as tracer of an oxidized upper mantle.
Mineralogy and Petrology. Roč. 108, č. 5 (2014), s. 645-662. ISSN 0930-0708. E-ISSN 1438-1168
Institucionální podpora: RVO:67985831
Klíčová slova: corundum * basalts * Eger (Ohře Rift)
Kód oboru RIV: DB - Geologie a mineralogie
Impakt faktor: 1.349, rok: 2014

Macrocrysts of corundum, ilmenite, and spinel-group minerals from alluvial deposits of the Eger Rift were studied for composition, texture, and mineral inclusions. All macrocrysts show usually magmatic corrosion textures indicating disequilibrium with the transporting alkali-basalt magma. Corundum grains, exclusively sapphires, were classified by trace-element signatures as magmatic and metamorphic types. Some sapphire grains show erratic compositions that may have resulted from a metasomatic overprint. The inclusion inventory of magmatic corundum suggests crystallization from a differentiated alkaline silicate melt. Corundum itself was never observed as an inclusion mineral. Magnesium- and Fe3+-rich ilmenite, described as typical mantle-derived species, is the dominant heavy mineral in almost all alluvial deposits of the Eger Rift. Most discrete macrocrysts are similar in appearance and composition to kimberlite- and basanite-related ilmenite. Ilmenite included in alluvial corundum and zircon grains differ from the bulk of discrete ilmenite grains by larger concentrations of Nb and Mn. The mantle origin of the Mg-Fe3+-rich ilmenite is confirmed by compositional and thermo-barometric comparison with ilmenite from clinoproxenitic and hornblenditic xenoliths, which probably originated in the Moho region. The Fe-Ti two-oxide geothermometry and oxygen-barometry of coexisting ilmenite-magnetite pairs yield equilibrium temperatures between 900 and 1,080 °C and oxygen fugacities log10fO2 between -0.1 and 1.1 (relative to the NNO buffer), which indicate that the upper mantle as well as the mantle/crust transition zone below the rift is at least partially oxidized. The ilmenite macrocrysts were transported from the source region to the surface by explosive alkali-basalt magmas, as implied by the presence of basaltic-pipe breccias in close vicinity to some placer deposits.
Trvalý link: http://hdl.handle.net/11104/0237003