Diffusion-driven magnesium and iron isotope fractionation in Hawaiian olivine
Diffusion-driven magnesium and iron isotope fractionation in Hawaiian olivine
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DOI：
10.1016/j.epsl.2011.06.003
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发表时间：
2011-08
影响因子：
5.3
通讯作者：
F. Teng;N. Dauphas;R. T. Helz;Shan Gao;Shichun Huang
中科院分区：
文献类型：
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作者：
F. Teng;N. Dauphas;R. T. Helz;Shan Gao;Shichun Huang
扩散在地球科学中起着重要的作用，以估计地质过程的时间尺度，如侵蚀，沉积物埋藏和岩浆冷却。在火成岩系统中，这些扩散过程以晶体环带的形式记录下来。然而，对这些特征的有意义的解释常常受到这样一个事实的阻碍，即它们不能明确地归属于单个过程（例如，岩浆分馏、晶体或液体中的扩散限制传输）。在这里，我们表明，镁和铁同位素分馏橄榄石晶体中可以用来跟踪岩浆系统中的扩散过程。夏威夷玄武岩中60多个橄榄石碎片的Mg和Fe同位素分馏值与世界玄武岩相比，26 Mg/24 Mg比值变化最大为0.4‰，56 Fe/54 Fe比值变化最大为1.6‰。Mg和Fe同位素组成的线性和负相关[即，δ 56 Fe =（−3.3±0.3）×δ 26 Mg]、Mg和Fe同位素组成与橄榄石碎片Fe/Mg比值的协变以及基于Mg和Fe元素剖面的模拟结果表明，Mg和Fe同位素分馏耦合是岩浆分异过程中环带橄榄石中Mg-Fe相互扩散的表现。这一特征可以用来限制火成岩和变质岩中矿物分带的性质，从而确定晶体在岩浆中的停留时间、原生熔体的组成以及变质事件的持续时间。随着方法的改进，原位同位素填图将成为岩石学中识别晶体扩散的重要工具。
Diffusion plays an important role in Earth sciences to estimate the timescales of geological processes such as erosion, sediment burial, and magma cooling. In igneous systems, these diffusive processes are recorded in the form of crystal zoning. However, meaningful interpretation of these signatures is often hampered by the fact that they cannot be unambiguously ascribed to a single process (e.g., magmatic fractionation, diffusion limited transport in the crystal or in the liquid). Here we show that Mg and Fe isotope fractionations in olivine crystals can be used to trace diffusive processes in magmatic systems. Over sixty olivine fragments from Hawaiian basalts show isotopically fractionated Mg and Fe relative to basalts worldwide, with up to 0.4‰ variation in26Mg/24Mg ratios and 1.6‰ variation in56Fe/54Fe ratios. The linearly and negatively correlated Mg and Fe isotopic compositions [i.e., δ56Fe=(−3.3±0.3)×δ26Mg], co-variations of Mg and Fe isotopic compositions with Fe/Mg ratios of olivine fragments, and modeling results based on Mg and Fe elemental profiles demonstrate the coupled Mg and Fe isotope fractionation to be a manifestation of Mg–Fe inter-diffusion in zoned olivines during magmatic differentiation. This characteristic can be used to constrain the nature of mineral zoning in igneous and metamorphic rocks, and hence determine the residence times of crystals in magmas, the composition of primary melts, and the duration of metamorphic events. With improvements in methodology, in situ isotope mapping will become an essential tool of petrology to identify diffusion in crystals.