Ilmenite from lithospheric mantle beneath Siberian craton - the formation ways

Ilmenite is found in lithospheric mantle rocks ranges from 1.5% (Udachnaya pipe, center of Siberian craton, Yakutian kimberlite province) to 4-7% (Obnazhennaya pipe, northeastern margin of the craton). In the Mir and Obnazhennaya pipes, this mineral occurs as small, rounded and elongated inclusions (up to 20-50 μm in size) in garnet and clinopyroxene, also needles, and lamellae (up to 20-40 μm thick), following the crystallographic orientation of the host mineral. These are presumably exsolution structures. Lamellas show a wide range of chemical composition, from 39.7 to 57.6 wt.% TiO₂ and 4.2-12.5 wt.% MgO. Large variations in the compositions of ilmenite lamellas from pyroxene and garnet crystals suggest that these ilmenites formed as exsolution structures during the gradual cooling of initial pigeonite megacrystals. Ilmenite from mantle rocks forms relatively large (0.3–2 mm) isometric grains with thin elongations parallel to the banding, and lenticular porphyroclasts with features of mosaic polygonality, indicating the initial stage of rock deformation. Ilmenite from kimberlite xenoliths in the central Siberian Craton occurs in polymictic breccias and exsolution structures in other minerals and is predominantly of cumulative origin. Ilmenite from mantle xenoliths from northeast of Yakutia has a variety of morphologies, which allows us to distinguish several generations and indicates a multi-stage genesis.

The rates of their cooling and the P-T of final crystallization were different, which is reflected in the difference in ilmenite compositions. Diffusion of elements from the host mineral could also affect compositional variations, since the sizes of small inclusions are up to 20-40 μm. Moreover, some of the compositions of ilmenite lamellas from Mir pipe xenoliths are close to the compositions of late fine-grained ilmenites of the bulk of kimberlites. Polymictic peridotite rocks with Phl-Ilm cement and accessory rutile and zircon, termed polymictic breccias, were described in the Udachnaya pipe. It is assumed that Phl-Ilm and Ilm parageneses crystallized in equilibrium with residual asthenospheric melts remaining after the crystallization of most of the megacrysts of the low-chromium association and the formation of deformed garnet peridotites, and are of cumulative origin. Residual magmatic liquids are enriched in potassium, titanium, iron and volatiles. Moreover, the age of phlogopite deformation coincides with the age of the kimberlite pipes formation - 367.1 ± 1.4 Ma.

In addition, large (up to 100-200 μm) rounded inclusions of ilmenite in garnet and pyroxene, intergrowths of ilmenite with garnet - such samples were not found in our collection of xenoliths from the central parts of the craton. Ilmenite also forms individual isomorphic crystals (often intergrown with isomorphic phlogopite plates). Rounded inclusions from the Obnazhennaya pipe are distinguished by narrow compositional variations - 49.9-52.5 wt.% TiO2 and are close to the compositions of mantle, asthenospheric ilmenites. Presumably, they formed as a result of the influence of alkaline basaltic melts enriched in iron and titanium (FeO - up to 12-15 wt.%; TiO2 - up to 5-9 wt.%). The formation of several generations of ilmenite and phlogopite, zoning of associated minerals, suggests that the impact of such melt-fluids was repeated.