Chemical and petrographic characterization of ilmenite and magnetite in oxide-rich cumulates of the Sokndale Region, Rogaland, Norway
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- Artikler 
Igneous ilmenite and magnetite in some oxide-rich cumulates of the Sokndal region show chemical features and microtextures that reflect the progress of magmatic evolution and subsolidus re-equilibration. These features also relate to the patterns of aeromagnetic anomalies in the region. The most primitive oxides studied occur in the Tellnes ilmenite norite. These are followed by oxides in the Bjerkreim-Sokndal layered intrusion, first in ilmenite norites and finally in magnetite-rich norites. The trend of magmatic oxide crystallization in Bjerkreim-Sokndal, is previously determined by Wilson et al. (1996), was from hematite-rich ilmenite with minor end-member magnetite to hematite-poor ilmenite with titanomagnetite, a trend indicating progressive diminution of Fe2O3. This trend was accompanied by decreasing MgO and Cr2O3, and by gradually increasing V2O3 to the point where magnetite became a primary magmatic precipitate: beyond this V2O3 also decreased. In primitive samples the ilmenite has abundant hematite exsolution lamellae, whereas the magnetite has end-member composition and is free or nearly free of lamellae. Highly evolved samples contain near end-member ilmenite without exsolution, and titanomagnetite with fine exsolution of ulvospinel subsequently oxidized to ilmenite and\/or oxidation-exsolution lamellae of Ilmenite. MgO is most strongly concentrated in ilmenite; Cr2O3 and V2O3 are strongly concentrated in hematite exsolution lamellae, but even more strongly in coexisting magnetite. The most chemically pure ilmenite, with the exception of MgO, occurs in small abundance in the most evolved titanomagnetite-rich cumulates.