THE DETERMINATION OF REPRESENTATIVE OXYGEN ISOTOPE COMPOSITIONS FOR NEODYMIUM OXIDE TIMS MEASUREMENTS

Oxygen isotope ratios are constants used in the correction of isobaric interferences during NdO+ isotope analysis. Precise determination of the 18O/16O and 17O/16O is critical to calculating accurate Nd isotope values from NdO+ measurements. As part of establishing a new NdO+ analytical protocol at the Boston College Center for Isotope Geochemistry, several approaches were used to determine representative oxygen isotope ratios and to examine their consistency between different elements and load sizes. Two solutions, a Pr ICPMS standard and an enriched 150Nd solution, were used for oxygen isotope measurements and prepared for analysis using the NdO+ method, which includes loading on single outgassed Re filaments with a H₃PO₄+Ta₂O₅ activator solution. Measurements were made on an Isotopx Phoenix TIMS at Boston College. Sixteen total measurements of the two solutions were analyzed: five 8ng 150Nd loads, three 100ng Pr loads, three 10ng Pr loads, and five mixed 150Nd+Pr solutions. Both solutions were run at low and high signal intensities (2.5 and 8.5 V on mass 166 and 157 respectively) maintained throughout the analysis. The single element solutions were analyzed using three-step dynamic sequences to verify that collector efficiencies did not alter the measured values. The results for both solutions are comparable to values previously reported. The 150Nd solution yields 18O/16O = 0.002070±0.000003 (2SD) and 17O/16O = 0.000386±0.000004 (2SD). The Pr solutions yield similar results but with greater uncertainty, likely the result of instrument induced mass fractionation that is most apparent in the 10ng Pr loads (though notably observed in all analyses). The mixed solutions were analyzed using a two-step sequence from PrO+ to 150NdO+ masses. The mixed solutions yield similar results to the individual solutions and are internally consistent, suggesting that the oxygen values are valid for different oxides on the same filament.

Our results demonstrate that accurate oxygen isotope measurements can be made using Pr or Nd solutions. Instrument induced mass fractionation of oxygen isotopes should be considered when determining oxygen isotope compositions and conducting high precision NdO+ analysis. Additionally, oxygen isotope values remain consistent regardless of the presence of Pr during NdO+ measurements.