Chromiferous spinels of the elephant's head dike

Analysis by optical, X-ray diffraction and microprobe methods, of essentially unzoned, disseminated spinels within cumulus picrites and olivine gabbros reveals an unbroken range of compositions from aluminian chromite (34% Cr₂O₃) to chromian magnetite (4% Cr₂O₃). TiO₂ contents vary between 0.5 and 7.7%. Exsolution of ilmenite indicates originally higher TiO₂ contents. Quenched contact-facies rocks with abundant olivine phenocrysts contain strongly zoned spinels in which a titaniferous chromian magnetite rim (16% Cr₂O₃, 10% TiO₂) encloses cores of weakly titanian chromite (40% Cr₂O₃). Platy dendrites of exsolved spinel occur in abundance within cumulus olivines. The evidence suggests that crystallization of the disseminated spinels occurred under the influence of an increase in oxygen fugacity towards the interior of the intrusion, and that the compositional diversity has stemmed from the homogenization of originally zoned grains mantled to varying degrees by high-Ti, low-Cr rims.