Silicate mineral inclusions in chromite from the Eastern Bushveld complex: Implications for the origin and evolution of hydrous melt during chromite mineralization in the Critical Zone

In this work, we systematically describe silicate mineral inclusion morphology and genesis in chromite from UG3, MG2 and LG6 chromitite layers of the Eastern Limb of the Bushveld Complex. Rounded to subhedral inclusions of phlogopite, amphibole and, to a lesser extent, olivine and orthopyroxene, are concentrated at or near the center of chromite grains. The most common occurrences are tabular amphibole forming planar contact with orthopyroxene, phlogopite, and phlogopite surrounding amphibole and olivine. Several features indicate re-equilibration reactions between the host chromite and the inclusions including high Mg# values in orthopyroxene inclusions (81.9 to 88.4; average of 85.0), low MgO (5.16 to 8.15 wt%) and high FeO (28.66 to 32.54 wt%) contents in the host chromite, and higher Cr2O3 content in amphibole (0.32 to 3.60 wt%) and phlogopite (0.29 to 2.29 wt%) inclusions relative to cumulus orthopyroxene, amphibole and phlogopite in the silicate rocks. Furthermore, the high H2O (4.16 to 4.45 wt%) and Na2O (0.07 to 6.12 wt%) contents in phlogopite inclusions, low transition element (V, Ni, Co and Zn) contents in the orthopyroxene from chromitite seams, and the deviation from mantle δ18O values (5.7 to 6.7 ‰) in inclusions, indicates alteration by or reaction with, hydrous melt. In addition, the similar orthopyroxene δ18O (5.7–6.8 ‰) in the MG2 and LG6 hanging wall and footwall, and in amphibole inclusions (δ18O = 5.9–6.7 ‰) from the LG6, MG2 and UG3 chromitite seams, suggest that the hydrous melt which formed hydrous mineral inclusions in chromite, and in the hanging wall and footwall silicate minerals, had a similar origin. The high δ18O values, LG6 olivine inclusions, and the lower H2O and Li contents in Critical Zone (CZ) orthopyroxene from chromitite compared to orthopyroxene from the overlying and underlying rocks, as well as previous experiments and chemical data, all indicated that a pyroxene remelted hydrous alkali-melt was probably the main source of the trapped melt in chromite.