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

• Main Authors: Novella, D, Maclennan, J, Shorttle, O, Prytulak, J, Murton, BJ
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier BV 2020
• Subjects: mantle; oxygen fugacity; MORB; Fe3+/Fetot; vanadium; isotopes; Reykjanes; Iceland
• Online Access: https://www.repository.cam.ac.uk/handle/1810/299149; https://doi.org/10.17863/CAM.46213

Mantle oxygen fugacity (fO2) 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 fO2 proxies: Fe3+/Fetot, 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 fO2 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+/Fetot. 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 δ51VAA composition of basaltic glasses along the Reykjanes Ridge covers only a narrow range (δ51VAA= -1.09 to -0.86‰; 1SD= 0.02-0.09) and does not co-vary with fractionation-corrected Fe3+/Fetot (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 δ51VAA 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+/Fetot is constant at 5% and that account for the increased mantle potential temperature nearer the plume center and the fO2 dependent partitioning of V, can reproduce the V/Sc data. However, while these melting models predict that basalt Fe3+/Fetot ratios should decrease with increasing mantle potential temperature towards Iceland, fractionation-corrected Fe3+/Fetot 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 ...