Post-melting oxidation of highly primitive basalts from the southern Andes
Post-melting oxidation of highly primitive basalts from the southern Andes
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DOI：
10.1016/j.gca.2020.01.042
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发表时间：
2020-03
影响因子：
5
通讯作者：
S. Tassara;M. Reich;C. Cannatelli;B. Konecke;D. Kausel;D. Morata;F. Barra;A. Simon;A. Fiege;E. Morgado;M. Leisen
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文献类型：
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作者：
S. Tassara;M. Reich;C. Cannatelli;B. Konecke;D. Kausel;D. Morata;F. Barra;A. Simon;A. Fiege;E. Morgado;M. Leisen
地球上地幔及其熔融产物的氧逸度（fO2）是弧岩浆地球化学演化及其与大陆地壳构造和生长联系的重要参数。一些研究集中在橄榄岩捕虏体、相对年轻的弧环境中的原始熔体和洋中脊玄武岩（MORB）的fo2上，但很少有研究试图研究成熟弧中原始岩浆的早期氧化还原历史。因此，我们对俯冲旋回中o2的性质和演化的认识仍然有限。在这里，我们研究了来自智利中南部Los Hornitos单成因锥体的玄武岩tephra，它是南安第斯山脉报道的最原始的物质之一（橄榄石Mg# ≤ 92.5，镍 ≤ 5000 μ g·g−1）。这些特征提供了一个独特的机会，通过研究橄榄石斑晶及其包含的晶体和熔体来探索安第斯弧下的fo2条件。我们整合了EPMA， LA-ICP-MS和µ-XANES分析，通过三种不同的独立方法来约束玄武岩中记录的氧化还原条件。首先，我们根据橄榄石-尖晶石平衡测定了fo2，得到平均值ΔFMQ + 1.3 ± 0.4 （1σ）。其次，我们使用Feµ-XANES数据和氧化还原依赖的橄榄石-熔体钒分配来约束熔体包裹体的fo2条件。校正捕获后结晶和弥漫性铁损失后，Feµ-XANES数据表明，熔体包裹体在ΔFMQ +2.5 ± 0.5 （1σ）处平均被捕获。熔体包裹体中橄榄石-熔体钒分配氧计的结果与Feµ-XANES数据一致，平均ΔFMQ值为+2.6 ± 0.3 （1σ）。为了测试熔体包裹体的其他圈闭后修饰可能影响喷发前的fo2的潜在影响，我们使用橄榄石中的Mg-Fe互扩散模型评估了这些岩浆的停留时间。与钒再平衡模型相比，较短的停留时间（<200 天）强烈表明熔体包裹体在其夹持期间保持了当前的fo2条件。熔体包裹体主要元素组成与Feµ-XANES测定的fo2之间的相关性以及V/Sc模型揭示了LHC岩浆熔融后氧化的情况。我们认为原始弧岩浆在其早期地球化学演化过程中表现为一个开放体系。我们的数据表明了安第斯山脉南部原始熔体的复杂的早期fo2历史，并为直接外推原始熔体的fo2值到它们的地幔源提供了一个警告。
The oxygen fugacity (fO2) of the Earth’s upper mantle and its melting products is an important parameter in the geochemical evolution of arc magmas and their connection with the continental crustal construction and growth. Several works have focused on thefO2of peridotite xenoliths, primitive melts in relatively young arc settings, and mid-ocean ridge basalts (MORB) but few studies have attempted to examine the early redox history of primitive magmas in mature arcs. Hence, our understanding of the nature and evolution offO2during the subduction cycle remains limited. Here, we investigate the basaltic tephra from the Los Hornitos monogenetic cones in central-southern Chile, which are among the most primitive materials reported in the Southern Andes (olivine Mg# ≤ 92.5, and Ni ≤ 5000 µg·g−1). These features offer a unique opportunity to explore thefO2conditions below the Andean arc by studying olivine phenocrysts and their contained crystal and melt inclusions. We integrated EPMA, LA-ICP-MS, and µ-XANES analyses to constrain the redox conditions recorded in the basaltic tephra by three different and self-reliant methods. First, we determined thefO2based on the olivine-spinel equilibrium, yielding average values ΔFMQ + 1.3 ± 0.4 (1σ). Second, we constrained thefO2conditions of melt inclusions using Fe µ-XANES data and the redox dependent olivine-melt vanadium partitioning. After correcting for post-entrapment crystallization and diffusive iron loss, the Fe µ-XANES data indicate that the melt inclusions were trapped in average at ΔFMQ +2.5 ± 0.5 (1σ). Results using the olivine-melt vanadium partitioning oxybarometer in melt inclusions are in agreement with Fe µ-XANES data, yielding average ΔFMQ values of +2.6 ± 0.3 (1σ). In order to test the potential effects of other post-entrapment modifications of the melt inclusions that could have affected thefO2prior to eruption, we assessed the residence time of these magmas using Mg-Fe interdiffusion modelling in olivine. The short residence times (<200 days) compared to vanadium re-equilibration models strongly suggest that the melt inclusions preserve the prevailingfO2conditions during their entrapment. Correlations between melt inclusions major element composition and theirfO2determined by Fe µ-XANES, as well as V/Sc modelling reveal a case of post-melting oxidation of the LHC magmas. We argue that primitive arc magmas behave as an open system with respect tofO2during their early geochemical evolution. Our data indicate a complexfO2early history of primitive melts in the southern Andes and provide a cautionary note on the direct extrapolation of primitive meltsfO2values to that of their mantle source.