Lead and Strontium Stable Isotope Measurements by Using a High Lateral Resolution Secondary Ion Mass Spectrometer(NanoSIMS)

Abstract

The method of Pb and Sr isotope measurements at about 5 mu m resolution was developed by using a high lateral resolution secondary ion mass spectrometer (NanoSIMS) NS50). Since the both elements have radiogenic neuclides such as Pb-206, Pb-207, and Sr-87, natural variations of isotopic ratios are large. It is possible to detect a meaningful variation in a terrestrial sample, even though the experimental error is relatively large. In the case if monozite U-Pb dating, a 4 nA O- primary beam was used to sputter the sample and secondary positive ions were extracted for mass analysis using a Mattauch-Herzog geometry. The multi-collector system was modified to detect Ce-110(+), Pb-201(+), Pb-206(+), (UO+)-U-238-O-16, and (UO2+)-U-238-O-16 ions simultaneously. Based on the monozite standard from North-Central Madagascar, we have determined the Pb-206/U-238 ratios of samples. Pb-207/Pb-206 ratios were measured by a magnet scanning with a single collector mode. 44 monazite grains extracted from a sedimentary rock in Taiwan were analysed. Observed ages were consistent with the U-Th-Pb chemical ages by EPMA. Then NanoSIMS has been used to measure Sr-87/Sr-86 ratios in natural calcium carbonate samples. Multi-collector system was adjusted to detect Ca-13(+), Ca-80(+), Sr-86(+), and Sr-87(+) ions at the same time. Magnetic field was scanning for the EM#4 counter to detect Rb-85(+), Sr-86(+), Sr-87(+), while the EM#4b can measure Sr-86(+), Sr-87(+), Sr-88(+), respectively. Repeated analysis of a coral skeleton standard (JCp-1) show that Sr-87/Sr-86 ratio agrees well with the seawater signature, after the series of corrections such as Ca dimer, Rb-87, and a mass bias estimated by Sr-87/Sr-86 ratio. The method is applied to an otolith from ayu (pleco-glossus altivelis altivelis) collected from the Yodo river, Japan. The spatial variation of Sr-87/Sr-86 ratios was consistent with amphidromous migration of the fish, namely, born in the lake and grown in the coastal sea and finally collected in a river.