Orientation of Tabular Mafic Intrusions Controls Convective Vigour and Crystallization Style
Orientation of Tabular Mafic Intrusions Controls Convective Vigour and Crystallization Style
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板状镁铁质侵入体的方向控制对流强度和结晶方式
DOI：
10.1093/petrology/egx081
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发表时间：
2017
影响因子：
3.9
通讯作者：
Holness M
中科院分区：
文献类型：
--
作者：
Holness M
玄武岩岩墙的显微结构与岩床和熔岩湖的显微结构有很大的不同。对于给定的侵入体宽度（或熔岩湖深度），垂直板状体比水平体的粒度更粗，在整个侵入体中具有不变的斜长石形状。当比较平均粒度相同的岩床和岩脉样品时，岩脉样品比岩床样品含有更少的小颗粒和更少的大颗粒。与此相反，中位数单斜辉石-斜长石-斜长石二面角在堤坝的变化精确地与观察到的窗台，是一个函数的扩散热损失率。这些模式可以解释，如果结晶的早期阶段在岩墙主要涉及孤立的颗粒悬浮在一个良好的混合对流岩浆的生长，与最后阶段（在此期间二面角的形式）发生在一个富含晶体的静态岩浆中，其中热损失主要是扩散。相比之下，结晶在窗台主要发生在边缘固化前，这表明任何对流运动是不足以夹带晶体从边缘糊状层，并保持他们悬浮，而他们的成长。穆尔孤立岩脉的成员提供了这种一般模式的一个例外，这些岩脉从苏格兰的穆尔古近纪火成岩中心向东南方向延伸100-1000 km，穿过浅地壳。这些岩墙的取样距离穆尔>100 km，其显微结构与厚度相当的岩床的显微结构难以区分。我们认为，足够的成核和结晶发生在这些堤坝，以增加粘度足以阻尼对流，一旦单向流动已经停止。
The microstructure in basaltic dykes is significantly different from that in sills and lava lakes of the same bulk composition. For a given width of intrusion (or depth of lava lake), vertical tabular bodies are coarser grained than horizontal bodies, with an invariant plagioclase shape across the intrusion. When comparing samples from sills and dykes for which the average grain size is the same, the dyke samples contain fewer small grains and fewer large grains than the sill samples. In contrast, the variation of median clinopyroxene–plagioclase–plagioclase dihedral angles in dykes correlates precisely with that observed in sills and is a function of the rate of diffusive heat loss. These patterns can be accounted for if the early stages of crystallization in dykes primarily involve the growth of isolated grains suspended in a well-mixed convecting magma, with the final stage (during which dihedral angles form) occurring in a crystal-rich static magma in which heat loss is primarily diffusive. In contrast, crystallization in sills occurs predominantly in marginal solidification fronts, suggesting that any convective motions are insufficient to entrain crystals from the marginal mushy layers and to keep them suspended while they grow. An exception to this general pattern is provided by members of the Mull Solitary Dykes, which propagated 100–1000 km SE from the Mull Palaeogene Igneous Centre, Scotland, through the shallow crust. These dykes, where sampled >100 km from Mull, have a microstructure indistinguishable from that of a sill of comparable thickness. We suggest that sufficient nucleation and crystallization occurred in these dykes to increase the viscosity sufficiently to damp convection once unidirectional flow had ceased.
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影响因子：
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作者：
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DOI：
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DOI：
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发表时间：
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