High-precision determination of the oxidation state of komatiite lavas using vanadium liquid-mineral partitioning
High-precision determination of the oxidation state of komatiite lavas using vanadium liquid-mineral partitioning
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DOI：
10.1016/j.chemgeo.2016.04.011
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发表时间：
2016-09
期刊：
影响因子：
3.9
通讯作者：
R. Nicklas;I. Puchtel;R. Ash
中科院分区：
文献类型：
--
作者：
R. Nicklas;I. Puchtel;R. Ash
地幔氧逸度（fO 2）是地球科学中一个重要的强度变量，在整个地球历史中基本上不受约束。现代玄武岩的氧逸度是通过检测新鲜火山玻璃的Fe+ 3/Fe+2比值来确定的，但这种方法不能应用于经历了岩浆后蚀变和/或变质作用的较古老的熔岩。使我们能够确定的fO 2的幔源熔岩的精度优于0.10 logfO 2单位。这种新的方法使用橄榄石和/或铬铁矿和科马提质熔体之间的氧化还原敏感的过渡金属钒的分配行为作为氧气压计。为了获得准确和精确的结果，在芬诺斯坎迪亚的分化的科马提质玄武岩维多利亚的熔岩湖收集了一系列全岩样品。特别注意确保熔岩中含有与侵位熔体成分平衡的岩浆橄榄石和铬铁矿，并且在熔岩分异过程中没有发生再平衡。采用标准加入溶液ICP-MS法测定全岩样品中钒和其它过渡金属丰度，其精密度优于5%（2SD）;采用激光烧蚀ICP-MS法测定液相线橄榄石和铬铁矿中钒和其它过渡金属丰度，其精密度优于5%（2SE）。MgO和过渡金属丰度的侵位科马提质玄武岩熔岩精确计算使用几个独立的方法，这些方法提供了一致的结果，同意在该方法的不确定性。然后计算分配系数DVOl、Chr-Liq，并用于确定熔岩湖的fO 2分别为− 0.24 ± 0.04和− 0.21 ± 0.03 ΔNNO对数单位（2SE）。这两个独立的估计值在各自的不确定性范围内是相同的，并证明了该方法的准确性;平均值为− 0.22 ± 0.04 ΔNNO log units（2SD）代表了我们对熔岩湖氧化还原状态的最佳估计。校正4%的地壳污染，原始科马提岩的氧化还原状态计算为− 0.29 ± 0.04 ΔNNO log单位。新方法提供了一个高分辨率的工具，约束地球历史上地幔的氧化还原状态的演变。
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+ 2ratios 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 thefO2of mantle-derived lavas with a precision of better than 0.10 logfO2units. 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 coefficientsDVOl, Chr-Liqwere then calculated and used to determine thefO2of 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.