[BCG06-P13] Pb-Pb chronology of Fe-Ti oxides
キーワード：鉛ー鉛年代、Fe-Ti酸化物、アカスタ片麻岩体、マフィック岩
Fe-Ti oxides appear in a wide range of igneous and metamorphic rocks. These minerals have been widely used for investigating the temperature and oxidation state of igneous and metamorphic processes. In addition, their potential as a U–Pb chronometer was demonstrated: Burton and O’Nions (1990) reported Pb isotope compositions of Fe-Ti oxides in amphibolite- and granulite-facies orthogneisses in Sri Lanka, whereas Noyes et al. (2011) presented Fe-Ti oxide Pb isotope analysis of the Monastery kimberlite in South Africa. The results of these previous studies illustrated that the potential utility of Fe-Ti oxides as a reliable U-Pb chronometer due to having high U/Pb value and high closure temperature of Pb (~ 800 ℃). Nevertheless, the application of Fe-Ti oxide chronology remains highly limited. This is mainly because Fe-Ti oxides generally have only moderately high U contents (~0.1 ppm) so that the presence of small amounts of contaminant Pb degrades the age precision and accuracy. In this study, we utilized the acid leaching technique to improve the precision and accuracy of Fe-Ti oxide chronometer.
We analyzed Fe- Ti oxides from the anorthosite FC1 in the Duluth complex, which yields the widely used zircon U-Pb standard (1,099 ± 0.6 Ma : Paces & Miller. (1993)). We evaluated the resistance of Fe-Ti oxides to acid leaching using SEM observation. In addition, we analyzed acid washed Fe-Ti oxides for Pb isotopes by MC-ICPMS. The results show that Fe-Ti oxides have high U/Pb ratio (> 189) compared with the whole rock fraction (3.29) and yield a reliable Pb–Pb age (1107 ± 102 Ma: MSWD = 15).
Recently, we have applied Fe-Ti oxide Pb-Pb chronology to an amphibolite in the Acasta Gneiss Complex, Canada. The Acasta Gneiss Complex is an Archean terrane containing the oldest known rocks (Bowring & Williams. (1999)) and is composed of felsic orthogneisses and amphibolites. Although the formation and metamorphic ages of the felsic gneisses have been extensively investigated, no reliable chronological data have been obtained from the amphibolites. We are in the process of determining Pb–Pb ages of Fe-Ti oxides from the amphibolites in an attempt to get the reliable chronological information.
We analyzed Fe- Ti oxides from the anorthosite FC1 in the Duluth complex, which yields the widely used zircon U-Pb standard (1,099 ± 0.6 Ma : Paces & Miller. (1993)). We evaluated the resistance of Fe-Ti oxides to acid leaching using SEM observation. In addition, we analyzed acid washed Fe-Ti oxides for Pb isotopes by MC-ICPMS. The results show that Fe-Ti oxides have high U/Pb ratio (> 189) compared with the whole rock fraction (3.29) and yield a reliable Pb–Pb age (1107 ± 102 Ma: MSWD = 15).
Recently, we have applied Fe-Ti oxide Pb-Pb chronology to an amphibolite in the Acasta Gneiss Complex, Canada. The Acasta Gneiss Complex is an Archean terrane containing the oldest known rocks (Bowring & Williams. (1999)) and is composed of felsic orthogneisses and amphibolites. Although the formation and metamorphic ages of the felsic gneisses have been extensively investigated, no reliable chronological data have been obtained from the amphibolites. We are in the process of determining Pb–Pb ages of Fe-Ti oxides from the amphibolites in an attempt to get the reliable chronological information.