Variations in Fe3+/ΣFe of Mariana Arc Basalts and Mantle Wedge fO2
Variations in Fe3+/ΣFe of Mariana Arc Basalts and Mantle Wedge fO2
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DOI：
10.1093/petrology/egu065
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发表时间：
2014-12-01
影响因子：
3.9
通讯作者：
Cottrell, E.
中科院分区：
文献类型：
--
作者：
Brounce, M. N.;Kelley, K. A.;Cottrell, E.
弧玄武岩比洋中脊玄武岩更容易被氧化，但目前尚不清楚这种差异是由于地壳的分化过程，还是由于它们的地幔源氧逸度的根本差异。区分这两种假设对于理解与弧岩浆作用有关的氧化还原敏感过程，从而更广泛地了解地球物质如何在全球范围内循环是很重要的。我们结合来自马里亚纳五个弧火山和马里亚纳海槽两个地区的橄榄石含矿玻璃包裹体和海底玻璃中的Fe3+/Sigma Fe比值，给出了主要、挥发性和微量元素浓度。对于单次喷发，Fe3+/Sigma Fe比值沿着下降的液体线变化，要么是轻微氧化（橄榄石+斜辉石+斜长石分馏，CO2+ /- H2O脱气），要么是还原性（橄榄石+斜辉石+斜长石+/-磁铁矿分馏，CO2+H2O+S脱气）。马里亚纳样品与钙碱性亲和度与岩浆水和岩浆氧逸度的整体关系一致，其中更湿润、氧逸度更高的岩浆对钙碱性分化表现出更大的亲和度。然而，我们发现低压分异不能解释马里亚纳弧玄武岩中Fe3+/Sigma Fe比值的大部分变化，需要岩浆氧逸度的主要差异。相对于洋中脊玄武岩（类似于QFM，其中QFM为石英-费雅石-磁铁矿缓冲层），在熔体分离压力和温度下计算的原始地幔熔体氧逸度在QFM+1.0 ~ QFM+1.6之间，而弧后相关样品记录的原始地幔氧逸度在QFM+0.1 ~ QFM+0.5之间。马里亚纳弧样品组包括多种俯冲作用的影响，从弧后均匀的富氢组分的较小影响，到沿弧的沉积物熔融和流体主导的影响。初级熔体氧逸度与沉积物熔体贡献（如Th/La）没有显著相关性，也不能归因于弧后先前的熔体提取。从马里亚纳海槽到马里亚纳弧，初级熔体氧挥发度与板块流体指数（如Ba/La）密切相关，相对于洋中脊玄武岩，Ba/La增加了10倍，增加了1.5个数量级。这些结果表明，板块对地幔楔的贡献可能是马里亚纳弧玄武岩记录的高氧逸度的原因，并且板块流体可能是非常氧化的。
Arc basalts are more oxidized than mid-ocean ridge basalts, but it is unclear whether this difference is due to differentiation processes in the Earth's crust or to a fundamental difference in the oxygen fugacity of their mantle sources. Distinguishing between these two hypotheses is important for understanding redox-sensitive processes related to arc magmatism, and thus more broadly how Earth materials cycle globally. We present major, volatile, and trace element concentrations in combination with Fe3+/Sigma Fe ratios determined in olivine-hosted glass inclusions and submarine glasses from five Mariana arc volcanoes and two regions of the Mariana Trough. For single eruptions, Fe3+/Sigma Fe ratios vary along liquid lines of descent that are either slightly oxidizing (olivine+clinopyroxene+plagioclase fractionation, CO2 +/- H2O degassing) or reducing (olivine+clinopyroxene+plagioclase +/- magnetite fractionation, CO2+H2O+S degassing). Mariana samples are consistent with a global relationship between calc-alkaline affinity and both magmatic H2O and magmatic oxygen fugacity, where wetter, higher oxygen fugacity magmas display greater affinity for calc-alkaline differentiation. We find, however, that low-pressure differentiation cannot explain the majority of variations observed in Fe3+/Sigma Fe ratios for Mariana arc basalts, requiring primary differences in magmatic oxygen fugacity. Calculated oxygen fugacities of primary mantle melts at the pressures and temperatures of melt segregation are significantly oxidized relative to mid-ocean ridge basalts (similar to QFM, where QFM is quartz-fayalite-magnetite buffer), ranging from QFM+1.0 to QFM+1.6 for Mariana arc basalts, whereas backarc related samples record primary oxygen fugacities that range from QFM+0.1 to QFM+0.5. This Mariana arc sample suite includes a diversity of subduction influences, from lesser influence of a homogeneous H2O-rich component in the back-arc, to sediment melt-and fluid-dominated influences along the arc. Primary melt oxygen fugacity does not correlate significantly with sediment melt contributions (e.g. Th/La), nor can it be attributed to previous melt extraction in the back-arc. Primary melt oxygen fugacity correlates strongly with indices of slab fluids (e.g. Ba/La) from the Mariana Trough through the Mariana arc, increasing by 1.5 orders of magnitude as Ba/La increases by a factor of 10 relative to mid-ocean ridge basalts. These results suggest that contributions from the slab to the mantle wedge may be responsible for the elevated oxygen fugacity recorded by Mariana arc basalts and that slab fluids are potentially very oxidized.