Magmaticfractionation by compositionalconvection in a sheet-like magma body
Magmaticfractionation by compositionalconvection in a sheet-like magma body
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片状岩浆体中成分对流的岩浆分馏
DOI：
10.1093/petrology/egr034
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发表时间：
2011
期刊：
影响因子：
--
通讯作者：
K.
中科院分区：
文献类型：
--
作者：
Simura;. and Ozawa;K.
根据对日本北方野海岬岩体的观测，研究了席状岩浆体的分馏机制。这是根室半岛的一系列晚白垩世钾玄岩岩床之一，具有复合结构，包括含有1050体积%堆晶的底部堆晶带和不含此类堆晶的上覆中间带。这些带夹在上部和下部冷却带和边缘带的斑状岩之间。据推测，大量的晶体沉淀或浮动从岩浆最初负载的晶体（原生晶）在片状岩浆体中的20%（体积），形成了积云堆和上覆的无晶体熔体层。上、下缘带部分冻结的复合构造形成后，侵入体内部缓慢凝固。在此期间，侵入岩片中部的熔体经历了分离结晶。间隙熔体组成的垂直变化模式表明了这一点。整体模式是倒置的（镜像）S形和S形的不兼容和兼容的元素，分别。不相容元素丰度在中间区域的底部附近显示出最大值，该区域对应于最终凝固层位。它们在堆积带中显示出极小值，这表明从晶体堆中排出了演化的熔体。分馏被推断发生通过成分对流没有沉淀的晶体生长后，入侵事件或崩溃的上边界层。中心熔体层中的熔体下降到可渗透的晶体堆中，抵消了由晶体堆中的孔熔体的结晶形成的浮力演化熔体的排出。这一结论是基于以下观察。(1)堆积带上方的岩浆侵入后，没有生长出晶体的堆积。(2)堆晶带中发育有浅色管状构造，是熔体演化的残留通道。这些结构的直径增加，数密度降低，不相容元素的丰度随高度增加。(3)堆晶带中的堆晶辉石具有富钙贫铝的溶解边缘。溶解的程度随地层高度的增加，这表明从上覆熔体层的富H2O熔体的下降。(4)没有证据表明，在堆积带的主要部分，伴随着孔隙熔融结晶的广泛压实。(5)估计的初始熔体组合物从基块的冷冻边缘岩石和平均组成的材料间隙的原生晶为整个窗台统计上是相同的。一个质量平衡模型的组成对流被构造来量化熔体输送过程。模型参数进行了优化，通过拟合观察到的间隙熔体组成的垂直变化。优化的结果表明，演化熔体排出的晶体堆和上升通过中央熔体层的羽毛与周围的熔体轻微混合。最初形成的化合物结构是组分对流的高度优先环境。
The mechanism of fractionation in a sheet-like magma body was investigated based on observations from the Nosappumisaki intrusion, northern Japan. This is one of a number of late Cretaceous shoshonite sills in the Nemuro peninsula and has a compound structure comprising a bottom cumulate zone containing ∼50 vol. % cumulus crystals and a overlying middle zone free of such cumulus crystals. These zones are sandwiched between porphyritic rocks of upper and lower chilled and marginal zones. It is inferred that massive crystal settling or flotation from a magma initially laden with ∼20 vol. % of crystals (primocrysts) in a sheet-like magma body formed a cumulus pile and an overlying crystal-free melt layer. After the formation of the compound structure with partial freezing of the upper and lower marginal zones, the internal part of the intrusion slowly solidified. During this period the melt in the central part of the intrusive sheet underwent fractional crystallization. This is shown by patterns of vertical variation in the composition of the interstitial melt. The overall patterns are inverted (mirror-image) S-shaped and S-shaped for incompatible and compatible elements, respectively. The incompatible element abundances show maxima near the bottom of the middle zone, which corresponds to the final solidification horizon. They show minima in the cumulate zone, which suggests discharge of an evolved melt from the crystal pile. Fractionation is inferred to have taken place via compositional convection without settling of crystals grown after the intrusion event or collapse of the upper boundary layer. The melt in the central melt layer descended into the permeable crystal pile counterbalancing discharge of buoyant evolved melt formed by crystallization of the pore melt in the crystal pile. This conclusion is based on the following observations. (1) There is no accumulation of crystals grown after the intrusion of the magma above the cumulate zone. (2) Leucocratic pipe-like structures, which represent relict pathways for the evolved melt, are developed in the cumulate zone. These structures increase in diameter, decrease in number density, and increase in abundance of incompatible elements with height. (3) Cumulus augite in the cumulate zone has Ca-rich and Al-poor dissolved rims. The extent of dissolution increases with stratigraphic height, suggesting downwelling of an H2O-rich melt from the overlying melt layer. (4) There is no evidence for extensive compaction accompanying pore melt crystallization in the main part of the cumulate zone. (5) The initial melt composition estimated from the groundmass of the chilled margin rocks and the average composition of the material interstitial to the primocrysts for the whole sill are statistically identical. A mass-balance model for compositional convection was constructed to quantify the melt transportation processes. Model parameters were optimized by fitting the observed vertical variation of the interstitial melt compositions. The optimized results suggest that the evolved melts discharged from the crystal pile and rose through the central melt layer as plumes with minor mingling with the surrounding melt. The initially formed compound structure is a highly preferential environment for compositional convection.