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（vanadium，V）在岩浆作用过程中具有氧化还原敏感性，但钒同位素（vanadium isotope）能否作为地幔氧逸度（oxygen fugacity，fO2）的指示剂仍存在争议。在将钒同位素用作氧化还原代用指标（redox proxy）之前，明确地幔熔融（partial melting）过程中钒同位素分馏的行为及其控制因素至关重要。本研究针对中国东部高氧逸度的碱性玄武岩，提供了高精度的钒同位素数据，该类玄武岩由碳酸盐交代地幔发生低度部分熔融形成。我们的结果显示，其δ51V值介于-0.85‰至-0.61‰之间，高于洋中脊玄武岩（mid-ocean ridge basalts，MORBs）与全硅酸盐地球（Bulk Silicate Earth，BSE）的δ51V值。化学蚀变、地壳混染（crustal contamination）或分离结晶作用（fractional crystallization）对该类碱性玄武岩的δ51V值影响可忽略不计。尽管俯冲碳酸盐岩（subducted carbonates）参与了中国东部下方的地幔源区，但质量平衡计算（mass balance calculations）表明，碳酸盐的加入并未显著抬升碱性玄武岩的钒同位素组成。与之相反，观测到的高δ51V值可能反映了部分熔融作用对幔源熔体（mantle-derived melts）钒同位素组成的调控作用。碱性玄武岩的δ51V值与部分熔融指标（如Nb/Y比值与δ56Fe值）呈正相关，这表明钒同位素在幔部熔融过程中发生了分馏。部分熔融过程中，高价态钒整体较低价态钒更不相容（incompatible），因此玄武质熔体更易富集高价态钒；加之高价态钒与51V具有较强的亲和性，最终促使碱性玄武岩中51V发生富集。此外，在更低的部分熔融程度和/或更强的氧化条件下，钒同位素分馏效应更为显著。综上，本研究证实了地幔部分熔融过程中存在可被识别的钒同位素分馏现象，并探讨了利用钒同位素示踪岩浆体系（magmatic systems）氧化状态的应用潜力。