Comparative in-situ U-Th-Pb geochronology and trace element composition of baddeleyite and low-U zircon from carbonatites of the Palaeozoic Kovdor alkaline-ultramafic complex, Kola Peninsula, Russia

摘要

The most widely used geochronometer for dating geological events is the U-Pb isotope system used on zircon crystals. However, in silica-undersaturated ultramafic and alkaline rocks, baddeleyite (ZrO _2) is the predominant zirconium mineral. We present the results of 65 U-Th-Pb SIMS (SHRIMP-II) analyses of baddeleyite grains from carbonatite and phoscorite rocks of the Paleozoic Kovdor alkaline-ultramafic complex, Kola Peninsula, Russia. There are no significant differences in the obtained ages, either for baddeleyite from different host rocks or morphotypes of baddeleyite crystals, and dates vary little between different analytical sessions. The batch calculations of baddeleyite data show a concordant age of 379.1±3.7Ma, and a weighted mean ~(206)Pb/ ~(238)U age of 376.5±4.3Ma. Accessory zircons from the same host rocks are characterized by extremely low uranium content (mainly 1ppm) and extraordinarily high Th/U ratio (reaching up to 9050), resulting in relatively imprecise dates: 33 analyses give a U-Pb age of 342±31Ma, and weighted mean ~(208)Pb/ ~(232)Th age of 374±11Ma. Our data demonstrate that baddeleyite from carbonatite rocks is more suitable for accurate dating than co-existing zircon. We report the first in-situ trace element data for the Kovdor baddeleyite and compare these with analyses of the Phalaborwa baddeleyite. Geochemical affinities of the studied baddeleyites reveal a relatively narrow range of trace element contents, typical for carbonatites and in good agreement with known data for baddeleyite, whereas trace element compositions in Kovdor zircons are characterized by significant variations. This along with a complicated internal structure as revealed by cathodoluminescence reflects complex crystallization processes during baddeleyite growth. Our data agree well with published ages obtained by both solution and in-situ methods of analysis, such as SIMS and ICP-MS, and indicate that the Kovdor baddeleyite may be used as a U-Th-Pb standard for in-situ isotope analysis.