Similar V/Sc Systematics in MORB and Arc Basalts: Implications for the Oxygen Fugacities of their Mantle Source Regions

V/Sc systematics in peridotites, mid-ocean ridge basalts and arc basalts are investigated to constrain the variation of fO 2 in the asthenospheric mantle. V/Sc ratios are used here to ' see through' those processes that can modify barometric fO 2 determinations in mantle rocks and/or magmas: early fractional crystallization, degassing, crustal assimilation and mantle metasomatism. Melting models are combined here with a literature database on peridotites, arc lavas and mid-ocean ridge basalts, along with new, more precise data on peridotites and selected arc lavas. V/Sc ratios in primitive arc lavas from the Cascades magmatic arc are correlated with fluidmobile elements (e.g. Ba and K), indicating that fluids may subtly influence fO 2 during melting. However, for the most part, the average V/Sc-inferred f O 2 s of arc basalts, MORB and peridotites are remarkably similar (125 to 05 log units from the FMQ buffer) and disagree with the observation that the barometric f O 2 s of arc lavas are several orders of magnitude higher. These observations suggest that the upper part of the Earth's mantle may be strongly buffered in terms of fO 2 . The higher barometric fO 2 s of arc lavas and some arc-related xenoliths may be due respectively to magmatic differentiation processes and to exposure to large, time-integrated fluid fluxes incurred during the long-term stability of the lithospheric mantle.