Precise geochronology of phoscorites and carbonatites: The critical role of U-series disequilibrium in age interpretations

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Amelin, Yuri, Zaitsez, Anatoly N.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Pergamon-Elsevier Ltd
• Subjects: Keywords: carbonatite; dating method; disequilibrium; mineralogy; uranium series dating; Russian Federation; Kola Peninsula; Kovdor; kola peninsula
• Online Access: http://hdl.handle.net/1885/37761; https://doi.org/10.1016/S0016-7037(02)00831-1; https://openresearch-repository.anu.edu.au/bitstream/1885/37761/5/34_2002_Kovdor_GCA.pdf.jpg; https://openresearch-repository.anu.edu.au/bitstream/1885/37761/7/01_Amelin_Precise_geochronology_of_2002.pdf.jpg

We present the results of a comparative study of several geochronometer minerals (baddeleyite, zircon, apatite, phlogopite and tetraferriphlogopite) and isotopic systems (U-Pb, Th-Pb and Rb-Sr) from phoscorites (magnetite-forsterite-apatite-calcite rocks) and carbonites of the Kovdor ultramafic-alkaline-carbonatite massif, Kola Peninsula, Russia, Uranium, thorium and their decay products are extremely fractionated by minerals that crystallise from carbonatite and phoscorite magma. We obtain high-precision ages from different chronometers, compare their accuracy, and evaluate the role of geochronological pitfalls of initial radioactive disequilibrium, differential migration of radiogenic isotopes, and inaccurate decay constants. Apatite yielded concordant U-Th-Pb ages between 376 and 380 Ma. The accuracy of the apatite238U-206Pb ages is, however, compromised by uncertainty in the amount of radiogenic206Pb produced from initial excess230Th. The235U-207Pb ages are relatively imprecise due to large common Pb correction and the uncertainty in the initial Pb isotopic composition. The Th-Pb system yields a more precise age of 376.4 ± 0.6 Ma. Zircon from two carbonatite samples is characterised by moderate to low U contents, high Th contents, and very high Th/U ratios up to 9000. The206Pb*/238U systems in the zircon are strongly affected by the presence of excess206Pb*, produced by decay of initial230Th. The208Pb*/232Th ages of zircon from both carbonatite samples are uniform and yield a weighted average of 377.52 ± 0.94 Ma. Baddeleyite U-Pb analyses are 3 to 6% normally discordant and have variable207Pb*/206Pb* apparent ages. Eleven alteration-free baddeleyite fractions from three samples with no evidence for Pb loss yield uniform206Pb*/238U ages with a weighted average of 378.54 ± 0.23 Ma (378.64 Ma after correction for initial230Th deficiency), which we consider the best estimate for age of the phoscorite-carbonatite body of the Kovdor massif. The206Pb*/238U ages of baddeleyite fractions from five other samples ...