Accurate in situ C-Mg-Ca isotope ratio analysis in carbonates using SIMS

Abstract

Isotopic compositions of carbonate-forming elements (C, O, Mg, Ca) are widely used geological proxies. Non-traditional stable isotope ratios (Mg, Ca) of carbonates can constrain water-rock reactions and precipitation kinetics but are typically measured by bulk digestion techniques that lack textural context and can be contaminated by elements released from associated phases (e.g., phyllosilicates). This is particularly problematic for micron-scale, chemically zoned extraterrestrial carbonates from asteroid-returned samples, carbonaceous chondrites, and Martian meteorites. Secondary ion mass spectrometry (SIMS) provides a reliable in situ approach, but its application to Mg and Ca isotopes in carbonates has been limited by the lack of well-characterized standards spanning the dolomite-ankerite and magnesite-siderite solid-solution series. Here we present 21 carbonate standards (10 dolomite-ankerite, 10 magnesite-siderite, and 1 calcite) for coupled C-Mg-Ca isotope analysis at 10-20 µm spatial resolution using a Cameca IMS 7f-GEO ion microprobe. The instrumental mass fractionation (IMF) for each isotopic system varies nonlinearly with Fe+Mn content of carbonates, parameterized as (Fe+Mn)# [= molar (Fe+Mn)/(Fe+Mn+Mg)], and is modelled using polynomial or sigmoidal functions (R 2 >0.92) with calibration residuals ≤0.7‰ (1SD). Application of these calibrations to compositionally zoned secondary standards yields grain-averaged isotopic compositions that agree with independent bulk measurements within propagated 2SE uncertainties, validating the accuracy of the protocol at permille-level precision. This work enables accurate, texture-resolved coupled C-Mg-Ca isotopic microanalysis of complex terrestrial and extraterrestrial carbonates.