Ion microprobe Sr isotope analysis of carbonates with about 5 μm spatial resolution: An example from an ayu otolith

A high lateral resolution secondary ion mass spectrometer (NanoSIMS NS50 ion microprobe) has been used to measure 87Sr/86Sr ratios in natural CaCO3 samples. A ∼2 nA O− primary beam was used to sputter a 5-7-μm diameter crater on the sample surface and secondary positive ions were extracted for mass analysis using an accelerating voltage of 8 kV and a Mattauch-Herzog geometry. The multi-collector system was adjusted to detect 43Ca+ (by a detector EM#2), 80Ca2+ (EM#3), 86Sr+ (EM#4), and 87Sr+ (EM#4b) ions at the same time. Then the magnetic field was scanned for the EM#4 to detect 85Rb+, 86Sr+ and 87Sr+, while the EM#4b can measure 86Sr+, 87Sr+ and 88Sr+, respectively. A mass resolution of 3600 at 10% peak height was attained with a flat peak top, while the sensitivity of Sr was about 10 cps/nA/ppm. An aragonite sample (coral skeleton standard; JCp-1) was used as a reference for Sr isotope ratio calibration. Repeated analyses of the JCp-1 show that the 87Sr/86Sr ratio agrees well with the seawater signature within a precision of 0.3‰ at 2σ, after the series of corrections such as the Ca dimer, 87Rb, and a mass bias estimated by the 88Sr/86Sr ratio. The method was applied to an otolith (ear-stone) from ayu (Plecoglossus altivelis altivelis) collected from the Yodo river, Japan. Although experimental errors are relatively large, up to 3‰ at 2σ, the ratios of the core region are higher than the seawater signature while more distal values agree well with seawater. The very outermost part again shows the relatively higher 87Sr/86Sr ratios. The spatial variation of 87Sr/86Sr 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.