SIMS- and IRMS-based study of apatite reference materials reveals new analytical challenges for oxygen isotope analysis

Minerals of the apatite group, especially hydroxylapatite Ca₅(PO₄)₃OH, are valuable archives for reconstructing environmental conditions occurring throughout the Earth’s history (e.g., Joachimski et al. 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 δ¹⁸O phosphate SIMS measurements – Durango apatite – has been found to be heterogeneous (Sun et al. 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 ¹⁸O/¹⁶O homogeneity using SIMS. The major and trace element compositions were determined by electron probe microanalyses (FE-EPMA), while the contents of OH^- and CO₃^2- were assessed using thermogravimetric analysis (TG) and infrared spectroscopy (IR). The δ¹⁸O 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 Ag₃PO₄. 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 δ¹⁸O_SMOW 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 δ¹⁸O 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 δ¹⁸O 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.

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).

References

Joachimski et al. 2009. Earth and Planetary Science Letters, 284, 599-609. doi: 10.1016/j.epsl.2009.05.028

Sun et al. 2016. Chemical Geology, 440, 164-178. doi: 10.1016/j.chemgeo.2016.07.013