Analytical Sciences

Abstract − Analytical Sciences, 21(9), 1091 (2005).

Nano-SIMS Analysis of Mg, Sr, Ba and U in Natural Calcium Carbonate
Yuji SANO,* Kotaro SHIRAI,* Naoto TAKAHATA,* Takafumi HIRATA,** and Neil C. STURCHIO***
*Ocean Research Institute, The University of Tokyo, Nakanoku, Tokyo 164-8639, Japan
**Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguroku, Tokyo 152-8551, Japan
***Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois 60607-7059, USA
Concentrations of minor (Mg and Sr) and trace (Ba and U) elements in four natural calcium carbonate samples were first analyzed by inductively coupled plasma mass spectrometry (ICP-MS) after chemical dissolution and calibrated against a standard dolomite. Their homogeneities were checked by in situ laser ablation (LA) ICP-MS with 10 - 20 spots. The carbonate samples were measured by using a high lateral resolution secondary ion mass spectrometer (Nano-SIMS NS50). A ∼4 nA O- primary beam was used to sputter a 5 - 6-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. A multi-collector system was adjusted to detect 26Mg+, 43Ca+, 88Sr+, 138Ba+, 238U16O+ and 238U16O2+ ions at the same time. A resolving power of 2500 - 5000 at 10% peak height was attained by an entrance slit set at 40 µm, and each exit slit at 50 µm with adequate flat-topped peaks. The observed 26Mg/43Ca, 88Sr/43Ca, 138Ba/43Ca and 238U16O2/43Ca ratios agreed well with those measured by LA-ICP-MS. Foraminifera shells were analyzed at 5 - 6 µm scale by Nano-SIMS. There was a large variation of the Mg/Ca ratios, up to ±38%, even in a single fragment of the shell, suggesting that although the ratios provide a useful paleoceanographic proxy at bulk scale, they may reflect a more complex pattern at <10 µm scale.