Matrix rheology effects on reaction rim growth II: coupled diffusion and creep model
Matrix rheology effects on reaction rim growth II: coupled diffusion and creep model
复制标题
基质流变对反应边缘生长的影响 II：耦合扩散和蠕变模型
DOI：
10.1111/j.1525-1314.2008.00805.x
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发表时间：
2009
影响因子：
3.4
通讯作者：
R. Milke
中科院分区：
文献类型：
--
作者：
D. Schmid;R. Abart;Y. Podladchikov;R. Milke
化学反应和相变通常涉及体积变化。在受限环境中，这将导致围绕发生体积变化的反应位点的基质的机械变形。因此，矿物反应和岩石基质的力学响应是耦合的。本期的一篇配套论文结合石英和橄榄石在1 GPa和1000 °C的环境条件下反应形成顽火辉石反应环的实验说明了这种耦合。 已经证明，对于相同的运行条件，反应边缘的厚度取决于石英颗粒是否嵌入橄榄石基质或橄榄石颗粒是否包含在石英基质中。实验条件、结果的性质和反应的大体积变化（-6%）使得只有粘性蠕变作为反应进程的可行基质响应。一个模型是开发这个反应，它结合了扩散的化学成分通过不断增长的边缘和粘性蠕变的矩阵。由此产生的速率律反应边缘增长球形几何形状表明，进展速率是成比例的反应超越和控制的两个竞争过程中的较慢的扩散或蠕变。如果扩散是速率限制，则边缘增长与结果之间通常呈线性比例。然而，如果粘性蠕变是速率限制的，则反应速率降低并且可以变得有效地蠕变控制。相对于配套文件中的实验，可以推断，两种基质材料（即多晶石英和橄榄石）的有效粘度相差约一个数量级，其中石英更强。推导出的粘度的绝对值对应以及公布的流动定律。天然岩石的流变特性是很好的参数范围内的显着的机械控制反应边缘的增长预期。这意味着，自然反应轮辋和日冕结构的解释扩散和机械控制需要考虑。此外，在从边缘生长实验中反演互扩散系数时，还需要考虑机械效应。这也应该考虑到geoseedometry分析。此外，由于基质的缓慢蠕变，在较冷的地壳条件下，与实验相比，对反应速率的控制预计将更加重要，并且可能对仅部分完成的反应的频繁观察做出重大贡献。我们认为，这种现象被称为“机械关闭”，这可能是一个重要的机制，在动力学位移的相组合的稳定领域之间的边界。
Chemical reactions and phase changes generally involve volume changes. In confined settings this will cause a mechanical deformation of the matrix that surrounds the reaction sites where the volume change takes place. Consequently, mineral reactions and the mechanical response of the rock matrix are coupled. A companion paper in this issue illustrates this coupling with experiments where quartz and olivine react to form enstatite reaction rims under ambient conditions of 1 GPa and 1000 °C. It has been demonstrated that for identical run conditions, the thickness of the reaction rims depends on whether quartz grains are embedded in an olivine matrix or olivine grains are included in a quartz matrix. The experimental conditions, the nature of the results, and the large volume change of the reaction (−6%) leave only viscous creep as a viable matrix response to the reaction progress. A model is developed for this reaction, which combines diffusion of chemical components through the growing rim and viscous creep of the matrix. The resulting rate law for reaction rim growth in spherical geometry shows that the progress rate is proportional to the reaction overstepping and controlled by the slower of the two competing processes; either diffusion or creep. If diffusion is rate limiting the usual linear proportionality between rim growth and results. However, if viscous creep is rate limiting, then the reaction rates are reduced and may become effectively creep controlled. With respect to the experiments in the companion paper it is inferred that the effective viscosity of the two matrix materials, i.e. polycrystalline quartz and olivine, differ by approximately one order of magnitude with the quartz being the stronger one. The absolute values of the inferred viscosities correspond well to published flow laws. The rheological properties of natural rocks are well within the parameter range for which significant mechanical control on reaction rim growth is expected. This implies that for the interpretation of natural reaction rims and corona structures both diffusion and mechanical control need to be considered. In addition the mechanical effect also needs to be considered when interdiffusion coefficients are retrieved from rim growth experiments. This should also be considered for geospeedometry analyses. Furthermore, the control on reaction rate because of slow creep of the matrix is expected to be even more important, compared to the experiments, under colder crustal conditions and may contribute substantially to the frequent observation of only partially completed reactions. We suggest that this phenomenon is referred to as ‘mechanical closure’, which may be an important mechanism in the kinetic displacement of the boundaries between the stability fields of phase assemblages.