Inter‐laboratory Characterisation of Apatite Reference Materials for Oxygen Isotope Analysis and Associated Methodological Considerations

Abstract

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.

New Insights on Micro‐Scale Variations of Geochemical and Oxygen Isotope Compositions in Conodont and Shark Tooth Bioapatite

“…An influence of the different c-axis orientations for minerals of the apatite group was noted for fluorine mass fractions measured by EPMA (e.g., Goldoff et al 2012). However, no differences were reported for oxygen isotope analysis of crystallographically different orientations of analyses (e.g., Li et al 2021, Wudarska et al 2022. Assuming a similar microstructure as other lamniform specimens previously imaged (cf., Enax et al 2014, Wilmers et al 2021, the fluorine content was compared in shark teeth "parallel-bundled" enameloid tissue (PBE, Figure 2, Enault et al 2015).…”

“…Durango apatite is a distinctive fluorapatite from an open-pit iron ore mine at Cerro de Mercado, Durango City, Mexico (Chew et al 2016). Besides having a high inter-crystal oxygen isotope variation (up to 4.4‰, Sun et al 2016), the Durango apatite is compositionally different from conodont bioapatite and may have specific sources of isotopic bias when sputtered by an ion beam during in situ analyses (e.g., Trotter et al 2015, 2016a, Sun et al 2016, Wudarska et al 2022. To evaluate such matrix related effects (Eiler et al 1997), previous studies employed a second fluorapatite reference material (RM), for example a great white shark tooth, to be analysed in parallel with the Durango RM (Trotter et al 2008, Rigo et al 2012.…”

Apatite Reference Materials for SIMS Microanalysis of Isotopes and Trace Elements

“…2021, Wudarska et al . 2022). Even though this reduces the applicability of the apatite samples we have characterised, it still provides a set of apatite RMs with compositions that are applicable to a large fraction of igneous, metamorphic and sedimentary apatite.…”

“…We have chosen not to study biogenic apatite, or apatite with high Cl, CO 3 or OH contents as these substitutions onto specific structural sites in apatite may be associated with bias of SIMS data (Sun et al 2016, Li et al 2021, Wudarska et al 2022. Even though this reduces the applicability of the apatite samples we have characterised, it still provides a set of apatite RMs with compositions that are applicable to a large fraction of igneous, metamorphic and sedimentary apatite.…”

Barite, Anhydrite and Gypsum Reference Materials for In Situ Oxygen and Sulfur Isotope Ratio Measurements

“…Secondary Ion Mass Spectrometry (SIMS) can provide targeted in situ isotope ratio determinations of both oxygen and sulfur isotope ratios at a 5-20 lm scale, with test portion masses reaching down to the mid-picogram range (Ramsey and Wiedenbeck 2018) and repeatability approaching or even surpassing AE0.1‰ (1s; Yang et al 2018, Li et al 2019, Wudarska et al 2021, 2022. A further advantage of SIMS is its high sample throughput rate, where hundreds of isotope ratio results can be generated in a single, day-long measurement session.…”