Vanadium isotope composition of the Bulk Silicate Earth: Constraints from peridotites and komatiites

Abstract

In order to apply the vanadium (V) stable isotope system for studies of planetary accretion and evolution in the solar system and redox variations in terrestrial magmatic processes, the V isotope composition of the Bulk Silicate Earth (BSE) needs to be precisely constrained. Previous studies have shown that fertile peridotites have systematically higher 51V/50V ratios than MORB. This, however, is in conflict with the theoretical prediction that mantle melting residues should be enriched in 50V rather than 51V. To address these issues, a more precise estimate of the V isotope composition of the BSE is required. This study presents δ51V data for eleven peridotite xenoliths from two late Cenozoic eruption centers at Tariat in central Mongolia, ten komatiites from five localities ranging in age between 3.48 and 2.41 Ga, and four 1.98 Ga picrites from the Onega Plateau in Fennoscandia. The mean δ51V for fertile spinel lherzolites is −0.91 ± 0.06‰ (2SD, n = 8). They show no resolvable difference in V isotope compositions compared to three moderately to highly refractory peridotite xenoliths analyzed, with a mean δ51V of −0.93 ± 0.01‰ (2SD, n = 3). The mean δ51V for the komatiites is −0.91 ± 0.05‰ (2SD, n = 10), which is identical to that for the fertile peridotites. Based on the V isotope compositions of the peridotites and komatiites analyzed in this study, the mean δ51V of the BSE is estimated to be −0.91 ± 0.09‰ (2SD, n = 18). In contrast, the mean δ51V for the Onega Plateau picrites and related cumulates is −0.80 ± 0.05‰ (2SD, n = 4), which is identical to a recent δ51V estimate for MORB. The mean δ51V for picrites and MORB are somewhat higher than the new BSE estimate, indicating that low-degree (<10%) partial melts have higher δ51V than their mantle sources. The new estimate of δ51V for the BSE overlaps with current estimates for δ51V in the silicate Moon and Mars, which may imply that these bodies have a common V isotope composition.