Collaborative Research: Effects of Composition and Cooling Rate on Fe XANES Glass Calibrations

合作研究：成分和冷却速率对 Fe XANES 玻璃校准的影响

• 批准号: 1219850
• 负责人: Molly McCanta
• 金额: $ 10.35万
• 依托单位: Tufts University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-08-01 至 2016-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1219850&HistoricalAwards=false
• 关键词: Collaborative; Research; Effects; Composition; Cooling

Magmas are probes of the physical conditions and chemistry of the Earth?s interior. Exploring the mineralogy of these erupted melts provides insight into the continuing geologic evolution of the planet. In addition to basic geochemistry, parameters such as temperature and pressure are important to quantify to constrain the conditions present within the Earth. Of equal importance to the above physical parameters is oxygen fugacity (fO2), a measure of the oxidative environment a magma crystallized under. Of all the elements in magmas, iron is the only major rock-forming element that commonly exists in multiple valence states. Thus, accurate knowledge of the valence state (Fe3+/Fe2+) provides the best proxy for the amount of oxygen that was present when the minerals crystallized, and allows for estimation of magmatic oxygen fugacity. The oxygen fugacity of a magma is one of the principal parameters, along with temperature, pressure, and water content, that determines a magma?s crystallization path, as well as the composition of the resulting minerals. In order to decipher the origin of any igneous rock, it is essential to first understand the compositional differences, phase changes, and crystallization sequence variations that can be caused by magmatic processes in a closed system at a given fO2. Oxybarometers have been developed to quantify the fO2 of a magmatic system using the chemistry or partitioning behavior of multivalent elements such as iron within a mineral (e.g., FeTi oxides, pyroxene) or glass. The best-calibrated methods for evaluating Fe3+/Fe2+ are wet chemistry and Mössbauer spectroscopy, both of which use utilize bulk samples that obfuscate small-scale variations. Fe XANES allows for in situ Fe3+ measurements in glass, but there are significant analytical issues primarily related to changes in melt structure arising from composition and cooling history. Therefore composition-specific calibrations and sophisticated multivariate analysis techniques are necessary for robust microanalyses of Fe3+/Fe2+ in glasses. The goal of this proposal is thus to provide a broad range of independently-analyzed Fe3+/ΣFe standards for glasses spanning a broad range of geologically-feasible conditions. The investigators propose a wide ranging series of experiments under carefully controlled fO2 conditions with the resulting products subjected to multiple analytical techniques to quantify Fe3+. The results of these experiments will provide a robust calibration of the Fe XANES technique for a wide range of geologically-relevant compositions and cooling histories.

岩浆是地球内部物理条件和化学性质的探测器。探索这些喷发熔体的矿物学可以深入了解地球持续的地质演化。除了基本地球化学之外，温度和压力等参数对于量化以限制地球内部的条件也很重要。与上述物理参数同等重要的是氧逸度（fO2），它是岩浆结晶所处氧化环境的量度。在岩浆中的所有元素中，铁是唯一通常以多种价态存在的主要造岩元素。因此，准确了解价态 (Fe3+/Fe2+) 可以最好地反映矿物结晶时存在的氧含量，并可以估计岩浆氧逸度。岩浆的氧逸度是主要参数之一，与温度、压力和含水量一起，决定岩浆的结晶路径以及所得矿物的成分。为了破译任何火成岩的起源，必须首先了解在给定 fO2 的封闭系统中岩浆过程可能引起的成分差异、相变和结晶序列变化。氧压计已被开发用于利用多价元素的化学或分配行为来量化岩浆系统的 fO2，例如矿物（例如 FeTi 氧化物、辉石）或玻璃中的铁。评估 Fe3+/Fe2+ 的最佳校准方法是湿化学和穆斯堡尔光谱法，这两种方法都使用大块样品来混淆小尺度变化。 Fe XANES 允许对玻璃中的 Fe3+ 进行原位测量，但存在主要与成分和冷却历史引起的熔体结构变化相关的重大分析问题。因此，针对玻璃中 Fe3+/Fe2+ 的稳健微量分析需要特定成分的校准和复杂的多元分析技术。因此，该提案的目标是为涵盖广泛地质可行条件的玻璃提供广泛的独立分析的 Fe3+/ΣFe 标准。研究人员提出在仔细控制的 fO2 条件下进行一系列广泛的实验，并对所得产品进行多种分析技术来量化 Fe3+。这些实验的结果将为 Fe XANES 技术提供针对各种地质相关成分和冷却历史的可靠校准。