Thermal and rheological controls on the formation of mafic enclaves or banded pumice
Thermal and rheological controls on the formation of mafic enclaves or banded pumice
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DOI：
10.1007/s00410-013-0961-7
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发表时间：
2014-01-01
影响因子：
3.5
通讯作者：
Manga, Michael
中科院分区：
文献类型：
--
作者：
Andrews, Benjamin J.;Manga, Michael
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体。我们认为，对混合方式的关键控制是侵入岩浆中发展的晶体网络（赋予岩浆强度）与熔化和破坏宿主中的这些网络之间的竞争。 X 射线计算机断层扫描分析表明，1915 年拉森山喷发的带状浮石缺乏晶体网络。相比之下，来自混沌峭壁的镁铁质包体的流英安石主体含有发育良好的大型晶体网络。我们提出了一个一维传导冷却模型，该模型可以预测混合类型（延性或脆性），作为岩浆成分、温度和侵入岩墙尺寸的函数。我们的模型依赖于三个假设：（1）通过将热岩浆注入具有屈服强度的较冷岩浆体来引发混合； (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，实际上是固态的。重要的是，由于两种岩浆的成分不同，因此它们的结晶度和粘度随温度的变化并不相同。当入侵的岩浆冷却时，它会从外向内结晶，同时入侵者附近的主岩浆温度升高。当主岩浆被充分加热到（1）破坏晶体网络和（2）引发对流时，两种岩浆开始混合。如果对流主岩浆对岩脉施加的剪应力大于岩脉边缘的屈服强度（且岩脉结晶度不超过40%），则发生流体混合，否则因岩脉脆性变形而形成包体。将该模型应用于代表拉森岩和混沌岩岩浆成分的模型表明，岩墙的侵位
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