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

了解铬氧化还原系统学镁铁质岩浆：在岩浆 fO2 计算和铬铁矿溶解度中的应用

• 批准号: 1848654
• 负责人: Aaron Bell
• 金额: $ 10.45万
• 依托单位: University of Colorado at Boulder
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-06-04 至 2020-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1848654&HistoricalAwards=false
• 关键词: Understanding; Chromium; Redox; Systematics; Mafic

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.

产于层状镁铁质侵入体中的层状铬铁矿床是迄今为止最具经济价值的铬（Cr）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。虽然人们普遍认为层状镁铁质侵入体中发现的铬铁矿矿床是母岩浆体的结晶产物，但对这些矿床所在岩浆体中诱发铬铁矿结晶的基本地球化学过程仍然知之甚少，也是许多科学争论的主题。本研究的主要目标之一是量化变量，如硅酸盐熔体的化学组成和氧化态的铬溶解在硅酸盐熔体中的影响铬铁矿的结晶行为。从这项研究中产生的结果可以用来推断的化学环境下，镁铁质岩浆饱和和结晶铬铁矿。在更广泛的背景下，这项研究的结果也可以应用于定量的镁铁质岩浆的氧化态。 开发新的工具来量化玄武岩浆的氧化状态是理解地球氧化状态的一个基本方面。的地幔和一个重要的一步，了解地球深部和表层储层之间的挥发性元素的形态和流动性。 该项目将支持新墨西哥州大学的硕士或博士生，以及本科生论文。这两名学生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的研究。 了解铬铁矿的稳定性是开发与层状镁铁质侵入体有关的层状铬铁矿矿床形成机制模型的一个关键方面。铬铁矿在玄武岩浆中的稳定性受温度、压力、熔体的化学成分及其氧化态（即fO2）等变量的影响。本研究的主要目的是产生一个定量的肖像铬氧化还原行为和铬铁矿稳定性的镁铁质岩浆，以及它如何与上述变量。这项研究结合了高温高压岩石学实验在新墨西哥州大学进行的X射线吸收近边结构（XANES）测量铬价比在实验生长的橄榄石晶体和淬火熔体。 XANES测量将在阿贡国家实验室的高级光子源进行。 本研究所获得的相平衡和Cr价态数据将用于建立镁铁质岩浆中铬铁矿结晶的新地球化学模型。此外，橄榄石晶体中铬价的测量将用于开发评估玄武岩浆fO2的新工具。初步调查表明，铬价橄榄石记录的变化fO2可能发生在pheoncrysts生长。因此，橄榄石中铬的价态测量可以发展成为一种可靠的、定量的岩浆体系氧化还原计时器。