Nano-SIMS Analysis of Mg, Sr, Ba and U in Natural Calcium Carbonate

Abstract

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.