Vanadium isotope fractionation during mantle melting: Evidence from Cenozoic alkali basalts in eastern China

It is well known that vanadium (V) is redox-sensitive during magmatism, but whether V isotopes can be used as an oxygen fugacity (fO2) sensor of the mantle remains controversial. It is crucial to understand the behavior and controlling factors of V isotope fractionation during mantle melting before using V isotopes as a redox proxy. This study presents high-precision V isotopic data for alkali basalts with high fO2 from eastern China, which were generated by low degree partial melting of carbonated mantle. Our results show that their δ51V values (-0.85‰ to -0.61‰) are higher than those of mid-ocean ridge basalts (MORBs) and Bulk Silicate Earth (BSE). Chemical alteration, crustal contamination or fractional crystallization negligibly affect the δ51V values of alkali basalts. Although subducted carbonates are involved in the mantle source region beneath eastern China, mass balance calculations show that the incorporation of carbonates did not significantly increase the V isotopic compositions of alkali basalt. In contrast, the observed high δ51V probably reflects the control of partial melting on the V isotopic compositions of mantle-derived melts. The δ51V values of alkali basalts are positively correlated with indicators of partial melting such as the Nb/Y ratios and δ56Fe values, which indicates that V isotopes could be fractionated during mantle melting. Basaltic melts tend to be enriched in V with high valence as it is overall more incompatible than the V with low valence during partial melting, which contributes to the enrichment of 51V in alkali basalts because of the affinity of high valence V and 51V. Furthermore, the fractionation of V isotopes is more significant at a lower degree of melting and/or a more oxidizing condition. Therefore, this study validates discernable V isotope fractionation during mantle partial melting and examines the potential of using V isotopes to trace the oxidation state of magmatic systems.

众所周知，钒（V）在岩浆作用过程中具有氧化还原敏感性，然而，钒同位素是否可用作地幔氧分压（fO2）传感器的议题仍存在争议。在将钒同位素作为氧化还原指标之前，理解钒同位素在地幔熔融过程中的行为及其控制因素至关重要。本研究提供了来自中国东部高fO2碱性玄武岩的高精度钒同位素数据，这些数据是通过碳酸盐地幔的低程度部分熔融产生的。我们的结果表明，它们的δ51V值（-0.85‰至-0.61‰）高于中洋脊玄武岩（MORBs）和全硅酸盐地球（BSE）。化学蚀变、地壳污染或分数结晶对碱性玄武岩的δ51V值的影响微乎其微。尽管在东部中国地幔源区下方存在俯冲碳酸盐，但质量平衡计算表明，碳酸盐的加入并未显著增加碱性玄武岩的钒同位素组成。相反，观察到的较高δ51V值可能反映了部分熔融对地幔来源熔融的钒同位素组成的影响。碱性玄武岩的δ51V值与部分熔融指标如Nb/Y比率和δ56Fe值呈正相关，这表明钒同位素在地幔熔融过程中可能发生分馏。在部分熔融过程中，高价态的钒相对于低价态的钒来说更不相容，因此，由于高价态钒与51V的亲和力，这导致了碱性玄武岩中51V的富集。此外，钒同位素的分馏在较低程度的熔融和/或更氧化的条件下更为显著。因此，本研究验证了地幔部分熔融过程中的可辨钒同位素分馏，并探讨了利用钒同位素追踪岩浆系统氧化态的潜力。