The Distribution of Olivine Compositions in Icelandic Basalts and Picrites
The Distribution of Olivine Compositions in Icelandic Basalts and Picrites
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DOI：
10.1093/petrology/egs083
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发表时间：
2012-12
影响因子：
3.9
通讯作者：
A. Thomson;J. Maclennan
中科院分区：
文献类型：
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作者：
A. Thomson;J. Maclennan
最近的一些研究利用岩浆晶体及其载体液体之间的成分关系来了解火山管道系统内的过程。在这里，大量的冰岛橄榄石和玻璃成分的电子探针数据被用来检查单个熔岩流中橄榄石大晶体中橄榄石含量的分布，以及这些橄榄石与其载体玄武岩液体的关系。研究人员检查了7836个橄榄石和233个玻璃点分析的数据集，确定了11个火山爆发，其中有玻璃数据和60多个晶体核心成分。与许多玄武岩套一样，单个橄榄石晶体通常具有均匀的核心成分和狭窄的正常带状边缘。因此，在冰岛11次喷发中的10次中，超过90%的橄榄石都是太强的橄榄石，无法与其载体玄武岩液体保持平衡。数据集的采样密度允许对橄榄石成分分布进行统计调查，其中包含的信息可用于提供岩浆过程的新约束。核密度估计和高斯混合模型的结果表明，11次喷发中的每一次都至少包含一个橄榄石成分的坚固峰。在这11次喷发中，8次为单峰分布，2次为双峰分布，1次为多峰分布。载体玻璃组分与橄榄石橄榄石含量分布之间关系的一个重要特征是，在11个流动中，有10个流动中橄榄石成分分布的强峰值出现在橄榄石含量比与载体液体平衡的橄榄石含量高2-3 mol %的地方。对于简单平衡或分式结晶模型产生的橄榄石，无法预测该偏移峰。相反，橄榄石成分的分布及其与载体液体的关系可以用三阶段模型来解释。在第一阶段，分步结晶和结晶沉降在岩浆房的底部形成一堆糊状物质。混合物中存在成分分层，其底部的橄榄石比顶部的橄榄石更具有森林质，反映了在分馏结晶过程中液体成分的演变。浆液最上部的橄榄石与腔室内部的玄武岩液体接近平衡。在第二阶段，局部扩散使糊状中的单晶均匀化，形成喷发时观察到的均匀岩心。同时，通过间隙熔融相的扩散改变了浆料堆全厚度上的化学梯度，减少了浆料中橄榄石岩心成分的差异。这一过程在冰岛流中从未完成，但确实在橄榄石成分中产生一个峰值，接近晶体堆中橄榄石的平均橄榄石含量。最后，在喷发前不久，糊状物被分解成舱内的载体液体，形成弥漫性边缘叠印。这一过程的定量模型表明，观察到的橄榄石成分偏移峰可以在42-8000年的泥堆扩散后产生，这取决于泥堆的厚度。因此，玄武岩中橄榄石成分分布的关键特征可以用一个简单的糊状生成和分解模型来解释。
A number of recent studies have used the compositional relationship between magmatic crystals and their carrier liquids to understand processes within volcanic plumbing systems. Here, an extensive compilation of electron microprobe data for Icelandic olivine and glass compositions is used to examine the distribution of the forsterite content of olivine macrocrysts within single lava flows, and the relationship of these olivines to their carrier basaltic liquids. A dataset of 7836 olivine and 233 glass point analyses was examined and 11 eruptions were identified where glass data and over 60 crystal core compositions were available. In common with many basaltic suites, single olivine crystals typically have uniform core compositions with narrow normally zoned rims. Accordingly, in 10 of the 11 Icelandic eruptions over 90% of the olivines are too forsteritic to be in equilibrium with their carrier basaltic liquids. The sampling density of the dataset permitted statistical investigation of the distribution of olivine compositions that contain information that can be used to provide new constraints on magmatic processes. The results of both kernel density estimates and Gaussian mixture modelling indicate that each of the 11 eruptions contained at least one robust peak in olivine compositions. Out of these 11 eruptions, eight show unimodal distributions of macrocryst olivine forsterite content, two are bimodal and one is polymodal. An important feature of the relationship between the carrier glass compositions and the distribution of olivine forsterite contents is that, for 10 of the 11 flows, a strong peak in the olivine compositional distribution occurs at forsterite contents that are 2–3 mol % higher than those expected for olivines in equilibrium with the carrier liquid. This offset peak is not predicted for olivines generated by simple equilibrium or fractional crystallization models. Instead, the distribution of olivine compositions and its relationship with the carrier liquids can be accounted for using a three-stage model. In the first stage, fractional crystallization and crystal settling generate a mush pile on the floor of a magma chamber. Compositional stratification is present in this mush, with the olivines at its base being more forsteritic than those at its top, reflecting the evolution of liquid compositions during fractional crystallization. The olivines in the uppermost part of the mush are close to equilibrium with basaltic liquid in the interior of the chamber. In the second stage local diffusion acts to homogenize single crystals in the mush, creating the uniform cores observed upon eruption. Concurrently the chemical gradient across the full thickness of the mush pile is altered by diffusion through the interstitial melt phase, reducing the variance of olivine core compositions in the mush. This process never reaches completion in the Icelandic flows but does generate a single peak in olivine compositions close to the mean forsterite content of the olivines in the crystal pile. Finally, the mush is disaggregated into the carrier liquid of the chamber interior shortly before eruption, creating the diffusional rim overprints. Quantitative models of this process indicate that the observed offset peak in olivine compositions can be generated after 42–8000 years of diffusion in a mush pile, depending on the mush thickness. Key features of the compositional distribution of olivines in basalts can therefore be accounted for using a simple model of mush generation and disaggregation.