Fractionation of Rare-Earth and High-Field Strength Elements by Melting of a Re-equilibrated Mantle

镁铁质岩和超镁铁质岩中稀土元素和高场强元素的丰度和分布特征为认识火成岩的成因和演化提供了重要线索。 地球化学中的一个基本观察结果是，在沿着大陆边缘喷发的玄武岩岩浆中，与REE和钍等其他微量元素相比，HFSE相对贫化。 地球化学家利用这一观测结果来解释俯冲带的岩浆过程。然而，导致这种相对消耗的机制仍然知之甚少。该项目旨在测试弧岩浆中HFSE的相对贫化是否是由俯冲带中的亚固相线再平衡和随后的部分熔融期间地幔矿物之间的REE和HFSE再分配引起的。该项目将通过为本科生提供研究机会并作为研究生的培训基地来促进STEM教育。 当残余矿物和部分熔体之间的化学交换动力学与熔融速率相比缓慢时，发生不平衡熔融。 由于固相线温度较低，它更可能发生在含水熔融过程中。 这项研究的主要目的是评估和量化的作用，subsolidus再平衡和随后的不平衡部分熔融的尖晶石二辉橄榄岩的分馏HFSE从稀土在岩浆成因。 该项目有两个密切相关的组成部分：（1）实验校准的参数化晶格应变模型之间的分配HFSE辉石和玄武岩熔体和斜方辉石和单斜辉石，（2）HFSE和稀土分馏在不平衡地幔熔融的数值和理论研究。 从建议的辉石熔体分配实验的结果也将被用来更新现有的分配模型的稀土元素和高场强辉石。 温度和成分依赖的分配模型允许调查人员使用自洽的初始条件HFSE和稀土元素丰度在开始辉石在他们的熔融模型，并进行模拟沿着熔融路径的温度和矿物的主要元素组成系统地改变。 地球化学正演和反演模拟将进行严格评估的地球化学后果的熔融subsolidusly再平衡地幔。

The abundance and distribution of rare earth elements (REE) and high field strength elements (HFSE) in mafic and ultramafic rocks provide important clues to understanding the origin and evolution of igneous rocks. A fundamental observation in geochemistry is that there is relative depletion of HFSE compared to REE and other trace elements such as thorium in basaltic magmas erupted along continental margins. Geochemists have used this observation to unravel magmatic processes in the subduction zones. However, the mechanisms leading to this relative depletion are still poorly understood. This project is designed to test if the relative depletion of HFSE in arc magmas is caused by redistribution of REE and HFSE between mantle minerals during subsolidus re-equilibration and subsequent partial melting in the subduction zone. The project would advance STEM education by providing research opportunities for undergraduate students and serving as a training ground for a graduate student. Disequilibrium melting happens when the kinetics of chemical exchange between a residual mineral and partial melt is sluggish compare to the rate of melting. It is more likely to take place during hydrous melting because the solidus temperature is lower. The primary objective of this proposed study is to assess and quantify the roles of subsolidus re-equilibration and subsequent disequilibrium partial melting of spinel lherzolite in the fractionation of HFSE from REE during magma genesis. The project has two closely related components: (1) experimental calibration of parameterized lattice strain models for the partitioning of HFSE between pyroxene and basaltic melt and between orthopyroxene and clinopyroxene, and (2) numerical and theoretical studies of HFSE and REE fractionation during disequilibrium mantle melting. Results from the proposed pyroxene-melt partitioning experiments will also be used to update existing partitioning models for REE and HFSE in pyroxenes. Temperature and composition dependent partitioning models allows the investigators to use self-consistent initial conditions for HFSE and REE abundances in starting pyroxenes in their melting model, and to conduct simulations along a melting path where temperature and mineral major element compositions change systematically. Forward and inverse geochemical modeling will be conducted to critically assess the geochemical consequences of melting of subsolidusly re-equilibrated mantle.