Collaborative Research: Resolving the Controversy of REE Diffusion in Olivine

这项早期探索性研究资助（EaGER）奖旨在解决目前关于稀土元素（REE）在矿物橄榄石（地球上地幔中最常见的阶段）中的扩散速率的争议。过去两项研究得出的REE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多快，以及在橄榄石包裹体中观察到的化学变化是否是地幔源非均质性的可靠记录等问题具有深远的意义。如果稀土元素在橄榄石中的扩散确实非常快，那么必须重新评估最近研究得出的结论。因此，岩石学和地球化学学界有兴趣迅速澄清这个问题。该项目将为培养一名博士后提供支持。该项目将综合运用理论、分析和实验方法来评价和确定两组数据差异的来源。研究小组假设，稀土元素在橄榄石中扩散的两个时间尺度可能在特定条件下起作用。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差异的根源。实验还将使用一种新的REE-in- oliolive -斜辉石温度计测定天然样品中橄榄石中的REE和Ca-Fe-Mg的闭合温度，以便更好地比较经历不同冷却速率的基性和超基性岩石中橄榄石中三价和二价阳离子的相对扩散速率。为了更好地理解两个实验室的扩散测量以及它们与自然样品观测的联系，研究小组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。本EaGER研究的最终目的不仅是解决围绕稀土元素在橄榄石中扩散的争议，而且为理解其他造岩矿物中微量元素和主元素在多组分扩散过程中的扩散时间尺度建立一个框架。

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.