The MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths in kimberlite

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 considere...

Magnesian Ilmenite and Its Role in Kimberlite Petrogenesis

Fractionation in the thermal ionization source

A discussion of the most important petrographic features of kimberlite is presented and a new definition of kimberlite is proposed. The discussion emphasizes those features of kimberlite which are of petrogenetic significance and points out that alnoitic and lamprophyric rocks associated with alkaline rock-carbonatite complexes are not true kimberlites. Consideration of the proposed relation of kimberlites to mica peridotites, carbonatites, basalts, potassic volcanic rocks, and melilitites reveals that kimberlites are not simply or closely related genetically to any of these rocks.

Kimberlite and Related Rocks: A Critical Reappraisal

Granulitic xenoliths, enclosed in Neogene alkali basalts from the Atakor and Adrar N'Addjer volcanic massifs in Central Hoggar, southern Algeria, include mafic metaigneous and felsic metasedimentary rocks. The main mineral phases of metaigneous xenoliths are plagioclase and pyroxenes whereas quartz, feldspars, sillimanite, garnet and sapphirine occur in metasedimentary granulites. The metaigneous xenoliths represent a magmatic sequence of cumulitic and non-cumulitic rocks which resembles gabbroic bodies associated with anorthosites and has calc-alkaline affinities. Similar types and distributions of granulitic xenoliths are widespread and are probably characteristic of the lower crust in general, suggesting its homogeneity on a large scale but heterogencity on a local scale.

J.R. Widdowson

Papers

The petrological environment of magnesium ilmenites