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

Vanadium isotope compositions (δ(⁵¹V)) in marine carbonates are a potential proxy to trace global redox states of ancient oceans. Although high-precision δ(⁵¹V) analyses are available for many geological materials, carbonate-hosted δ(⁵¹V) data have not been reported yet due to extremely high matrix elements and low V contents (generally below 10 μg 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 δ(⁵¹V) 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 δ(⁵¹V) standard, USTC-V. The mean δ(⁵¹V) 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 δ(⁵¹V) 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 δ(⁵¹V) 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 δ(⁵¹V) analyses is better than ±0.14‰ (2SD). Our method can be applied to measure carbonate-hosted δ(⁵¹V) to trace the evolution in global marine redox states throughout the Earth's history.