Vanadium Isotope Compositions of Mid-Ocean Ridge Lavas and Altered Oceanic Crust

Vanadium isotope compositions of igneous rocks have the potential to constrain variations of physicochemical conditions such as oxidation states during magmatism. Here, we present V isotope data for 27 fresh lavas (ranging from basaltic to dacitic compositions) from mid-ocean ridges, 31 altered basalts and gabbros from IODP site 1256 near the East Pacific Rise (EPR), and 2 back arc basin basalts (BABB). Our analyses of fresh mid-ocean ridge basalt (MORB) provide new constraints on the V isotope composition of MORBs, i.e. delta V-51 = -0.84 +/- 0.02 parts per thousand (2SE, n = 22). In addition, the mean delta V-51 of MORBs from individual segments is correlated with the mean ridge depth and Na-8.0 of the segment, which might reflect the effect of melting extent on V isotope fractionation during mantle melting. The mafic profile of intact altered oceanic crust (AOC) from the IODP site 1256 has delta V-51 ranging from -1.01 to -0.77 parts per thousand, similar to that of fresh MORBs, suggesting that V isotope fractionation is limited during alteration of oceanic crust. These results also indicate the V isotopic homogeneity of the bulk oceanic crust with average delta V-51 of -0.85 +/- 0.02 parts per thousand (2SE, n = 53), which is unaffected by ocean water and hydrothermal fluid alteration. Our results provide a guideline for application of V isotopes into studies of low and high temperature geochemical processes. The evolved lavas (basaltic andesites, andesites, and dacites) from the East Pacific Rise (EPR) show apparent shifts towards heavy delta V-51 values with increasing degree of differentiation, which can be explained by the crystal-liquid fractionation during crystallization with an inferred isotope fractionation factor of Delta V-51(mineral-melt) = -0.15 x 10(6)/T-2. The enrichment of V-51 with increasing differentiation degree for the 9 degrees N Overlapping Spreading Center (OSC) lavas is consistent with direction of the isotope shift observed in lavas from Anatahan Island (Northern Mariana Arc) and Hekla Volcano (Iceland), but the magnitude (0.3 parts per thousand) is much smaller than that (2 parts per thousand) reported in Prytulak et al. (2017). Modeling of V isotope fractionation between mineral and melt shows that variations in redox condition are important for controlling V isotope fractionation, but insufficient to explain the dramatically different Delta V-51(mineral-melt) between 9 degrees N OSC lavas and Anatahan/Hekla suites. More studies are necessary for better understanding of mechanisms of V isotope fractionation during magmatism. (C) 2018 Elsevier B.V. All rights reserved.