Diffusion-driven Fe-Mg and Li isotope fractionation in olivine: An experimental investigation and new modeling approach

在本项目中，我们的目标是实验研究岩浆晶体中的同位素分馏，由化学扩散产生。在第一个资助期，我们的目标是专注于橄榄石和Fe和Mg同位素分馏，由Fe-Mg交换扩散驱动，以及扩散驱动的Li同位素分馏。在第二个供资期内，这种调查可能会扩展到其他岩浆矿物，如辉石和斜长石，在第一个供资期内，研究单位的其他项目（#1和#2）将调查其扩散速率。同位素的优点是，在岩浆温度下，同位素只有显着分馏扩散过程中，而平衡同位素分馏是小的。因此，同位素分区可以用来明确地识别扩散驱动的分区，此外，结合化学分区，以解决复杂的岩浆的情况下，岩浆晶体经历了几个生长和扩散阶段。然而，扩散驱动的同位素分区的延伸还几乎不为人知，在天然橄榄石中观察到的同位素剖面仅适合于扩散建模过程中同位素分馏的假设模型。我们提出了精确的实验测定扩散驱动的同位素分馏和它的依赖参数，如温度，氧逸度，化学成分和结晶取向的橄榄石。这将导致更好的约束的初始边界条件，假设的扩散模型，从而导致更准确的约束岩浆过程的持续时间的确定。这也将有助于提高估计的扩散驱动的化学通量（在部分熔融或交代事件），矿物或甚至散装岩石同位素数据的基础上，我们将进一步开发3D模型，以预测同位素分馏剖面的各向异性，应允许识别自然晶体分区的切片效果。这一模型和关于扩散驱动同位素分馏的所有研究结果将被应用于扩散建模的用户友好型软件，该软件将在项目6中开发。在与项目#8的合作中，我们已经将本项目中确定的实验校准同位素分区应用于天然橄榄石的研究。

In this project of the proposed research unit, we aim to experimentally investigate isotope fractionation in magmatic crystals, generated by chemical diffusion. In the first funding period, we aim to focus on olivine and Fe and Mg isotope fractionation, driven by Fe-Mg exchange diffusion, as well as diffusion-driven Li isotope fractionation. In a second funding period such investigations may be extended to other magmatic minerals, such as pyroxene and plagioclase, for which the diffusion rate will be investigated in other projects (# 1 and #2) of the research unit, during the first funding period.The use of isotopic zoning has recently been developed as an additional complementary tool in diffusion chronology, including Fe-Mg and Li isotope zoning in olivine. The advantage of isotopes is that at magmatic temperatures isotopes only significantly fractionate during diffusion, while equilibrium isotope fractionation is small. Isotopic zoning can thus be used to unequivocally identify diffusion-driven zoning and furthermore, in combination with chemical zoning to resolve complex magmatic scenarios in which magmatic crystals experienced several growth and diffusion stages. However, the extend of diffusion-driven isotopic zoning is yet barely known and isotopic profiles observed in natural olivines are only fitted to assumed models of isotopic fractionation during diffusion modelling. We propose the precise experimental determination of diffusion-driven isotope fractionation and its dependence on parameters such as temperature, oxygen fugacity, chemical composition and crystallographic orientation of the olivine. This will result in much better constraints on the initial boundary conditions that are assumed for the diffusion model, and consequently lead to the determination of more accurate constraints on the duration of magmatic processes. It will also help to improve estimates of diffusion-driven chemical fluxes (during partial melting or metasomatic events), based on mineral- or even bulk rock isotopic data.We will furthermore develop 3D models to predict the anisotropy of isotope fractionation profiles that should allow to identify sectioning effects in natural crystals zoning. This model and all findings on diffusion-driven isotope fractionation will be implemented into the user friendly software for diffusion modelling, which will be developed in project #6. In cooperation with project #8, we will already apply the experimentally calibrated isotope zoning, as determined in this project, for the investigation of natural olivine.