High-precision determination of the oxidation state of komatiite lavas using vanadium liquid-mineral partitioning

Abstract

Oxygen fugacity of the mantle (fO2) is an important intensive variable in Earth sciences that is largely unconstrained throughout Earth history. Oxygen fugacity of modern basalts is determined by examining the Fe+3/Fe+2 ratios of fresh volcanic glass, but this method cannot be applied to older lavas that have experienced post-magmatic alteration and/or metamorphism.Here, we report the newly developed analytical techniques that, using the published experimental petrology data, have enabled us to determine the fO2 of mantle-derived lavas with a precision of better than 0.10 log fO2 units. This new method uses the partitioning behavior of the redox-sensitive transition metal vanadium between olivine and/or chromite and komatiitic melt as oxybarometers. In order to obtain accurate and precise results, a series of whole-rock samples was collected across differentiated komatiitic basalt Victoria's Lava Lake in Fennoscandia. Special attention was paid to ensure that the lava contained magmatic olivine and chromite that were in equilibrium with the emplaced melt composition, and that no re-equilibration has occurred during the lava differentiation. Vanadium and other transition metal abundances in the whole-rock samples were determined using standard addition solution ICP-MS technique with a precision of better than 5% (2SD), and in liquidus olivine and chromite by laser ablation ICP-MS, with a precision of better than 5% (2SE). The MgO and transition metal abundances in the emplaced komatiitic basalt lava were precisely calculated using several independent approaches; these approaches provided consistent results that agreed within the uncertainty of the method. The partition coefficients DVOl, Chr-Liq were then calculated and used to determine the fO2 of the lava lake to be −0.24±0.04 and −0.21±0.03 ΔNNO log units, respectively (2SE). The two independent estimates are identical within the respective uncertainties and attest to the accuracy of the method; the average value of −0.22±0.04 ΔNNO log units (2SD) represents our best estimate for the redox state of the lava lake. Corrected for 4% crustal contamination, the redox state of the original komatiite is calculated to be −0.29±0.04 ΔNNO log units. The new method provides a high-resolution tool for constraining the evolution of the redox state of the mantle over Earth's history.