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 δ18O and Fe# [molar Fe/(Mg + Fe)] in dolomite is reported. Based on
five newly developed dolomite O-isotopic 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 18O–16O–56Fe16O–24Mg16O measurements without additional chemical electron probe
microanalysis. A logistic equation was proposed as the best-fit curve
to represent the δ18O matrix effect based on the 56Fe16O/24Mg16O ratios. For
CTD-4 carbonatitic dolomite with variable Fe# but homogeneous oxygen
isotopes, the off-line method exhibited highly variable apparent δ18O values in the range of 5.74–10.11‰. The online
method yielded a homogeneous δ18O value of 7.94 ±
0.34‰ (2SD, n = 40), which is comparable with
that of bulk analysis (7.94 ± 0.20‰; 2SD). Comprehensive
analyses validated the online method as the best strategy for performing
accurate δ18O 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.