Magnesian Ilmenite and Its Role in Kimberlite Petrogenesis

Abstract

Textures observed in magnesian ilmenites from kimberlite pipes are attributed to high-temperature deformation and recrystallization of an ilmenite cumulate body during the initial stages of formation of a kimberlite magma. Subsolidus exsolution is rarely observed in such magnesian ilmenites and the general rarity is considered to be due to the quenching of possible exsolution processes during rapid ascent of a fluidized magma. Rutile-ilmenite intergrowths are described; the rutile is considered to be a primary phase. Compositional data reveal wide variations in MgO content within a single pipe (ca. 13%). This variation is attributed to fractional crystallization. The possibility of magnesian ilmenite being either a primary mineral or a xenocryst is discussed; the former is considered more feasible and it is proposed that magnesian ilmenite crystallizes from melts unusually rich in Ti and Mg (protokimberlite) formed by small amounts of partial melting of a five-phase lherzolite mantle. Melting is considered to involve only Ti-rich mica, chrome diopside, and garnet.