ADVANCING IN-SITU MICROANALYTICAL CALCIUM ISOTOPE GEOCHEMISTRY BY SIMS

Calcium isotope (δ^44/40Ca) measurements of marine carbonates are widely employed as proxies for paleoenvironmental processes, including chemical weathering, ocean acidification, and diagenesis. Most studies use ICP-MS and TIMS techniques to measure δ^44/40Ca values for bulk sediments, rocks, and fossils. However, important questions remain about Ca isotope variability at small spatial scales. SIMS analyses of marine carbonates can resolve significant variations in oxygen and carbon isotopes (δ¹⁸O and δ¹³C) at the micrometer scale. Few studies have analyzed δ^44/40Ca values by SIMS, mainly due to absence of a reliable calibration. Samples and standards analyzed by SIMS require similar chemical compositions and crystal structures, and the Ca isotope composition of standards must be known with a high degree of accuracy and precision.

Here, we present SIMS δ^44/40Ca measurements of 10 carbonate reference minerals, including calcite, aragonite, dolomite, and ankerite, and we compare them to high-precision TIMS measurements. We present results for two different SIMS methods. One method measures Ca⁺ ions, while another measures CaO^- ions. The CaO^- method is particularly important for carbonates with high [Sr], to avoid interference from ⁸⁸Sr²⁺. For the UWC-3 calcite standard, the Ca⁺ method yields an external precision of approximately ±0.30‰ (2SD) for 3-minute-long analyses and ~10 µm diameter spots, and the CaO^- method yields an external precision of approximately ±0.40‰ (2SD) for 5-minute-long analyses and ~15 µm diameter spots. The other 9 standards show similar reproducibility for both methods and display differences in instrumental mass fractionation that correspond to [Mg] and [Fe]. More development of the SIMS δ^44/40Ca method will foster future studies of diverse naturally occurring carbonates at the scale of 10’s of micrometers. For some samples, the current level of precision could sufficiently resolve whether diagenetic carbonate precipitated under closed- vs. open-system conditions.