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首页> 外文期刊>Contributions to Mineralogy and Petrology >Thermal and rheological controls on the formation of mafic enclaves or banded pumice
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Thermal and rheological controls on the formation of mafic enclaves or banded pumice

机译：热和流变学控制镁铁质飞地或带状浮石的形成

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Magma mixing can occur in a fluid manner to produce banded pumice or in a brittle manner to form enclaves. We propose that the critical control on mixing style is a competition between developing networks of crystals in the intruding magma that impart a strength to the magma and melting and disrupting those networks in the host. X-ray computed tomography analysis demonstrates that banded pumice from the 1915 Mt. Lassen eruption lacks crystal networks. In contrast, rhyodacite hosts with mafic enclaves from Chaos Crags contain well-developed networks of large crystals. We present a one-dimensional conductive cooling model that predicts mixing style, either ductile or brittle, as a function of magma compositions, temperatures, and the size of the intruding dike. Our model relies on three assumptions: (1) Mixing is initiated by the injection of a hot dike into a cooler magma body with a yield strength; (2) when magma crystallinity exceeds a critical value, 13 vol% plagioclase, the magma develops a yield strength; and (3) when total crystallinity exceeds 40 vol%, the magma has a penetrative crystal network and is effectively solid. Importantly, because the two magmas are of different compositions, their crystallinities and viscosities do not have the same variations with temperature. As the intruding magma cools, it crystallizes from the outside in, while simultaneously, host magma temperature near the intruder rises. Mixing of the two magmas begins when the host magma is heated sufficiently to (1) disrupt the crystal network and (2) initiate convection. If the shear stress exerted by the convecting host magma on the dike is greater than the yield strength of the dike margin (and dike crystallinity does not exceed 40 %), then fluid mixing occurs, otherwise enclaves form by brittle deformation of the dike. Application of the model to magma compositions representative of Lassen and Chaos Crags shows that emplacement of dikes <1 m thick should produce enclaves, whereas thicker dikes should generate fluid mixing and form banded pumice within days to weeks of emplacement. Similar relationships apply to other modeled magmatic systems, including Pinatubo, Unzen, and Ksudach/Shtuybel' volcanoes. For all studied systems, the absolute size of the intruding dike, not just its proportion relative to the host, influences mixing style.
机译：岩浆混合可以以流体方式发生以产生带状浮石，或以脆性方式发生以形成包壳。我们提出，对混合方式的关键控制是侵入岩浆中不断发展的晶体网络之间的竞争，从而使岩浆具有一定的强度，并融化并破坏宿主中的这些网络。 X射线计算机断层扫描分析表明该带状浮石来自1915年。拉森喷发缺乏晶体网络。相比之下，带有混沌岩体的镁铁质飞地的流纹岩主体包含发达的大晶体网络。我们提出了一种一维传导冷却模型，该模型预测了岩浆成分，温度和侵入堤坝的大小的函数，表明混合方式（韧性或脆性）。我们的模型基于三个假设：（1）通过将热堤注入到具有屈服强度的较冷岩浆体中来启动混合; （2）当岩浆结晶度超过临界值13vol％斜长石时，岩浆产生屈服强度。 （3）当总结晶度超过40vol％时，岩浆具有穿透性的晶体网络并且有效地为固体。重要的是，由于两个岩浆的成分不同，因此它们的结晶度和粘度不会随温度变化。随着侵入岩浆的冷却，它从外面结晶出来，同时，入侵者附近的宿主岩浆温度升高。当主岩浆被充分加热到（1）破坏晶体网络和（2）启动对流时，两个岩浆开始混合。如果对流主岩浆在堤防上施加的剪切应力大于堤防边缘的屈服强度（且堤防结晶度不超过40％），则会发生流体混合，否则堤坝会因脆性变形而形成包壳。该模型在代表拉森岩和混沌岩浆的岩浆成分中的应用表明，厚度小于1 m的堤防进驻将产生飞地，而厚度较大的堤防应引起流体混合，并在侵扰后数日至数周内形成带状浮石。类似的关系适用于其他建模岩浆系统，包括Pinatubo，Unzen和Ksudach / Shtuybel火山。对于所有研究的系统，入侵堤坝的绝对大小，而不仅仅是相对于主体的比例，都会影响混合方式。

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