Pb-Sr-Nd ISOTOPIC CHARACTERIZATION OF USGS REFERENCE MATERIALS BY TIMS AND MC-ICP-MS AT CPGeo-USP

Systematic analyses of Pb, Nd, and Sr isotopic compositions of USGS Reference Materials (BCR-2 and AGV-2) were carried out at the Research Center of Isotope Geochemistry and Geochronology (CPGeo-USP). These measurements were performed on a Triton Thermo Ionization Mass Spectrometer (TIMS) and a Neptune Multi Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS). All chemical procedures were performed in a class ISO 7 clean room equipped with class ISO 5 laminar flow hoods. About 50 mg of rock powder was dissolved with HF, HNO₃, and HCl in Savillex® beakers on a hot plate at 100 ^oC. Pb, Sr, and Nd from the same sample solution were purified using the ion exchange chromatographic technique. The first stage was the separation of Pb from the other elements using an anion exchange resin AG1-X8 (Biorad®). The rinses discarded from this column were then dried and reconstituted for exchange separation of Sr, using Sr Spec resin (Eichrom®). The rinses discarded from the Sr Spec column were then processed through RE resin (Eichrom®) to separate rare earth elements. Nd was separated using Ln resin (Eichrom®). Each solution with the eluted element was dried down and loaded onto filaments for mass spectrometry measurements.Pb and Sr isotopic compositions were measured on a multi-collector Thermo Triton Mass Spectrometer at ~1250 ^oC and ~1400 ^oC, respectively. Pb samples were loaded on degassed Re filaments (99.995 % H. Cross®) with H₃PO₄ and silica gel. In every barrel, four NBS981 were analyzed to calculate the daily mass fractionation factor using the values proposed by Todt (1996). Sr isotopic analyses were carried out using degassed single Ta filament (99.959 % H. Cross®) using TaO-activator and H₃PO₄. The ⁸⁷Sr/⁸⁶Sr ratios were normalized to the value of ⁸⁶Sr/⁸⁸Sr = 0.1194 (Steiger, 1977) using an exponential law. Also, these measured ratios were corrected for Rb interference.Nd isotopic analyses were performed on a Neptune MC-ICP-MS in 3% v/v HNO₃ solution. The ¹⁴³Nd/¹⁴⁴Nd ratios were normalized to the value of ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 (Wasserburg et al., 1981), using an exponential law. As an external correction, the ¹⁴³Nd/¹⁴⁴Nd factor of the daily average of JNdi-1 determined by MC-ICP-MS and the annual average obtained by TIMS were used.

Pb, Sr, and Nd Isotope Standards data from the last 12 months are presented. Analyses of SRM 981 common Pb standard (n=33) yielded ²⁰⁶Pb/²⁰⁴Pb = 16.902 ± 0.004, ²⁰⁷Pb/²⁰⁴Pb = 15.445 ± 0.005, ²⁰⁸Pb/²⁰⁴Pb = 36.554 ± 0.017, SRM 987 Sr standard (n=31) yielded ⁸⁷Sr/⁸⁶Sr value of 0.710273 ± 0.000009, and JNdi-1 Nd standard (n=64) showed¹⁴³Nd/¹⁴⁴Nd ratios of 0.512103 ± 0.000003.

Analyses of BCR-2 (n=18) yielded ²⁰⁶Pb/²⁰⁴Pb = 18.743 ± 0.014, ²⁰⁷Pb/²⁰⁴Pb = 15.606 ± 0.010, ²⁰⁸Pb/²⁰⁴Pb = 38.675 ± 0.044, ⁸⁷Sr/⁸⁶Sr value of 0.705036 ± 0.000007 and ¹⁴³Nd/¹⁴⁴Nd ratios of 0.512631 ± 0.000003. Some authors reported the following values for this RM: ²⁰⁶Pb/²⁰⁴Pb = 18.750 ± 0.011, ²⁰⁷Pb/²⁰⁴Pb = 15.615 ± 0.003, ²⁰⁸Pb/²⁰⁴Pb = 38.691 ± 0.021 (Woodhead et al., 2000), ⁸⁷Sr/⁸⁶Sr value of 0.705019 ± 0.000016 (Weis et al., 2006) and ¹⁴³Nd/¹⁴⁴Nd ratios of 0.512637 ± 0.000017 (Weis et al., 2006). Analyses of AGV-2 (n=20) yielded ²⁰⁶Pb/²⁰⁴Pb = 18.862 ± 0.012, ²⁰⁷Pb/²⁰⁴Pb = 15.605 ± 0.014, ²⁰⁸Pb/²⁰⁴Pb = 38.505 ± 0.044, ⁸⁷Sr/⁸⁶Sr value of 0.704007 ± 0.000007 and ¹⁴³Nd/¹⁴⁴Nd ratios of 0.512784 ± 0.000005. The reported values for this RM in the literature were: ²⁰⁶Pb/²⁰⁴Pb = 18.864 ± 0.007, ²⁰⁷Pb/²⁰⁴Pb = 15.609 ± 0.006, ²⁰⁸Pb/²⁰⁴Pb = 38.511 ± 0.020 (Woodhead et al., 2000), ⁸⁷Sr/⁸⁶Sr value of 0.703987 ± 0.000016 (Weis et al., 2006) and ¹⁴³Nd/¹⁴⁴Nd ratios of 0.512786 ± 0.000017 (Weis et al., 2006).The Pb, Nd, and Sr isotopic ratios obtained for BCR-2 and AGV-2 at CPGeo agree with the literature values.