Ion microprobe U-Pb dating of monazite with about five micrometer spatial resolution

We have developed ²³⁸U-²⁰⁶Pb and ²⁰⁷Pb-²⁰⁶Pb dating method of monazite by using a Cameca NanoSIMS NS50 ion microprobe. A ∼4 nA O^- primary beam was used to sputter a 5-7-μm-diameter crater and secondary positive ions were extracted for mass analysis using a Mattauch-Herzog geometry. The multi-collector system was modified to simultaneously detect ¹⁴⁰Ce⁺, ²⁰⁴Pb⁺, ²⁰⁶Pb⁺, ²³⁸U¹⁶⁺, and ²³⁸U¹⁶O₂⁺ ions. A mass resolution of 4100 at 1% peak height was attained with a flat peak top, while the sensitivity of Pb was about 4 cps/nA/ppm. A monazite from North-Central Madagascar with a U-Pb age of 524.9 ± 3.1 Ma (2σ) obtained by thermal ionization mass spectrometry was used as a reference for Pb⁺/UO⁺ - UO₂+/UO⁺ calibration. Based on the positive correlation, we have determined the ²⁰⁶Pb/²³⁸U ratios of samples. ²⁰⁷Pb/²⁰⁶Pb ratios were measured by a magnet scanning with a single collector mode. Then 44 monazite grains extracted from a sedimentary rock in Taiwan were analyzed. Observed ages were compared with the U-Th-Pb chemical ages by electron microprobe. ²³⁸U-²⁰⁶Pb ages agree well with those of the chemical ages except for some samples. The discrepancy may be due to an over-estimation of radiogenic Pb by the chemical method. ²⁰⁷Pb-²⁰⁶Pb ages also agree with the chemical ages while there are a few discordant samples. Taking into account the concordant samples, there are three main age groups, 230 Ma, 440 Ma and 1850 Ma of monazites. The age distribution suggests that the provenance of detrital monazites is possibly the North China Craton or the Qinling-Dabie-Sulu zone between the North China and South China blocks.