Multicomponent diffusion in silicate melts using eigen-component approach

该项目将使用一种新方法研究所有主要氧化物（定义为超过百分之一重量）在干燥的天然硅酸盐熔体中的扩散。在过去，研究人员只能成功地定量地研究氧化物在具有单调或平坦浓度分布的天然硅酸盐熔体中的扩散。但是具有非单调轮廓的氧化物呢？该项目的目标是开发一种更简单的方法来量化干燥硅酸盐熔体中所有主要氧化物的扩散。新方法的一个应用将是模拟两个岩浆的混合。初步结果表明，这种混合可能导致某些金属意想不到的高浓度。如此高的浓度可能导致成矿。因此，实际应用之一是解释以前无法解释的矿床，并预测矿床的可能位置。新方法的其他应用包括更精确地定量晶体生长和溶解速率。该研究还将解决高温地球化学动力学中的一个重大理论问题。本项目将使用特征组分方法研究干燥天然硅酸盐熔体中的多组分扩散。硅酸盐熔体中的多组分扩散是火成岩过程中物质输运的基础，也是地球化学动力学家面临的最困难的实验和理论问题之一。多组分扩散在天然硅酸盐熔体中经常遇到，但由于在理论和特别是实验方面的困难程度而没有加以处理。该团队提取了玄武岩熔体在三种不同温度下的扩散矩阵，并大致验证了玄武岩熔体中的扩散特征向量矩阵与温度大致无关。他们现在假设，在干熔体中，扩散特征向量矩阵也大致与熔体成分无关。他们现在正在进一步改进特征向量矩阵，希望通过复合来验证特征向量矩阵是不变的。在特征组分方法中，将绘制特征组分的“浓度”（而不是氧化物wt%或摩尔分数）与距离的关系图。如果他们的假设是正确的，那么使用特征向量矩阵，所有的特征成分图将是单调的，并且扩散特征值（即特征成分的扩散率）将通过拟合特征成分剖面获得。在我们的假设框架内，如果某些实验中的某些特征分量图显示出非单调分布，则意味着特征向量矩阵不准确，因此需要对特征向量矩阵进行改进。为了完成对熔体中多组分扩散的研究，在每个温度和压力下进行几个精心设计的实验就足以确定所有特征值，从而确定完整的扩散矩阵。该团队的假设和新方法将有助于从根本上理解天然硅酸盐熔体中的扩散。这项工作的成功将从地球化学动力学家的名单中消除一个难题，并使未来的地球化学家能够现实地预测扩散控制过程，如岩浆混合，矿物生长和溶解，以及熔体包裹体与其宿主矿物之间的捕获后相互作用。他们将建立并发布一个在线资源来计算多组分扩散曲线。这将使感兴趣的学生和科学家能够在研究地质问题时使用多组分扩散。这个计划中的在线计算器将有助于促进岩石学和地球化学界使用他们的结果。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This project will investigate diffusion of all major oxides (defined to be more than one weight percent) in dry natural silicate melts using a new approach. In the past, researchers could only successfully and quantitatively study the diffusion of oxides in natural silicate melts with monotonic, or flat, concentration profiles. But what about oxides with non-monotonic profiles? The goal of this project is to develop an easier way to quantify diffusion of all major oxides in dry silicate melts. One application of the new method will be the modeling of the mixing of two magmas. Preliminary results show that such mixing may result in unexpected high concentrations of some metals. Such high concentrations may lead to ore formation. Hence, one of the practical applications is to explain previously unexplained ore formation, and to predict possible locations of ore deposits. Other applications of the new method include more accurate quantification of crystal growth and dissolution rates. This research will also solve a major theoretical problem in high-temperature geochemical kinetics. This project will investigate multicomponent diffusion in dry natural silicate melts using eigen-component approach. Multicomponent diffusion in silicate melts is fundamental to mass transport in igneous processes and is also one of the most difficult experimental and theoretical problems facing geochemical kineticists. Multicomponent diffusion is often encountered in natural silicate melts but not treated due to the level of difficulties in terms of both theory and especially experiments. This team has extracted a diffusion matrix in basaltic melts at three different temperatures, and roughly verified that the diffusion eigenvector matrix in basaltic melts is roughly independent of temperature. They now hypothesize that the diffusion eigenvector matrix is also roughly independent of melt composition in dry melts. They are now further improving the eigenvector matrix and hope to verify that the eigenvector matrix is invariant with composition. In the eigen-component approach, plots of “concentrations” of eigen-components (rather than oxide wt% or mole fractions) versus distance will be made. If their hypothesis is correct, then using the eigenvector matrix, all eigen-component plots would be monotonic, and diffusion eigenvalues (i.e., diffusivity for the eigen-components) will be obtained from fitting the eigen-component profiles. If some eigen-component plots in some experiments show nonmonotonic profiles, within the framework of our hypothesis, it would mean inaccuracy of the eigenvector matrix, and hence the need to improve the eigenvector matrix. To complete the investigation of multicomponent diffusion in a melt, a couple of well-designed experiments at each temperature and pressure will be enough to determine all eigenvalues, and hence the full diffusion matrix. This team's hypothesis and new approach will contribute to the fundamental understanding of diffusion in natural silicate melts. The success of the proposed work will remove one difficult problem from the list of geochemical kineticists and allow future geochemists to realistically predict diffusion-controlled processes, such as magma mixing, mineral growth and dissolution, and post-entrapment interaction between a melt inclusion and its host mineral. They will build and publish an online resource to compute multicomponent diffusion profiles. This would enable interested students and scientists to use multicomponent diffusion in their research of geological problems. This planned online calculator will help facilitate the use of their results by the petrological and geochemical community.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.