Collaborative Research: Resolving the Controversy of REE Diffusion in Olivine

这一早期探索性研究奖章旨在解决目前有关稀土元素在矿物橄榄石中的扩散速度的争议，橄榄石是地球上地幔中最常见的相。过去两次研究得到的稀土元素的扩散系数相差3~4个数量级，这对解释玄武岩岩浆从地球和行星内部深处上升的速度有深远的意义，以及在橄榄石赋存的熔体包裹体中观察到的化学变化是否可靠地记录了地幔来源的不均一性。如果稀土在橄榄石中的扩散确实非常快，那么从最近的研究中得出的结论必须重新评估。出于这个原因，岩石学和地球化学界有兴趣迅速澄清这一问题。这将为博士后的培养提供支持。该项目将结合理论、分析和实验方法，评估和确定两个数据集的差异来源。该团队假设，橄榄石中稀土元素扩散的两个时标可能在特定条件下运行。为了验证这一假设，该团队将进行一系列实验室扩散实验，以评估主要元素浓度梯度的存在、不同的稀土元素浓度以及替代机制是否是不同研究结果不一致的根源。在进行实验的同时，还将使用一种新的稀土橄榄石-单斜辉石温度计测定天然样品中橄榄石中的稀土元素和钙铁镁元素的封闭温度，以便更好地比较经历不同冷却速度的镁铁质和超镁铁质岩石中橄榄石中三价阳离子和二价阳离子的相对扩散速度。为了更好地了解这两个实验室的扩散测量结果及其与天然样品观测的联系，该团队还计划进行一项理论研究，考察主要元素浓度梯度和化学应变能对橄榄石中稀土元素扩散的影响。这项迫切的研究的最终目标不仅是解决围绕REE在橄榄石中扩散的争议，而且还为了建立一个框架，以了解微量元素和常量元素在其他造岩矿物中多组分扩散过程中的扩散时间尺度。

This Early Grant for Exploratory Research (EaGER) award is designed to resolve a current controversy regarding the rate of diffusion of rare earth element (REE) in the mineral olivine that is the most common phase in the Earth's upper mantle. Diffusivities of REE derived from two past studies differ by 3~4 orders of magnitude, which has profound implications for the interpretation of questions such as how fast basaltic magmas ascend from depths in the Earth and planetary interiors and if chemical variations observed in olivine-hosted melt inclusions are reliable records of mantle source heterogeneity. If REE diffusion in olivine is indeed very fast, conclusions derived from recent studies must be re-evaluated. For that reason, the petrology and geochemistry community is interested in clarifying the issue quickly. This EaGER will provide support for the training of a post-doctoral fellow. The project will integrate theoretical, analytical and experimental approaches to evaluate and determine the source of discrepancy in the two data sets. The team hypothesizes that there is a possibility that two time scales for REE diffusion in olivine may operate under specific conditions. In order to test this hypothesis, the team will conduct a series of laboratory diffusion experiments to evaluate if the presence of major element concentration gradients, differing REE concentrations, and substitutional mechanisms are at the source of the discrepant results from different studies. The experiments will be accompanied by closure temperatures determinations of REE and Ca-Fe-Mg in olivine from natural samples using a new REE-in-olivine-clinopyroxene thermometer, enabling a better comparison of relative diffusion rates between the trivalent and divalent cations in olivine from mafic and ultramafic rocks that experienced different cooling rates. To better understand diffusion measurements from the two laboratories and their connection to observations in natural samples, the team also plans to undertake a theoretical study examining the effects of major element concentration gradient and chemical strain energy on REE diffusion in olivine. The ultimate goal of this EaGER study is not only to resolve the controversy surrounding REE diffusion in olivine, but also to develop a framework for understanding the time scales of diffusion for the trace elements and major elements during multicomponent diffusion in other rock-forming minerals.