Ilmenite as a recorder of kimberlite history from mantle to surface: examples from Indian kimberlites
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Five compositional-textural types of ilmenite can be distinguished in nine kimberlites from the Eastern Dharwar craton of southern India. These ilmenite generations record different processes in kimberlite history, from mantle to surface. A first generation of Mg-rich ilmenite (type 1) was produced by metasomatic processes in the mantle before the emplacement of the kimberlite. It is found as xenolithic polycrystalline ilmenite aggregates as well as megacrysts and macrocrysts. All of these ilmenite forms may disaggregate within the kimberlite. Due to the interaction with low-viscosity kimberlitic magma replacement of pre-existing type 1 ilmenite by a succeeding generation of geikielite (type 2) along grain boundaries and cracks occurs. Another generation of Mg-rich ilmenite maybe produced by exsolution processes (type 3 ilmenite). Although the identity of the host mineral is unclear due to extensive alteration and possibility includes enstatite. Type 4 Mn-rich ilmenite is produced before the crystallization of groundmass perovskite and ulvöspinel. It usually mantles ilmenite and other Ti-rich minerals. Type 5 Mn-rich ilmenite is produced after the crystallization of the groundmass minerals and replaces them. The contents of Cr and Nb in type 2, 4 and 5 ilmenites are highly dependent on the composition of the replaced minerals, they may not be a good argument in exploration. The highest Mg contents are recorded in metasomatic ilmenite that is produced during kimberlite emplacement, and cannot be associated with diamond formation. The higher Mn contents are linked to magmatic processes and also late processes clearly produced after the crystallization of the kimberlite groundmass, and therefore ilmenite with high Mn contents cannot be considered as a reliable diamond indicator mineral (DIM) and kimberlite indicator mineral (KIM).
KeywordsMagnesian ilmenite Manganoan ilmenite Geikielite Kimberlite India
Authors acknowledge the Scientific and Technical Centres of the University of Barcelona (CCiTUB) for assistance with SEM-EDS (Javier García Veigas, Eva Prats Miralles and David Artiaga Torres) and EMPA (Xavier Llovet). We also thank Bruce Wyatt and Adrian Van Rythoven for their detailed reviews. The authors appreciate the careful editorial work of Casey Michael Hetman. This research was supported by the CGL2006-12973 and CGL2009-13758 projects of the Ministerio de Ciencia e Innovación of Spain, the AGAUR 2014SGR01661 and 2017SGR707 (Generalitat de Catalunya), and a FI grant to Jingyao Xu (coded FI_B 00904, Departament d’Educació i Universitats, Generalitat de Catalunya).
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