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: 1550929
• 负责人: Aaron Bell
• 金额: $ 28.58万
• 依托单位: University of New Mexico
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-07-01 至 2018-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1550929&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.

赋存于层状镁铁质侵入体中的层状铬铁矿矿床是迄今为止最重要的铬元素经济来源。铬是制造不锈钢合金所需的关键成分，也是无数其他具有重要技术意义的“高温合金”。虽然人们普遍认为，在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，但在这些矿床所在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，也是许多科学争论的主题。这项研究的主要目的之一是量化变量，如硅酸盐熔体的化学成分和溶解在硅酸盐熔体中的铬的氧化状态如何影响铬铁矿的结晶行为。这项研究的结果可用于推断镁铁质岩浆饱和并结晶铬铁矿的化学环境。在更广泛的背景下，这项研究的结果也可以用于量化镁铁质岩浆的氧化状态。开发新的工具来量化玄武岩岩浆的氧化状态，是了解地球S地幔氧化状态的基础方面，也是了解地球深部与地表储集层之间挥发性元素的形态和迁移性的重要一步。这个项目将支持新墨西哥大学的硕士或博士生，以及本科生的论文。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的研究。了解铬铁矿的稳定性是建立与层状镁铁质侵入岩有关的层状铬铁矿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温度、压力、熔体的化学成分及其氧化态(即fO2)等变量的影响。这项研究的主要目的是提供镁铁质岩浆中铬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射线吸收近边缘结构(XANES)测量实验生长的橄榄石晶体和淬火熔体中的铬价比。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铬价态数据将被用来建立镁铁质岩浆中铬铁矿结晶的新的地球化学模型。此外，橄榄石晶体中的铬价态测量将被用来开发一种新的工具来评估玄武岩岩浆的fO2。初步研究表明，橄榄石中的铬价记录了可能在晶体生长过程中发生的fO2变化。通过这种方法，橄榄石中铬的价态测量可以发展成为用于岩浆系统的可靠的定量氧化还原计时器。

项目成果

Quantifying and correcting the effects of anisotropy in XANES measurements of chromium valence in olivine: Implications for a new olivine oxybarometer

• DOI: 10.2138/am-2017-5911
• 发表时间: 2017-06
• 期刊: American Mineralogist
• 影响因子: 3.1
• 作者: A. Bell;C. Shearer;P. Burger;M. Ren;M. Newville;A. Lanzirotti