Two-point normalization for reducing inter-laboratory discrepancies in δ17O, δ18O, and Δ′17O of reference silicates

The δ17O and δ18O values of a number of terrestrial minerals and rocks have been determined using laser fluorination method worldwide. For the comprehensive and congruous interpretation of oxygen isotope data, the δ-values should be normalized by the two-point method (i.e., the VSMOW-SLAP scale) to eliminate inter-laboratory bias. In this study, the δ17O and δ18O values of VSMOW and SLAP were measured to calibrate our laboratory working standard O2 gas. The O2 gas liberated from the water samples was purified using the preparation line normally employed for solid samples, and analyzed by the same mass spectrometer. From the analyses of VSMOW and SLAP, the oxygen isotope compositions of the international silicate standards (UWG2 garnet, NBS28 quartz, and San Carlos olivine) were normalized to the VSMOW-SLAP scale (two-point calibration), and then the Δ′17O values were determined. Using the δ-values obtained in this way, the inter-laboratory discrepancy of the δ17O and δ18O results of the silicate standards could be reduced. The VSMOW-SLAP scaling for δ17O and δ18O analysis of silicates provides the most effective way to obtain accurate and precise data. In reporting the Δ′17O values, it is important to make the choice of the reference fractionation line into account because the Δ′17O value is quite variable owing to the slope and y-intercept of the linear relation of the δ-values. The reference fractionation line obtained from the measurement of the low- and high-δ18O reference silicates would help to compare ∆′17O values. We confirmed that the ∆′17O results of the international silicate standards based on the two-point silicate reference line were consistent with the results from other laboratories.

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

<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’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 δ<sup>18</sup>O phosphate SIMS measurements – Durango apatite – 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 δ<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 “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 δ<sup>18</sup>O<sub>SMOW</sub> 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 δ<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 δ<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 “ORIGINS” 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>

Accuracy and reproducibility of the triple oxygen isotope measurement of silicate micro-samples by laser-fluorination-IRMS

<p>Recent studies showed that the <sup>17</sup>O-excess of plant leaf biogenic silicates (phytoliths) can be used to quantify the atmospheric relative humidity occurring during leaf water transpiration. The <sup>17</sup>O-excess vs ∂<sup>18</sup>O signature of phytoliths can also be used to trace back to the signature of leaf water. In a similar way, the signature of lacustrine diatoms is expected to record the signature of the lake water in which they formed. Therefore, the triple oxygen isotope composition of biogenic silicates extracted from well-dated sedimentary cores may bring new insights for past climate and hydrological reconstructions. However, for high time-resolution reconstructions, we need to be able to measure microsamples (300 to 800 µg) of biogenic silica. In another context, the triple oxygen isotope composition of micro-meteorites constitutes an efficient tool to determine their parent-body. In this case too, micro-samples need to be handled.</p><p>Here we report the results of new ∂<sup>18</sup>O and ∂<sup>17</sup>O measurements of macro- and micro-samples of international and laboratory silicate standards (e.g. NBS28 quartz, San Carlos Olivine, Boulangé quartz, MSG phytoliths and PS diatoms). Molecular O<sub>2</sub> is extracted from silica and purified in a laser-fluorination line, passed through a 114°C slush to condense potential interfering gasses and sent to the dual-inlet Isotope Ratio Mass Spectrometer (IRMS) Thermo-Scientific Delta V. In order to get sufficient 34/32 and 33/32 signals for microsamples the O<sub>2</sub> gas is concentrated within the IRMS in an additional auto-cooled 800 ml microvolume tube filled with silica gel. Accuracy and reproducibility of the ∂<sup>18</sup>O, ∂<sup>17</sup>O and <sup>17</sup>O excess measurements are assessed. Attention is payed to determine the concentration from which O<sub>2</sub> gas yields offsets in ∂<sup>18</sup>O, ∂<sup>17</sup>O and <sup>17</sup>O-excess are measured and whether these offsets are reproducible and can be corrected for.</p>

Fingerprinting Paranesti Rubies through Oxygen Isotopes

In this study, the oxygen isotope (δ18O) composition of pink to red gem-quality rubies from Paranesti, Greece was investigated using in-situ secondary ionization mass spectrometry (SIMS) and laser-fluorination techniques. Paranesti rubies have a narrow range of δ18O values between ~0 and +1‰ and represent one of only a few cases worldwide where δ18O signatures can be used to distinguish them from other localities. SIMS analyses from this study and previous work by the authors suggests that the rubies formed under metamorphic/metasomatic conditions involving deeply penetrating meteoric waters along major crustal structures associated with the Nestos Shear Zone. SIMS analyses also revealed slight variations in δ18O composition for two outcrops located just ~500 m apart: PAR-1 with a mean value of 1.0‰ ± 0.42‰ and PAR-5 with a mean value of 0.14‰ ± 0.24‰. This work adds to the growing use of in-situ methods to determine the origin of gem-quality corundum and re-confirms its usefulness in geographic “fingerprinting”.