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Igneous differentiation by deformation

机译：通过变形变化

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In a paper published in 1920, Bowen conceived of a situation where forces acting on a crystalline mesh could extract the liquid phase from the solid, and in doing so cause variations in chemistry distinct from the purely gravitational effects of fractional crystallisation. His paper was a call-to-arms to explore the role of deformation as a cause of variation in igneous rocks, but was never followed-up in a rigorous way. Inspired by this, we have developed a quantitative model showing how shear deformation of a crystallised dense magma (phi > 70%) with poro-elastic properties is analogous to a granular material. The critical link between the mechanics and associated compositional changes of the melt is the degree to which the crystallising magma undergoes dilation (volume increase) during shear. It is important to note that the effect can only take place after the initial loose solid material has undergone mechanical compaction such that the grains comprising the rigid skeleton are in permanent contact. Under these conditions, the key material parameters governing the dilatancy effect are the physical permeability, mush strength, the shear modulus and the contact mechanics and geometry of the granular assemblage. Calculations show that dilation reduces the interstitial fluid (melt) pressure causing, in Bowen's words, "the separation of crystals and mother liquor" via a suction effect. At shear strain rates in excess of the tectonic background, deformation-induced melt flow can redistribute chemical components and heat between regions of crystallising magma with contrasting rheological properties, at velocities far in excess of diffusion or buoyancy forces, the latter of course the driving force behind fractional crystallisation and viscous compaction. Influx of hotter, less evolved melt drawn internally from the same magma body into regions where crystallisation is more advanced (auto-intrusion), may result in reverse zoning and/or resorption of crystals. Because dilatancy is primarily a mechanical effect independent of melt composition, evolved, chemically distinct melt fractions removed at this late stage may explain miarolitic alkaline rocks, intrusive granophyres in basaltic systems and late stage aplites and pegmatites in granites (discontinuous variations), as proposed by Bowen. Post-failure instabilities include hydraulic rupture of the mush along shear zones governed by the angles of dilation and internal friction. On the macro-scale, a combination of dilatancy and fracturing may provide a means to extract large volumes of chemically evolved melt from mush columns on short (< 1000 year) geological timescales.
机译：在1920年发表的纸张中，鲍文构思了作用在结晶网格上的力可以从固体中提取液相，并且在这样做的情况下导致化学的变化不同于分数结晶的纯度重力作用。他的论文是一种呼叫武器，可以探讨变形的作用，作为发火岩石的变化的原因，但从未以严谨的方式随访。由此启发，我们开发了一种定量模型，示出了具有孔弹性性质的结晶致密岩浆（PHI> 70％）的剪切变形是如何类似于颗粒材料。熔体的力学和相关组成变化之间的关键联系是结晶岩浆在剪切期间经历扩张（体积增加）的程度。重要的是要注意，在初始松散的固体材料经过机械压实之后，效果只能发生这种效果，使得包含刚性骨架的颗粒是永久接触的。在这些条件下，控制膨胀效应的关键材料参数是物理渗透性，糊状强度，剪切模量和颗粒组合的接触力学和几何形状。计算表明，扩张降低了通过抽吸效果的鲍文的单词“晶体和母液分离”的间质液（熔体）压力降低。在超越构造背景下的剪切应变率，变形诱导的熔体流动可以在具有对比的流变性质的结晶岩浆区域之间重新分配化学成分和热量，其速度远远超过扩散或浮力，后者当然是驱动力背后的分数结晶和粘性压实。从相同的岩浆体内部涌入更热，较较小的熔体进入结晶更先进的区域（自入侵），可能导致晶体的反向分区和/或吸收。因为膨胀性主要是熔体组合物无关的机械效果，所以在该晚期除去的化学不同的熔体部分可以解释MiArolic碱性岩石，玄武岩系统中的侵入性颗粒状和花岗岩中的晚期的消耗性（不连续变化），如所提出的鲍文。失败后的不稳定性包括沿着剪切区域的液压破裂，由扩张和内部摩擦的角度控制。在宏观尺度上，膨胀性和压裂的组合可以提供从短（<1000年）地质时间尺度的糊色谱柱中提取大量的化学演化熔体的方法。

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