Composition and petrogenesis of Ti-oxides associated with diamonds

Among Ti-bearing oxides, rutile is common in eclogitic (E-type) diamonds an d diamondiferous eclogites of varied bulk chemistry, but rarely occurs in a ssociation with ultramafic (or peridotitic) (U-type) diamonds, especially i n Iherzolitic assemblages. Roughly 100 rutile samples associated with diamo nds as inclusions (71 samples), intergrowths with polycrystalline diamond a ggregates (20 samples), and extracted from diamondiferous eclogites (11 sam ples) were studied by electron microprobe techniques for Fe, Al, Mg, Ca, Mn , Cr, Nb, and Zr contents. Rutiles included in diamonds from Yakutia and Ve nezuela dominate the study. Compositions of rutiles associated with diamond and/or coesite from crustal ultrahigh-pressure (UHP) metamorphic rocks fro m the Kokchetav massif, northern Kazakhstan, and the Dora Maim massif in th e Italian Alps are presented for comparison. Titanates of the LIMA (lindsleyite-mathiasite) series and perovskite are ra re accessories in U-type diamonds. Ilmenite of a wide range of compositions , containing up to 80% geikielite, is closely associated with rutile in bim ineralic intergrowths in a number of diamonds and diamondiferous eclogites. Diamond-associated rutiles contain up to 0.8 wt% Al2O3, 2.5 wt% Fe2O3, 1.4 wt% Nb2O5, 0.45 wt% ZrO2, and 2.3 wt% Cr2O3 tin the U type). The majority of rutiles, including diamond inclusions, are heterogeneous with widely var ying Al, Fe, and Mg contents. These heterogeneities are mainly caused by th e presence of sigmoidal oriented lamellae of ilmenite, picroilmenite (geiki elite), and corundum; these are documented for the first time in diamond-as sociated rutiles and may represent exsolution from a UHP precursor. These f eatures reflect the complex P-T and f(O-2) history of the rutiles. The pres ence of complex titanates of the LIMA series and perovskite in diamond-hear ing assemblages reflects the metasomatizing conditions of diamond formation .