Development of CCMb : A Low‐Mg Calcite Reference Material for In Situ Measurement of δ 18 O , δ 13 C and Selected Trace Elements

A calcite reference material (CCMb) derived from Carrara marble was developed for calibrating in situ mass fraction measurements of selected minor and trace elements (Mg, Sr) and the isotope ratios of carbon (δ 13 C) and oxygen (δ 18 O). Chemical and isotopic properties were characterised by bulk solution and in situ techniques, with ten laboratories participating in an analytical round‐robin. Grains of CCMb were analysed by ion microprobe (SIMS) against a suite of calcite reference materials, including international reference material IAEA‐603, to evaluate the influence of minor amounts of Mg on the instrumental mass fractionation of δ 18 O values. CCMb consists of an assemblage of tightly interlocking calcite crystallites measuring 100–300 μm across. Fluid‐inclusion trails are abundant along crystallite grain boundaries. An offset was observed between δ 18 O values of CCMb determined by gas‐source isotope ratio mass spectrometry (GS‐IRMS) and SIMS measurement. The results of this study indicate the offset could be related to the presence of isotopically light calcite material in grain boundary regions and micro‐fissures, domains only several micrometres thick. When used for calibrating SIMS measurements, CCMb should be assigned a reference δ 18 O value of +28.81 ± 0.15‰ (VSMOW, 2 s, based on SIMS results), representative of the integrated crystallite volume less the contribution of the inferred low δ 18 O phase occupying intercrystalline space. Measurements of δ 13 C by ion microprobe reproduced the bulk‐grain, GS‐IRMS derived value of +1.95 ± 0.06‰ (VPDB, 2 s ). This study did not find evidence for matrix effects related to Mg‐substitution at mass fractions ≤ 3000 μg g ‐1 . The preferred reference values for the Mg and Sr mass fractions in CCMb are 0.36 ± 0.05% m/m (2 s ) and 156 ± 8 μg g ‐1 (2 s ), respectively.