<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&#8217;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 &#948;<sup>18</sup>O phosphate SIMS measurements &#8211; Durango apatite &#8211; 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 &#948;<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 &#8220;bulk&#8221; 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&#8240; (1 standard deviation, 1s) for the different RMs, confirming good homogeneity at the nanogram scale. The IRMS-determined &#948;<sup>18</sup>O<sub>SMOW</sub> values, which fall between ~5 and ~22&#8240; 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&#8240;. The &#948;<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 &#948;<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 &#8220;ORIGINS&#8221; 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>
Beryl Reference Materials for In Situ Oxygen Isotope Determination
