ARCHEAN SULFIDE INCLUSIONS IN PALEOZOIC ZIRCON MEGACRYSTS FROM THE MIR KIMBERLITE, YAKUTIA: IMPLICATIONS FOR THE DATING OF DIAMONDS

Abstract

Zircon megacrysts from the Mir kimberlite, Yakutia, contain inclusions of chromite, chrome diopside, magnesian olivine, Ni-rich monosulfide solid solution and phlogopite. The mineral chemistry of the inclusion suite suggests that the zircons grew in a metasomatized peridotite matrix. Twenty-three zircons were chosen for U–Pb dating, Hf isotope and trace element determinations. The trace element data are typical of kimberlitic zircons worldwide. LAM-ICPMS U–Pb dating yields a weighted mean 206Pb/236U age of 353.6±2.5 Ma. Hf isotope measurements by LAM-MC-ICPMS yield ϵHf values of 3.0–9.2, and model ages (TDM) of 600–800 Ma. These data constrain the crystallization of the zircons to between 350 and 600 Ma. However, LAM-MC-ICPMS microanalysis of Os isotopes in sulfides included in three zircons yields TRD model ages of 2.37–2.92 Ga (TMA=2.39–3.19 Ga). To explain the discordance between the ages of the zircons and their sulfide inclusions, we suggest that these zircons grew in a metasomatized peridotite, which contained sulfides that were residual from ancient melting events. These sulfides, together with other peridotite phases, were trapped in the metasomatic zircon with little modification of their elemental or isotopic composition. This model has important implications for the interpretation of Re–Os model ages of sulfide inclusions in diamonds. Diamonds also could capture and preserve older sulfides during their growth or regrowth in mantle rocks, and the inclusions therefore do not necessarily date the formation of the diamond.