Paragenesis and U-Pb systematics of baddeleyite (ZrO2)

摘要: The mineral baddeleyite (ZrO2) occurs in many terrestrial and lunar samples some achondrite meteorites as a trace constituent that most often crystallizes the late-stage, chemically fractionated portions of mafic magmas together with apatite, ilmenite, ± zircon, zirconolite. In rocks these interstitial regions commonly contain K-feldspar, Cl-rich mica, amphibole small amount quartz. addition, tektites dissociation product zircon during meteorite impact rims on mantle megacrysts recovered from kimberlites. Baddeleyite is major carrier Hf, Ti, Fe U strongly fractionates Hf Zr it can have very high Zr/Hf (> 50). xenocrysts unique chemical composition significantly higher TiO2 lower FeO contents. Baddeleyite an ideal for U-Pb dating because has abundant (up to 3000 ppm), negligible initial common Pb, rarely xenocrysts, and, unmetamorphosed samples, experiences Pb loss. As example reproducibility conventional U-Pb-baddeleyite dating, several individual fragments large crystal obtained Phalaborwa carbonatite, South Africa, yielded remarkable consistency mean 207Pb206Pb age 2059.8 0.8 Ma (2σ), interpreted carbonatite emplacement. Perhaps best geological Mackenzie dyke swarm North America where eight sill collected over distances > 2000 km, all consistent ages 1267 1268 Ma. ability obtain precise (± 1−2 Ma) accurate magmatic outbursts Earth, such event, paramount importance attempting make supercontinent reconstructions global correlation ancient activity worldwide. In metamorphosed rocks, also survive grades metamorphism but develops rim polycrystalline metamorphic zircon. width grain size aggregate increase when subjected progressively metamorphism. A particularly interesting variably Hurwitz gabbro sill, Northwest Territories, Canada. relatively sample, 2111.2 0.6 (2σ) four fractions growth preserves crystallization while middle upper greenschist-facies sample along strike, likely within same contains significant development corresponding discordance between 12% 31% depending rims. It noteworthy both primary be gabbroic rocks. amphibolite- granulite-facies coronitic Grenville Province, Ontario, Canada, retain 1152 2 pseudomorphs after record at ∼ 1050