Accurate and Efficient SIMS Oxygen Isotope Analysis of Composition-Variable Minerals: Online Matrix Effect Calibration for Dolomite

High-quality oxygen isotope analysis of composition-variable minerals (e.g., ubiquitous carbonates) using secondary ion mass spectrometry (SIMS) is extremely challenging. The classical off-line procedure, which requires additional electron probe microanalyzer (EPMA) chemical compositions for calibrating instrumental mass fractionation (IMF), is inherently inaccurate and analytically inefficient. In this study, the first accurate and paired SIMS analysis of delta O-18 and Fe# [molar Fe/(Mg + Fe)] in dolomite is reported. Based on five newly developed dolomite Oisotopic standards with an Fe# range of 0.01-0.35 obtained by SIMS, a novel accurate and rapid online matrix effect calibration method for dolomite O-isotope analysis was developed using concurrent SIMS O-18-O-16-(FeO)-Fe-56-O-16-(MgO)-Mg-24-O-16 measurements without additional chemical electron probe microanalysis. A logistic equation was proposed as the best-fit curve to represent the delta O-18 matrix effect based on the (FeO)-Fe-56-O-16/(MgO)-Mg-24-O-16 ratios. For CTD-4 carbonatitic dolomite with variable Fe# but homogeneous oxygen isotopes, the off-line method exhibited highly variable apparent delta O-18 values in the range of 5.74-10.11 parts per thousand. The online method yielded a homogeneous delta O-18 value of 7.94 +/- 0.34 parts per thousand(2SD, n = 40), which is comparable with that of bulk analysis (7.94 +/- 0.20 parts per thousand; 2SD). Comprehensive analyses validated the online method as the best strategy for performing accurate delta O-18 analysis of samples with highly heterogeneous compositions. Based on its accuracy, simplicity, and economic feasibility, this method has potential applications in the analysis of composition-complex dolomites, detrital dolomites, and other precious terrestrial and extraterrestrial materials.

Automated summary

This study introduces a novel and rapid online matrix effect calibration method for dolomite oxygen isotope analysis, avoiding the inaccuracies and analytical inefficiency associated with traditional off-line procedures, making it the best strategy for accurate oxygen isotope analysis of samples with highly heterogeneous compositions.