Abstract

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 <1. These examples show that crystallization and diffusion can be distinguished by concave-up and concave-down trends in Fo–Ni diagrams.

中文翻译：

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橄榄石的Fo和Ni关系区分结晶和扩散趋势

抽象的

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在Fo-Ni图上导致凹入趋势。“富镍”橄榄石成分被认为是落在这种结晶趋势之上的成分。为了解释富镍的橄榄石晶体，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现象如何与演化出的（低镁，低镍）再平衡。 ）通过扩散融化。这些模型描述了在长期扩散过程中，各种晶体形状以及镍和铁镁的相对扩散率不同时，橄榄石芯中的Fo和Ni逐渐消失的情况。如果Ni的扩散率低于Fe-Mg互扩散的扩散率，然后，受长期扩散影响的橄榄石表晶形成了下陷的趋势，与下陷的结晶趋势形成了鲜明的对比。不同简单几何形状的模型表明，扩散趋势的凹度不取决于晶体的大小，而仅取决于晶体的形状。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大小。因此，扩散各向异性和晶体形状都不会显着改变凹向下扩散趋势。使用一系列具有各向异性的更复杂的，真实的橄榄石形态的三维数值扩散模型证实了这一结论。因此，向下扩散趋势的曲率主要由Ni和Fe-Mg扩散系数之比确定。扩散趋势的起点和终点反过来由铁镁质和更多演化的熔体之间的成分对比决定，这些熔体混合后会引起橄榄岩芯与周围熔体之间的不平衡。我们提供了堪察加半岛弧形玄武岩橄榄石测量的几个例子，以及来自非俯冲环境（红土岩和金伯岩）的镁铁质岩浆的橄榄石数据集，这些数据与扩散控制的Fo-Ni行为一致。在每种情况下，Ni和Fe–Mg扩散系数之比均表示为<1。这些例子表明，结晶和扩散可以通过Fo-Ni图的上凹和下凹趋势来区分。