Figure 6
From: Chromitite layers indicate the existence of large, long-lived, and entirely molten magma chambers
Two contrasting scenarios for the textural evolution of a 3D framework of chromite crystals in massive chromitites of the Bushveld Complex. (a–c) UG1 chromitite starts forming as a 3D framework of touching chromite crystals (> 500,000 grains per 1 cm3) which self-nucleate on the anorthosite floor of a magma chamber. This initial framework has been preserved owing to the early growth of plagioclase (and pyroxene) oikocrysts from an interstitial melt. The early nucleation of plagioclase/pyroxene oikocrysts is likely due to a short interval of chromite-only crystallization of the parental melt. (d–f) LG6 chromitite also starts forming as a 3D framework of touching chromite crystals which self-nucleate on the orthopyroxenite floor of a magma chamber. However, in this case the initial framework has been obliterated by adcumulus growth of chromite (up to 100% chromite) which was not arrested by crystallization of plagioclase/pyroxene oikocrysts. No nucleation and growth of silicate phases oikocrysts is likely due to a much longer interval of chromite-only crystallization of the parental melt. In both scenarios, the growth of a 3D framework of touching chromite crystals has been accompanied by settling of chromite grains that grew in situ at the crystal-liquid interface but were torn loose by flowing melt and collapsed downwards into the open space of the crystal framework (a & d). Some of the grains have landed on and were captured by growing oikocrysts (b) and, therefore, occur there as entirely discrete grains. Crystal sizes are not to scale. The figure is prepared using CorelDRAW (version 18.1.0.690).
