Diagenesis of titaniferous minerals in Jurassic sandstones from the Norwegian Sea

Ti-oxides (anatase and, in some cases, rutile) that attain various textures and modes of occurrence are minor, yet very common authigenic minerals in the Jurassic sandstones of the Haltenbanken oil field, offshore central Norway. These Ti-oxides (± pyrite) commonly occur as pseudomorphs after detrital Fe-Ti oxides (usually ilmenite, titanomagnetite and titanohematite). Pseudomorphs comprised of Ti-oxides as well as quartz and/or calcite are believed to result upon alteration of detrital sphene. Ti-oxides have also resulted upon kaolinization of detrital biotite and, less significantly, muscovite. Authigenic sphene occurs in very small amounts in some of the Jurassic sandstones (at burial depths > 2000 m). It displays three major crystallization habits: (1) as almond-shaped crystals that have, most commonly, crystallized on authigenic calcite; (2) as discrete rhombic crystals that have crystallized adjacent to altered Fe-Ti oxides and biotite; and (3) as aggregates of euhedral and subhedral crystals. These crystallization habits suggest that sphene has been formed by reaction of Si and Ti ions with calcite, crystallization from pore fluids, and replacement of Fe-Ti oxides, respectively. Microprobe analyses of authigenic sphene revealed that Al and Fe³⁺ substitute Ti in appreciable amounts (average 0.08 and 0.04 moles per 5 oxygen atoms, respectively), whereas substitutions of Ca by Mn, K and Na and of Ti by Mg are negligible.