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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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