Understanding Chromium Redox Systematics Mafic Magmas: Applications to Magmatic fO2 Calculations and Chromite Solubility

层状基性侵入体中的层状铬铁矿是迄今为止经济上最重要的铬元素来源。铬是制造不锈钢合金以及无数其他技术上重要的“超级合金”所需的关键成分。虽然人们普遍认为，在层状基性侵入岩中发现的铬铁矿矿床是母岩浆体的结晶产物，但在承载这些矿床的岩浆体中诱导铬铁矿结晶的基本地球化学过程仍然知之甚少，并且是许多科学争论的主题。本研究的主要目的之一是量化诸如硅酸盐熔体的化学成分和硅酸盐熔体中溶解的Cr的氧化态等变量如何影响铬铁矿的结晶行为。研究结果可用于推断镁基岩浆被铬铁矿饱和并结晶的化学环境。在更广泛的背景下，本研究的结果也可以应用于定量镁质岩浆的氧化状态。开发新的工具来量化玄武岩岩浆的氧化状态是了解地球氧化状态的一个基础方面。这是了解地球深部和地表储层之间挥发性元素的物种形成和流动性的重要一步。该项目将支持一名新墨西哥大学的硕士或博士研究生，以及一篇本科生论文。这两名学生都将接受最先进的分析和实验技术培训，并协助在美国同步加速器设施进行研究。了解铬铁矿的稳定性是建立与层状基性侵入相关的层状铬铁矿床形成机制模型的关键方面。玄武岩岩浆中铬铁矿的稳定性受到温度、压力、熔体化学成分及其氧化状态（即fO2）等变量的影响。本研究的主要目的是对镁质岩浆中cr -氧化还原行为和铬铁矿稳定性的定量描述，以及它与上述变量的关系。这项研究结合了在新墨西哥大学进行的高温高压岩石学实验和x射线吸收近边结构（XANES）测量实验生长的橄榄石晶体和淬火熔体中的Cr价比。XANES测量将在阿贡国家实验室的先进光子源进行。本研究获得的相平衡和Cr价数据将用于建立一种新的镁基岩浆铬铁矿结晶地球化学模型。此外，橄榄石晶体中Cr价的测定将为玄武岩岩浆的fO2评价提供新的工具。初步研究表明，橄榄石中的cr价记录了可能在晶体生长过程中发生的fO2变化。通过这种方式，橄榄石中的铬价测量可以发展成为岩浆系统中可靠的定量氧化还原计时器。

Stratiform chromite deposits hosted in layered mafic intrusions are by far the most economically important source of the element Chromium (Cr). Chromium is a key ingredient required for the manufacturing of stainless steel alloys, as well as a myriad of other technologically important 'super alloys'. While it is widely accepted that the chromite deposits found in layered mafic intrusions are a crystallization product of a parental magma body, the fundamental geochemical processes that induce the crystallization of chromite in the magma bodies that host these deposits are still poorly understood and the subject of much scientific debate. One of the main objectives of this study is to quantify how variables such as the chemical composition of a silicate melt and the oxidation state of the Cr dissolved in the silicate melt affect the crystallization behavior of chromite. The results generated from this study can be used to deduce the chemical circumstances under which mafic magmas become saturated with and crystallize chromite. In a broader context, the results of this study can also be applied to quantifying the oxidation state of mafic magmas. Developing new tools to quantify the oxidation state of basaltic magmas is a foundational aspect of understanding the oxidation state of the earth?s mantle and a major step toward understanding the speciation and mobility of volatile elements between the deep earth and surficial reservoirs. This project will support a masters or doctoral student at the University of New Mexico, as well as an undergraduate student thesis. Both of these students will be trained in state-of-the-art analytical and experimental techniques and assist in research conducted at a U.S. synchrotron facility. Understanding chromite stability is a critical aspect of developing models for the formation mechanisms of stratiform chromite ore deposits associated with layered mafic intrusions. Chromite stability in basaltic magmas is influenced by variables such as temperature, pressure, the chemical composition of the melt, and its oxidation state (i.e. fO2). The primary objective of this study is to produce a quantitative portrait of Cr-redox behavior and chromite stability in mafic magmas and how it relates to the aforementioned variables. This study combines high temperature-pressure petrology experiments conducted at the University of New Mexico with X-ray Absorption Near Edge Structure (XANES) measurements of Cr valence ratios in the experimentally grown olivine crystals and quenched melts. The XANES measurements will be conducted at the Advanced Photon Source, Argonne National Laboratory. The phase equilibria and Cr valence data generated by this study will be used to develop a new geochemical model of chromite crystallization in mafic magmas. Furthermore, the Cr valence measurements in olivine crystals will be used to develop a new tool for assessing the fO2 of basaltic magmas. A preliminary investigation has shown that the Cr-valence in olivine records changes in fO2 that may have occurred during pheoncryst growth. In this way, Cr valence measurements in olivine may be developed into a robust, quantitative redox chronometer for magmatic systems.