Cr-rich rutile: A powerful tool for diamond exploration

Abstract

Mineralogical studies and U–Pb dating have been carried out on rutile included in peridotitic and eclogitic garnets from the Internatsionalnaya pipe, Mirny field, Siberian craton. We also describe a unique peridotitic paragenesis (rutile+forsterite+enstatite+Cr-diopside+Cr-pyrope) preserved in diamond from the Mir pipe, Mirny field. Compositions of rutile from the heavy mineral concentrates of the Internatsionalnaya pipe and rutile inclusions in crustal almandine-rich garnets from the Mayskaya pipe (Nakyn field), as well as from a range of different lithologies, are presented for comparison. Rutile from cratonic mantle peridotites shows characteristic enrichment in Cr, in contrast to lower-Cr rutile from crustal rocks and off-craton mantle. Rutile with Cr2O3>1.7wt% is commonly derived from cratonic mantle, while rutiles with lower Cr2O3 may be both of cratonic and off-cratonic origin.New analytical developments and availability of standards have made rutile accessible to in situ U–Pb dating by laser ablation ICP-MS. A U–Pb age of 369±10Ma for 9 rutile grains in 6 garnets from the Internatsionalnaya pipe is consistent with the accepted eruption age of the pipe (360Ma). The equilibrium temperatures of pyropes with rutile inclusions calculated using Ni-in-Gar thermometer range between ~725 and 1030°C, corresponding to a depth range of ca ~100–165km. At the time of entrainment in the kimberlite, garnets with Cr-rich rutile inclusions resided at temperatures well above the closure temperature for Pb in rutile, and thus U–Pb ages on mantle-derived rutile most likely record the emplacement age of the kimberlites.The synthesis of distinctive rutile compositions and U–Pb dating opens new perspectives for using rutile in diamond exploration in cratonic areas.