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.

Automated summary

A method was developed to measure the delta(V-51) values in marine carbonates, and data for carbonate reference materials were successfully obtained, indicating the high reliability of the method for tracing the evolution of global marine redox states throughout Earth's history.