Inter‐laboratory Characterisation of Apatite Reference Materials for Chlorine Isotope Analysis

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 Ca10(PO4)6(F,Cl,OH)2 solid solution series. Numerous apatite specimens, obtained from mineralogical collections, were initially screened for 37Cl/35Cl homogeneity using SIMS followed by δ37Cl 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‰ (1s) for the five samples with > 0.5 % m/m Cl and ± 0.19‰ (1s) 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 37Cl/35Cl ratios measured on three oriented apatite fragments. Furthermore, the results of GS‐IRMS analyses show small but systematic offset of δ37ClSMOC values between the three laboratories. Nonetheless, all studied samples have comparable chlorine isotope compositions, with mean 103δ37ClSMOC values between +0.09 and +0.42 and in all cases with 1s ≤ ± 0.25.

High-precision apatiteδ37Cl measurement by SIMS with a 1012Ω amplifier Faraday cup

“…40 Subsequently, the new generation of the large-radius SIMS (CAMECA IMS 1280/1280-HR) was utilised for this purpose, and two Faraday cups (FCs) in the multi-collection mode were used to collect 35 Cl and 37 Cl signals simultaneously. This greatly improves the reproducibility to 0.5‰ (2SD), 41 and even to 0.04‰ (2SD) 42 for a high-Cl (6 wt%) sample. For most samples (with < 2 wt% Cl), the analytical uncertainty can only reach ∼0.2‰ (2SD).…”

“…The internal structure and major element compositions of these apatite minerals were previously obtained by electron probe microanalysis (Table 1). 42,43 No textural or compositional zoning was observed. The green fluorapatite TUBAF#37 contains minor inclusions of pyrite, quartz, calcite, Ca–Mg silicates, Mg–Fe-aluminosilicates, Ba-/Ca-sulfates, and zircon.…”

“…In this study, we analysed four natural apatite reference samples (TUBAF#37, 42 Durango, 42 MGMH#128441A, 42 and Eppawala-AP 43 ), which have been demonstrated to be homogeneous in chlorine isotopes at the micrometer-level, and were used as standards for previous SIMS chlorine isotope measurement. 42,43…”

“…The chlorine mass fraction of TUBAF#37 varies slightly with the electron beam current (median 0.27 wt% (for 5 nA) and 0.26 wt% (for 40 nA)), and the recommended δ 37 Cl value is +0.20 ± 0.26‰ (2SD). 42 The yellow/yellowish-green apatite Durango (from the Cerro de Mercado Fe deposit, Mexico) has 0.47 wt% Cl on average. Same fragments of this Durango sample have been used for SIMS δ 37 Cl measurements, with the recommended δ 37 Cl value of +0.19 ± 0.12‰ (2SD).…”

“…For most samples (with < 2 wt% Cl), the analytical uncertainty can only reach ∼0.2‰ (2SD). 42,43 This level of analytical uncertainty can meet some isotopic systems with large fractionation ( e.g. , oxygen and sulfur) but not for chlorine.…”