Composition and Petrogenesis of Ti-Oxides Associated with Diamonds |
| |
| Abstract: | Among Ti-bearing oxides, rutile is common in eclogitic (E-type) diamonds and diamondiferous eclogites of varied bulk chemistry, but rarely occurs in association with ultramafic (or peridotitic) (U-type) diamonds, especially in lherzolitic assemblages. Roughly 100 rutile samples associated with diamonds as inclusions (71 samples), intergrowths with polycrystalline diamond aggregates (20 samples), and extracted from diamondiferous eclogites (11 samples) were studied by electron micro-probe techniques for Fe, Al, Mg, Ca, Mn, Cr, Nb, and Zr contents. Rutiles included in diamonds from Yakutia and Venezuela dominate the study. Compositions of rutiles associated with diamond and/or coesite from crustal ultrahigh-pressure (UHP) metamorphic rocks from the Kokchetav massif, northern Kazakhstan, and the Dora Maira massif in the Italian Alps are presented for comparison. Titanates of the LIMA (lindsleyite-mathiasite) series and perovskite are rare accessories in U-type diamonds. Ilmenite of a wide range of compositions, containing up to 80% geikielite, is closely associated with rutile in bimineralic 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 (in the U type). The majority of rutiles, including diamond inclusions, are heterogeneous with widely varying Al, Fe, and Mg contents. These heterogeneities are mainly caused by the presence of sigmoidal oriented lamellae of ilmenite, picroilmenite (geikielite), and corundum; these are documented for the first time in diamond-associated rutiles and may represent exsolution from a UHP precursor. These features reflect the complex P-T and f(O2) history of the rutiles. The presence of complex titanates of the LIMA series and perovskite in diamond-bearing assemblages reflects the metasomatizing conditions of diamond formation. |
| |
| Keywords: | |
|
|
