Fo and Ni Relations in Olivine Differentiate between Crystallization and Diffusion Trends
Fo and Ni Relations in Olivine Differentiate between Crystallization and Diffusion Trends
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DOI：
10.1093/petrology/egaa083
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发表时间：
2020-09-01
影响因子：
3.9
通讯作者：
Worner, Gerhard
中科院分区：
文献类型：
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作者：
Gordeychik, Boris;Churikova, Tatiana;Worner, Gerhard
镍是橄榄石中的强相容元素，因此橄榄石的分步结晶通常会导致 Fo-Ni 图上的凹上趋势。 “富镍”橄榄石组合物被认为是那些落在这种结晶趋势之上的组合物。为了解释富镍橄榄石晶体，我们开发了一套理论和计算模型来描述来自母体（高镁、高镍）玄武岩的原始橄榄石斑晶如何通过扩散与演化的（低镁、低镍）熔体重新平衡。这些模型描述了各种晶体形状以及 Ni 和 Fe-Mg 的不同相对扩散率在长时间扩散过程中橄榄石核中 Fo 和 Ni 的逐渐损失。当Ni的扩散率低于Fe-Mg相互扩散的情况下，受延长扩散影响的橄榄石斑晶形成下凹趋势，这与上凹结晶趋势相反。不同简单几何形状的模型表明，扩散趋势的凹度不依赖于晶体的尺寸，仅微弱地依赖于它们的形状。我们还发现扩散各向异性对趋势凹度的影响与晶体形状的影响具有相同的程度。因此，扩散各向异性和晶体形状都不会显着改变下凹扩散趋势。使用一系列更复杂、更真实且具有各向异性的橄榄石形态的三维数值扩散模型证实了这一结论。因此，下凹扩散趋势的曲率主要由Ni和Fe-Mg扩散系数之比决定。扩散趋势的起点和终点依次由镁铁质和更演化的熔体之间的成分对比决定，这些熔体混合导致橄榄石核心和周围熔体之间的不平衡。我们提供了几个来自堪察加半岛弧玄武岩的橄榄石测量示例，并发布了来自非俯冲环境（镁铁岩和金伯利岩）的镁铁质岩浆的橄榄石数据集，这些橄榄石数据集与扩散控制的 Fo-Ni 行为一致。在每种情况下，Ni 和 Fe-Mg 扩散系数之比表示为
Nickel is a strongly compatible element in olivine, and thus fractional crystallization of olivine typically results in a concave-up trend on a Fo-Ni diagram. 'Ni-enriched' olivine compositions are considered those that fall above such a crystallization trend. To explain Ni-enriched olivine crystals, we develop a set of theoretical and computational models to describe how primitive olivine phenocrysts from a parent (high-Mg, high-Ni) basalt re-equilibrate with an evolved (low-Mg, low-Ni) melt through diffusion. These models describe the progressive loss of Fo and Ni in olivine cores during protracted diffusion for various crystal shapes and different relative diffusivities for Ni and Fe-Mg. In the case when the diffusivity of Ni is lower than that for Fe-Mg interdiffusion, then olivine phenocrysts affected by protracted diffusion form a concave-down trend that contrasts with the concave-up crystallization trend. Models for different simple geometries show that the concavity of the diffusion trend does not depend on the size of the crystals and only weakly depends on their shape. We also find that the effect of diffusion anisotropy on trend concavity is of the same magnitude as the effect of crystal shape. Thus, both diffusion anisotropy and crystal shape do not significantly change the concave-down diffusion trend. Three-dimensional numerical diffusion models using a range of more complex, realistic olivine morphologies with anisotropy corroborate this conclusion. Thus, the curvature of the concave-down diffusion trend is mainly determined by the ratio of Ni and Fe-Mg diffusion coefficients. The initial and final points of the diffusion trend are in turn determined by the compositional contrast between mafic and more evolved melts that have mixed to cause disequilibrium between olivine cores and surrounding melt. We present several examples of measurements on olivine from arc basalts from Kamchatka, and published olivine datasets from mafic magmas from non-subduction settings (lamproites and kimberlites) that are consistent with diffusion-controlled Fo-Ni behaviour. In each case the ratio of Ni and Fe-Mg diffusion coefficients is indicated to be