SIMS- and IRMS-based study of apatite reference materials reveals new analytical challenges for oxygen isotope analysis
Authors: Wudarska Alicja , Wiedenbeck Michael , Słaby Ewa , Harris Chris , Joachimski Michael M. , Lécuyer Christophe , MacLeod Kenneth G. , Pack Andreas , Vennemann Torsten , Couffignal Frédéric , Glodny Johannes , Kusebauch Christof , Lempart Małgorzata , Sun Yadong , Wilke Franziska 
Published: March 2020

&lt;p&gt;Minerals of the apatite group, especially hydroxylapatite Ca&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;OH, are valuable archives for reconstructing environmental conditions occurring throughout the Earth&amp;#8217;s history (e.g., Joachimski &lt;em&gt;et al.&lt;/em&gt; 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 &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O phosphate SIMS measurements &amp;#8211; Durango apatite &amp;#8211; has been found to be heterogeneous (Sun &lt;em&gt;et al.&lt;/em&gt; 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 &lt;sup&gt;18&lt;/sup&gt;O/&lt;sup&gt;16&lt;/sup&gt;O homogeneity using SIMS. The major and trace element compositions were determined by electron probe microanalyses (FE-EPMA), while the contents of OH&lt;sup&gt;-&lt;/sup&gt; and CO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt; were assessed using thermogravimetric analysis (TG) and infrared spectroscopy (IR). The &amp;#948;&lt;sup&gt;18&lt;/sup&gt;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&lt;sub&gt;3&lt;/sub&gt;PO&lt;sub&gt;4&lt;/sub&gt;. The first method provides the information on &amp;#8220;bulk&amp;#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&amp;#8240; (1 standard deviation, 1s) for the different RMs, confirming good homogeneity at the nanogram scale. The IRMS-determined &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;SMOW&lt;/sub&gt; values, which fall between ~5 and ~22&amp;#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&amp;#8240;. The &amp;#948;&lt;sup&gt;18&lt;/sup&gt;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 &amp;#948;&lt;sup&gt;18&lt;/sup&gt;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.&lt;/p&gt;&lt;p&gt;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 &amp;#8220;ORIGINS&amp;#8221; and the German Academic Exchange Service (DAAD).&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;p&gt;Joachimski &lt;em&gt;et al.&lt;/em&gt; 2009. Earth and Planetary Science Letters, 284, 599-609. doi: 10.1016/j.epsl.2009.05.028&lt;/p&gt;&lt;p&gt;Sun &lt;em&gt;et al.&lt;/em&gt; 2016. Chemical Geology, 440, 164-178. doi: 10.1016/j.chemgeo.2016.07.013&lt;/p&gt;