Abstract:
New measurements of partition of vanadium (V) between spinel, garnet and a pigeonite-like high pressure (P) pyroxene and magnesian liquids on the mantle solidus as a function of oxygen fugacity (fO2) are presented. The spinel-liquid experiments show an effect of Cr/Al in spinel on partitioning, and further suggest that V exists as V3+, V4+ and V5+ in melts at terrestrial fO2. Vanadium is mildly incompatible in both ‘pigeonitic’ pyroxene and garnet between 4.5 and 6.5 GPa on the mantle solidus. Analysis of the fO2-sensitive partitioning for V between mantle minerals and melts are combined with compositional data from peridotite melting experiments to model the covariation of V and Al in peridotite residues produced by non-modal, fractional melting under different fO2 from 1.0 to 7.0 GPa. The partial melting models at 1.0 to 3.0 GPa fit the covariation of V and Al in abyssal peridotites quite well at fO2s similar to those of mid-ocean ridge basalt. Many orogenic massifs and spinel lherzolite xenoliths represent mantle that formed at fO2 higher than that produced at mid-ocean ridges in a range of tectonic environments. A large proportion of spinel-facies Archean cratonic lithosphere formed at fO2s significantly higher than those of abyssal peridotites possibly linking its formation to a convergent margin (arc) tectonic setting. The case for garnet-facies cratonic mantle is equivocal; it may have formed at significantly higher pressures (7.0 GPa), or within the spinel-facies at lower pressures but at significantly higher fO2 than is observed for abyssal peridotites. The imbrication of both oceanic garnet-facies mantle with spinel-facies arc mantle may explain the datasets for some Archean cratons. Overall, the data for Archean mantle melts and residues make clear that models cannot look to reduced, mantle-derived volcanic gases containing H2 and CO to engender early life synthesis, or to promote hydrogen escape and gradual oxygenation of the Archean earth system.
