Ilmenite in peridotite and pyroxenite xenoliths from Siberian kimberlite pipes: morphology and genesis

Ilmenite is a diamond associate mineral from kimberlites and present in other ultramafic and basic alkaline rocks. In kimberlites ilmenite is common in the form of monomineral nodules-megacrysts, as well as phenocrysts in a fine-grained groundmass. The amount ilmenite-containing rocks from the total mantle xenoliths make up 4-7%, and it can be connected with proto-kimberlite melts. Ilmenite also occurs in a whole group of mantle rocks (called ilmenite hyperbasites), is presented as individual euhedral crystals, rounded grains, is also observed as inclusions in pyroxene, garnet and sometimes forms sideronite structures (intergrowths with silicates) and veinlets in a fine-grained olivine matrix.

In this work, mantle xenoliths were studied from the Mir (Mirny field) and Obnazhennaya (Kuoika field) kimberlite pipes of the Yakutian kimberlite province. These pipes are located in different parts of the Siberian craton and have different ages. The chemical composition of the ilmenite lamellaes and rounded inclusions from two pipes is discussed. A wide range of values ​​is observed for lamellae from both pipes - from 39.7 to 57.6 wt.% TiO₂. Rounded inclusions from the Obnazhennaya pipe are distinguished by narrow composition variations - 53-56 wt.% TiO₂. At the same time, they are close to megacrystalline and xenogenic (lithospheric) ilmenites from kimberlites. Large variations in the compositions of ilmenite lamellae from pyroxene and garnet crystals suggest that these ilmenites formed as disintegration and exsolution structures during gradual cooling of the initial megacrystals. Their cooling velocity and P-T final crystallization were different to reflect the difference in ilmenite compositions. Diffusion of elements from the host mineral could also affect composition variations, since the sizes of small inclusions are up to 20-40 μm. At the same time, some of the compositions of ilmenite lamellae from Mir pipe xenoliths close into the composition field of late fine-grained ilmenites of the main mass of kimberlites. This fact may indicate deep differentiation of melts enriched in iron and titanium, possibly longer processes of lithospheric mantle evolution under the Mir kimberlite pipe than under the northeast of the craton (Obnazhennaya pipe). Rounded inclusions of ilmenite in garnet and pyroxene from peridotites of the Obnazhennaya pipe have a different genesis. Their chemical compositions on the MgO-TiO₂ diagram form a compact group and are close to the region of the original, asthenospheric ilmenites. It was formed from melts, but in terms of formation time they are later than ilmenites from lamellas, which indicates a more complex history of formation and heterogeneity of the lithospheric mantle beneath the Obnazhennaya pipe.

The research was supported by Russian Science Foundation grant № 22-77-10073.