A multi-proxy investigation of mantle oxygen fugacity along the Reykjanes Ridge

Mantle oxygen fugacity (fO(2)) governs the physico-chemical evolution of the Earth, however current estimates from commonly used basalt redox proxies are often in disagreement. In this study we compare three different potential basalt fO(2) proxies: Fe3+/Fe-tot, V/Sc and V isotopes, determined on the same submarine lavas from a 700 km section of the Reykjanes Ridge, near Iceland. These samples provide a valuable test of the sensitivities of fO(2) proxies to basalt petrogenesis, as they formed at different melting conditions and from a mantle that towards Iceland exhibits increasing long-term enrichment of incompatible elements. New trace element data were determined for 63 basalts with known Fe3+/Fe-tot. A subset of 19 lavas, covering the geographical spread of the ridge transect, was selected for vanadium Isotope analyses. Vanadium is a multi-valence element whose isotopic fractionation is theoretically susceptible to redox conditions. Yet, the delta V-51(AA) composition of basaltic glasses along the Reykjanes Ridge covers only a narrow range (delta V-51(AA) = -1.09 to -0.86 parts per thousand; 1SD = 0.02-0.09) and does not co-vary with fractionation-corrected Fe3+/Fe-tot (0.134-0.151; 1SD = 0.005) or V/Sc (6.6-8.5; 1SD = 0.1-1.3) ratios. However, on a global scale, basaltic delta V-51(AA) may be controlled by the extent of melting. The V/Sc compositions of primitive (MgO > 7.5 wt%) basalts show no systematic change along the entire length of the Reykjanes Ridge. Typical peridotite melting models in which source Fe3+/Fe-tot is constant at 5% and that account for the increased mantle potential temperature nearer the plume center and the fO(2) dependent partitioning of V, can reproduce the V/Sc data. However, while these melting models predict that basalt Fe3+/Fe-tot ratios should decrease with increasing mantle potential temperature towards Iceland, fractionation-corrected Fe3+/Fe-tot of Reykjanes Ridge lavas remain nearly constant over the ridge length. This discrepancy is explained by source heterogeneity, where an oxidized mantle pyroxenite component contributes to melting with increasing proximity to Iceland. Comparison of observed and modeled Fe3+/Fe-tot indicate that source variation in fO(2) is present under the Reykjanes Ridge, with higher Fe3+/Fe-tot closer to Iceland. This source variability in fO(2) cannot be resolved by V isotopes and redox-sensitive trace element ratios, which instead appear to record magmatic processes. (C) 2019 Elsevier B.V. All rights reserved.