Liquid ilmenite or liquidus ilmenite: a comment on the nature of ilmenite vein deposits

The interstitial character of ilmenite in common gabbroidic rocks has led to the idea that the last drop of liquid to crytallize in a rock could be pure ilmenite. When injected into enclosing rocks this liquid could give rise to ilmenite-pure veins. Fractional crystallization or immiscibility cannot produce a pure ilmenite melt. Actually, ilmenite is a liquidus mineral, which is the first or among the first mafic minerals to crystallize in Fe-Ti rich magmas. This is clearly shown by the sequence of crystallization in the Bjerkreim-Sokndal layered intrusion, by quantitative modelling of the liquid line of descent of jotunitic (hypersthene monzodioritic) magmas, and by experimental work. The interstitial character of ilmenite is therefore acquired subsolidus. Deformation can enhance the process. The synemplacement deformation linked to the diapiric uprise of anorthosite massifs is a suitable environment for the formation of layers of Cr- and Mg-rich ilmenite and their subsequent deformation and mobilization in discordant veins of pure ilmenite.