Collaborative Research: Resolving the Controversy of REE Diffusion in Olivine

这个早期探索研究资助（EaGER）奖旨在解决目前关于稀土元素（REE）在矿物橄榄石中扩散速率的争议，橄榄石是地球上地幔中最常见的相。两次研究结果的稀土元素扩散系数相差3~4个数量级，这对解释玄武岩岩浆从地球深部和行星内部上升的速度有多快，以及橄榄岩熔体包裹体中的化学变化是否是地幔源区不均一性的可靠记录等问题具有重要意义。如果稀土元素在橄榄石中的扩散确实非常快，则必须重新评估最近研究得出的结论。因此，岩石学和地球化学界有兴趣迅速澄清这个问题。EaGER将为博士后研究员的培训提供支持。该项目将综合理论、分析和实验方法，以评价和确定两个数据集差异的来源。该团队假设，在特定条件下，橄榄石中REE扩散的两个时间尺度可能会起作用。为了验证这一假设，研究小组将进行一系列实验室扩散实验，以评估主要元素浓度梯度、不同REE浓度和替代机制的存在是否是不同研究结果不一致的根源。实验将伴随着封闭温度测定的稀土和钙铁镁橄榄石从天然样品中使用一种新的稀土在橄榄石单斜辉石温度计，使三价和二价阳离子之间的相对扩散速率更好地比较橄榄石从镁铁质和超镁铁质岩石经历不同的冷却速率。为了更好地了解两个实验室的扩散测量结果及其与天然样品观察结果的联系，该团队还计划进行一项理论研究，研究主要元素浓度梯度和化学应变能对橄榄石中REE扩散的影响。EaGER研究的最终目标不仅是解决围绕橄榄石中稀土元素扩散的争议，而且还建立了一个框架，用于理解其他成岩矿物中多组分扩散过程中微量元素和主量元素扩散的时间尺度。

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.