Inter‐laboratory Characterisation of Apatite Reference Materials for Chlorine Isotope Analysis
Alicja Wudarska, Ewa Słaby, Michael Wiedenbeck, Jaime D. Barnes, Magali Bonifacie, Neil C. Sturchio, Gérard Bardoux, Frédéric Couffignal, Johannes Glodny, Linnea J. Heraty, Timm John, Christof Kusebauch, Sathish Mayanna, Franziska Wilke, Ewa Deput
Abstract
Here we report on a set of six apatite reference materials (chlorapatites MGMH#133648, TUBAF#38 and fluorapatites MGMH#128441A, TUBAF#37, 40, 50) which we have characterised for their chlorine isotope ratios; these RMs span a range of Cl mass fractions within the apatite Ca 10 (PO 4 ) 6 (F,Cl,OH) 2 solid solution series. Numerous apatite specimens, obtained from mineralogical collections, were initially screened for 37 Cl/ 35 Cl homogeneity using SIMS followed by δ 37 Cl characterisation by gas source mass spectrometry using both dual‐inlet and continuous‐flow modes. We also report major and key trace element compositions as determined by EPMA. The repeatability of our SIMS results was better than ± 0.10‰ (1 s ) for the five samples with > 0.5 % m / m Cl and ± 0.19‰ (1 s ) for the low Cl abundance material (0.27% m / m ). We also observed a small, but significant crystal orientation effect of 0.38‰ between the mean 37 Cl/ 35 Cl ratios measured on three oriented apatite fragments. Furthermore, the results of GS‐IRMS analyses show small but systematic offset of δ 37 Cl SMOC values between the three laboratories. Nonetheless, all studied samples have comparable chlorine isotope compositions, with mean 10 3 δ 37 Cl SMOC values between +0.09 and +0.42 and in all cases with 1 s ≤ ± 0.25.