The stable vanadium isotope composition of the mantle and mafic lavas

Vanadium exists in multiple valence states under terrestrial conditions (2(+), 3(+), 4(+), 5(+)) and its isotopic composition in magmas potentially reflects the oxidation state of their mantle source. We present the first stable vanadium isotope measurements of 64 samples of well-characterized mantle-derived mafic and ultramafic rocks from diverse localities. The delta V-51 ranges from -0.27 parts per thousand to -1.29 parts per thousand, reported relative to an Alfa Aesar (AA) vanadium solution standard defined as 0 parts per thousand. This dataset is used to assess the effects of alteration, examine co-variation with other geochemical characteristics and define a value for the bulk silicate Earth (BSE). Variably serpentinised peridotites show no resolvable alteration-induced delta V-51 fractionation. Likewise, altered mafic oceanic crustal rocks have identical delta V-51 to fresh hand-picked MORB glass. Intense seafloor weathering can result in slightly (similar to 0.2-0.3 parts per thousand) heavier isotope compositions, possibly related to late-stage addition of vanadium. The robustness of delta V-51 to common alteration processes bodes well for its potential application to ancient mafic material. The average delta V-51 of mafic lavas, including MORB, Icelandic tholeiites and lavas from the Shatsky Rise large igneous province is -0.88 +/- 0.27 parts per thousand 2sd. Peridotites show a large range in primary delta V-51 (-0.62%. to -1.17 parts per thousand), which covaries positively with vanadium concentrations and indices of fertility such as Al2O3. Although these data suggest preferential extraction of heavier isotopes during partial melting, the isotope composition of basalts (delta V-51=-0.88 +/- 0.27 parts per thousand 2sd) and MORB glass in particular (delta V-51=-0.95 +/- 0.13 parts per thousand 2sd) is lighter than fertile peridotites and thus difficult to reconcile with a melt extraction scenario. Determination of fractionation factors between melt and mineral phases such as pyroxenes and garnet are necessary to fully understand the correlation. We arrive at an estimate of delta V-51(BSE)=-0.7 +/- 0.2 parts per thousand (2sd) for the bulk silicate Earth by averaging fertile, unmetasomatised peridotites. This provides a benchmark for both high and low temperature applications addressing planet formation, cosmochemical comparisons of the Earth and extraterrestrial material, and an inorganic baseline for future biogeochemical investigations. Whilst delta V-51 could relate to oxidation state and thus oxygen fugacity, further work is required to resolve the isotopic effects of oxidation state, partial melting, and mineral fractionation factors. (C) 2013 Elsevier B.V. All rights reserved.