SIMS Bias on Isotope Ratios in Ca‐Mg‐Fe Carbonates (Part III): δ18O and δ13C Matrix Effects Along the Magnesite–Siderite Solid‐Solution Series

This study explores the effects of cation composition on mass bias (i.e., the matrix effect), which is a major component of instrumental mass fractionation (IMF) in the microanalyses of delta C-13 and delta O-18 by SIMS in carbonates of the magnesite-siderite solid-solution series (MgCO3-FeCO3). A suite of twelve calibration reference materials (RMs) was developed and documented (calibrated range: Fe# = 0.002-0.997, where Fe# = molar Fe/[Mg+Fe]), along with empirical expressions for regressing calibration data (affording residuals < 0.5 parts per thousand relative to certified reference material NIST-19). The calibration curves of both isotope systems are non-linear and have, over a 2-year period, fallen into one of two distinct but largely self-consistent shape categories (data from ten measurement sessions), despite adherence to well-established analytical protocols for carbonate delta C-13 and delta O-18 analyses at WiscSIMS (CAMECA IMS 1280). Mass bias was consistently most sensitive to changes in composition near the magnesite end-member (Fe# 0-0.2), deviating by up to 4.5 parts per thousand (delta C-13) and 14 parts per thousand (delta O-18) with increasing Fe content. The cause of variability in calibration curve shapes is not well understood at present and demonstrates the importance of having available a sufficient number of well-characterised RMs so that potential complexities of curvature can be adequately delineated and accounted for on a session-by-session basis.