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

在拟建研究单位的这个项目中，我们的目标是通过实验研究化学扩散在岩浆晶体中产生的同位素分馏。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。在第二个资助期内，这种调查可能会扩展到其他岩浆矿物，如辉石和斜长石，在第一个资助期内，研究单位的其他项目（# 1和#2）将对其扩散速率进行调查。同位素分带的使用最近被发展为扩散年代学的额外补充工具，包括橄榄石中的Fe-Mg和Li同位素分带。同位素的优点是，在岩浆温度下，同位素在扩散过程中只发生明显的分馏，而平衡同位素分馏很小。因此，同位素分带可以明确地识别扩散驱动的分带，进一步，与化学分带相结合，可以解决岩浆晶体经历几个生长和扩散阶段的复杂岩浆情景。然而，扩散驱动的同位素分带的范围尚不清楚，在天然橄榄石中观察到的同位素剖面仅适用于扩散模拟过程中假设的同位素分馏模型。我们提出了精确的实验测定扩散驱动同位素分馏及其对诸如温度、氧逸度、化学成分和橄榄石晶体取向等参数的依赖。这将导致对扩散模型所假定的初始边界条件的更好的约束，从而导致对岩浆过程持续时间的更精确的约束的确定。它还将有助于改进基于矿物甚至块状岩石同位素数据的扩散驱动的化学通量（在部分熔融或交代事件期间）的估计。我们将进一步开发3D模型来预测同位素分馏剖面的各向异性，这将允许识别自然晶体分带的切片效应。该模型和所有关于扩散驱动同位素分馏的发现将被应用到扩散建模的用户友好软件中，该软件将在项目#6中开发。在与项目8的合作中，我们已经将该项目确定的实验校准同位素分带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.