Resolving poly-metamorphic Paleoarchean ages by chemical dating of monazites using multi-spectrometer U, Th and Pb analyses and sub-counting methodology

摘要

Chemical U-Th-total Pb dating method is a low-cost and reliable technique to estimate the age of monazites of detrital and diagenetic origin or those found in low to high-T metamorphic or magmatic rocks using electron microprobe (EMP) analyses. The method has been commonly applied for age estimation in Phanerozoic and Proterozoic rocks and less commonly in Neoarchean rocks. Monazite dates from the Paleoarchean era are rare and the associated error % of spot age (=. [2σ error/absolute age]. ×. 100) is high, i.e., between 2 and 26%. The errors associated with chemical dating of monazite are significantly larger compared to isotope methods, which impede precise age estimation and age resolution in polygenetic monazites of Neoarchean and older rocks.In this study, simultaneous element analyses in multiple spectrometers and the sub-counting method of analyses are employed to obtain precise estimates of U, Th and Pb concentrations in monazite. Using vanadinite [Pb_5(VO_4)_3Cl] as the Pb standard (instead of PbS), the method yields an age of 3118+/-28Ma with Mount Narryer Mesoarchean monazite (Thermal Ionization Mass Spectrometry [TIMS] age: 3132+/-1Ma) and 516+/-12Ma with Moacyr monazite (TIMS age: 509+/-1Ma; EMP monazite age: 497+/-10Ma) as the reference samples. The modifications led to ~40% reduction in spot age errors in the Mount Narryer sample relative to those obtained by conventional monazite dating method. The reduced errors in spot ages helped in resolving statistically un-mixed age populations (3152+/-16Ma, 3322+/-15Ma and 3420+/-140Ma) in polygenetic monazites within muscovite-quartz schists from the Singhbhum Craton, Eastern India. The significance of these dates is briefly discussed.