Analytical Sciences

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.

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