Vanadium isotope measurement by MC-ICP-MS

We present a method to measure vanadium(V) isotopic composition for terrestrial rocks in this study. Vanadium was efficiently separated from matrix elements by a chromatographic technique using cation- and anion-exchange resin columns, avoiding the expensive TRU Spec resin. Vanadium isotope ratios were measured using a Thermo Scientific Neptune Plus MC-ICP-MS employing a sample-standard bracketing method. The increase in instrument sensitivity significantly reduces the amount of V required for the isotope analysis. Potential effects of acid molarities and concentration mismatch on instrumental analyses were rigorously evaluated. In addition, we performed Cr- and Ti-doping experiments to ensure the precision and accuracy of V isotope measurement. The delta V-51 values of mono-elemental V standards (BDH and USTC-V) relative to an Alfa Aesar (AA) standard solution (defined as delta V-51 = [(V-51/V-50)(sample) / (V-51/V-50)(AA) -1] x 1000) measured in our laboratory were -1.23 +/- 0.08 parts per thousand(2 SD, n = 197) and 0.07 +/- 0.07 parts per thousand (2 SD, n = 112), respectively. Analyses of synthetic standard solutions (element doping + matrix spiking) obtained the same delta V-51 for the pure V solutions with a precision better than +/- 0.1 parts per thousand (2 SD). Vanadium isotopic compositions of 12 reference materials, including igneous rocks (with mafic to felsic compositions) and manganese nodules, were measured using this method. These reference materials including basalts: BCR-2,-0.78 +/- 0.08 parts per thousand (2 SD, n = 36); BHVO-2,-0.83 +/- 0.09 parts per thousand (2 SD, n = 22); BIR1, -0.92 +/- 0.09 parts per thousand (2 SD, n = 52); JB-2, -0.87 +/- 0.06 parts per thousand (2 SD, n = 20); diabase: W-2, -0.94 +/- 0.08 parts per thousand (2 SD, n = 15); andesites: AGV-1, -0.71 +/- 0.10 parts per thousand (2 SD, n = 6); AGV-2, -0.70 +/- 0.10 parts per thousand (2 SD, n = 37); JA-2, -0.80 +/- 0.07 parts per thousand (2 SD, n = 15); quartz latite: QLO-1, -0.61 +/- 0.03 parts per thousand (2 SD, n = 3); granodiorite: GSP-2, -0.62 +/- 0.07 parts per thousand (2 SD, n = 26); and manganese nodules: NOD-P, -1.65 +/- 0.06 parts per thousand (2 SD, n = 10); NOD-A, -0.99 +/- 0.10 parts per thousand (2 SD, n = 19). Based on repeated analyses of the rock standards, the long-term external precision of ourmethod is better than +/- 0.1 parts per thousand (2 SD) for delta V-51. Such precision allows us to identify V isotope fractionation in high-temperature terrestrial samples, suggesting that V isotope geochemistry can be more widely used to study magmatism as well as supergene processes. (C) 2015 Elsevier B.V. All rights reserved.