Determination of Vanadium Isotope Compositions in Carbonates Using an Fe Coprecipitation Method and MC-ICP-MS

Vanadium isotope compositions (delta(V-51)) in marine carbonates are a potential proxy to trace global redox states of ancient oceans. Although high-precision delta(V-51) analyses are available for many geological materials, carbonate-hosted delta(V-51) data have not been reported yet due to extremely high matrix elements and low V contents (generally below 10 mu g g(-1)). In this study, we developed an Fe coprecipitation method combined with an Fe column to preconcentrate V from the major matrix elements and subsequent four-step chromatographic procedures to further purify V in carbonates. The delta(V-51) values were measured using a sample-standard bracketing method by MC-ICP-MS. The robustness of this method was assessed by analyzing element-doped and matrix-spiked synthetic carbonate solutions containing an in-house delta(V-51) standard, USTC-V. The mean delta(V-51) value of the synthetic carbonate solutions (0.06 +/- 0.08%; 2SD, n = 33) is in good agreement with the recommended value of the USTC-V relative to the Oxford AA solution (0.07 +/- 0.08%; 2SD, n = 347). In addition, the consistency in the delta(V-51) value of the igneous carbonatite standard, COQ-1, which was processed in parallel with the whole purification (- 0.48 +/- 0.04%; 2SD, n = 3) and a four-step chromatographic procedure (-0.43 +/- 0.08%; 2SD, n = 3), further validates the robustness of our method. For the first time, we obtained delta(V-51) values of four carbonate reference materials: JDo-1, -0.56 +/- 0.09% (2SD, n = 27); JLs-1, -0.61 +/- 0.14% (2SD, n = 33); GBW07217a, -0.79 +/- 0.09% (2SD, n = 6); GBW07214a, -0.51 +/- 0.13% (2SD, n = 48). The long-term external precision of carbonate-hosted delta(V-51) analyses is better than +/- 0.14% (2SD). Our method can be applied to measure carbonate-hosted delta(V-51) to trace the evolution in global marine redox states throughout the Earth's history.