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 fluid-mobile 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 fO(2)s of arc basalts, MORB and peridotites are remarkably similar (-1.25 to +0.5 log units from the FMQ buffer) and disagree with the observation that the barometric fO(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.