Volcanic clasts from the International Ocean Discovery Program Site U1431 in the South China Sea (SCS) were analyzed for V isotopic compositions in this study. These volcanic clasts represent mantle-derived carbonated melts and record the transition from carbonated melts to alkali basalts. Based on the formation sequence, these clast samples were divided into early-stage (> 8.3 Ma) and late-stage (< 8.3 Ma) clasts. The early-stage volcanic clasts have δV values of −0.76‰ to −0.67‰, which provide an estimate for the V isotopic composition of primary carbonated melts. Their V isotopic compositions are slightly heavier than those of mid-ocean ridge basalts (−0.84‰ ± 0.10‰) and bulk silicate Earth (−0.86‰ to −0.91‰), reflecting the control of low-degree partial melting under conditions of high oxygen fugacity. The late-stage volcanic clasts have δV values ranging from −0.62‰ to 0.29‰, which are systematically heavier than those of early-stage volcanic clasts. The carbonated melts rose through and reacted with the lithospheric mantle and were transformed into alkali basalts by dissolving orthopyroxene and precipitating olivine and/or clinopyroxene. Because the reactants and reaction products do not lead to significant V isotope fractionation, the V isotopes show limited variations during transformation from carbonated melts to alkali basalts. The alkali basalts were emplaced into shallow crust by multiple pulses, in which the magma experienced variable degrees of magma differentiation, with δV values close to those of the initial alkali basalts or increasing due to fractional crystallization of FeTi oxides. The change in V isotopic compositions from early-stage volcanic clasts to late-stage volcanic clasts was driven by the varying magnitude of influence exerted by different magmatic processes en route from the deep mantle to the shallow crust.

中文翻译：

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碳酸盐熔体向碱性玄武岩转变的钒同位素记录


本研究对南海 (SCS) 国际海洋发现计划站点 U1431 的火山碎屑进行了 V 同位素组成分析。这些火山碎屑代表了地幔来源的碳化熔体，并记录了从碳化熔体到碱性玄武岩的转变。根据形成顺序，这些碎屑样品分为早期（> 8.3 Ma）和晚期（< 8.3 Ma）碎屑。早期火山碎屑的δV值为-0.76‰至-0.67‰，这为原生碳化熔体的V同位素组成提供了估计。其V同位素组成略重于洋中脊玄武岩（-0.84‰±0.10‰）和块状硅酸盐地球（-0.86‰至-0.91‰），反映了在高温度条件下对低度部分熔融的控制。氧逸度。晚期火山碎屑的δV值范围为-0.62‰至0.29‰，系统地比早期火山碎屑重。碳化熔体上升穿过岩石圈地幔并与岩石圈地幔发生反应，并通过溶解斜方辉石和沉淀橄榄石和/或单斜辉石而转化为碱性玄武岩。由于反应物和反应产物不会导致显着的 V 同位素分馏，因此在从碳酸盐熔体转变为碱性玄武岩的过程中，V 同位素显示出有限的变化。碱性玄武岩在多次脉冲作用下侵位到浅层地壳中，岩浆经历了不同程度的岩浆分异，δV值接近初始碱性玄武岩的δV值或因FeTi氧化物的分异结晶而增加。 从早期火山碎屑到晚期火山碎屑的V同位素组成的变化是由深部地幔到浅部地壳过程中不同岩浆过程所产生的不同程度的影响所驱动的。