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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.

Here we report on the oxygen isotope compositions of four proposed apatite reference materials (chlorapatite MGMH#133648 and fluorapatite specimens MGMH#128441A, MZ-TH and ES-MM). The samples were initially screened for 18 O/ 16 O homogeneity using secondary ion mass spectrometry (SIMS) followed by δ 18 O determinations in six gas source isotope ratio mass spectrometry laboratories (GS-IRMS) using a variety of analytical protocols for determining either phosphate-bonded or "bulk" oxygen compositions. We also report preliminary δ 17 O and Δ' 17 O data, major and trace element compositions collected using EPMA, as well as CO 3 2and OHcontents in the apatite structure assessed using thermogravimetric analysis and infrared spectroscopy. The repeatability of our SIMS measurements was better than AE 0.25 ‰ (1s) for all four materials that cover a wide range of 10 3 δ 18 O values between +5.8 and +21.7. The GS-IRMS results show, however, a significant offset of 10 3 δ 18 O values between the "phosphate" and "bulk" analyses that could not be correlated with chemical characteristics of the studied samples. Therefore, we provide two sets of working values specific to these two classes of analytical methodologies as well as current working values for SIMS data calibration.

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<p>Minerals of the apatite group, especially hydroxylapatite Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>OH, are valuable archives for reconstructing environmental conditions occurring throughout the Earth’s history (e.g., Joachimski <em>et al.</em> 2009). Apatite oxygen isotope compositions have proved useful in studies of conodonts as well as fish and mammalian teeth and bones. Secondary ion mass spectrometry (SIMS) is a rapid and precise technique that enables the investigation of small and heterogeneous samples. However, this method is constrained by the availability of matrix-matched reference materials (RMs). The most commonly used RM for calibrating δ<sup>18</sup>O phosphate SIMS measurements – Durango apatite – has been found to be heterogeneous (Sun <em>et al.</em> 2016); therefore, we have undertaken this study, in which we have characterized a new suite of RMs for oxygen isotope analyses of apatite. Four potential apatite RMs obtained from various sources were assessed for <sup>18</sup>O/<sup>16</sup>O homogeneity using SIMS. The major and trace element compositions were determined by electron probe microanalyses (FE-EPMA), while the contents of OH<sup>-</sup> and CO<sub>3</sub><sup>2-</sup> were assessed using thermogravimetric analysis (TG) and infrared spectroscopy (IR). The δ<sup>18</sup>O reference values have now been determined in six independent laboratories using isotope ratio mass spectrometry (IRMS) and applying different analytical protocols, which fall into two groups: laser fluorination and high-temperature reduction of Ag<sub>3</sub>PO<sub>4</sub>. The first method provides the information on “bulk” oxygen compositions, while the second determines the composition of phosphate-bound oxygen. The repeatability of SIMS measurements on random crystal fragments was better than 0.25‰ (1 standard deviation, 1s) for the different RMs, confirming good homogeneity at the nanogram scale. The IRMS-determined δ<sup>18</sup>O<sub>SMOW</sub> values, which fall between ~5 and ~22‰ for the different samples, cover almost the full range of compositions found in igneous, metamorphic and biogenic apatite samples. However, the IRMS data collected using different techniques show offsets of ~1-2‰. The δ<sup>18</sup>O values obtained using laser fluorination are, in most cases, lower than those acquired by high-temperature reduction. Furthermore, the data collected within each group of IRMS methods reveal differences between laboratories, which do not correlate with the chemical composition of the apatite crystals. This suggests a more complex behavior of apatite during sample processing for conventional δ<sup>18</sup>O analyses as compared to other minerals such as tourmaline, and highlights the importance of the characterization of RMs with the support of multiple laboratories applying different protocols.</p><p>This research was partially funded by the Polish NCN grant no. 2013/11/B/ST10/04753 and the IGS PAS grant for the early career researchers as well as supported by the COST Action TD 1308 “ORIGINS” and the German Academic Exchange Service (DAAD).</p><p>References</p><p>Joachimski <em>et al.</em> 2009. Earth and Planetary Science Letters, 284, 599-609. doi: 10.1016/j.epsl.2009.05.028</p><p>Sun <em>et al.</em> 2016. Chemical Geology, 440, 164-178. doi: 10.1016/j.chemgeo.2016.07.013</p>