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

岩浆是对地球内部物理条件和化学成分的探测--S。探索这些喷发的熔岩的矿物学有助于洞察地球持续的地质演化。除了基本的地球化学外，温度和压力等参数对于量化以限制地球内部存在的条件也很重要。与上述物理参数同样重要的是氧逸度(FO2)，这是一种衡量岩浆在其下结晶的氧化环境的指标。在岩浆中的所有元素中，铁是唯一一种主要的成岩元素，通常以多种价态存在。因此，对价态(Fe3+/Fe2+)的准确了解提供了矿物结晶时存在的氧量的最佳替代，并允许估计岩浆氧逸度。岩浆的氧逸度与温度、压力和水含量一起，是决定岩浆S结晶路径以及由此产生的矿物组成的主要参数之一。为了破译任何火成岩的起源，首先必须了解在给定fO2的封闭系统中岩浆作用可能引起的成分差异、相变和结晶序列变化。已经开发出氧气压计，以利用多价元素的化学或分配行为来量化岩浆系统的fO2，例如铁在矿物(例如，FeTi氧化物、辉石)或玻璃中的分配行为。评估Fe3+/Fe2+的最佳校准方法是湿化学法和穆斯堡尔光谱学，这两种方法都使用了混淆小范围变化的大样本。FeXANES允许原位测量玻璃中的Fe3+，但主要与成分和冷却历史引起的熔体结构变化有关的重大分析问题。因此，对于玻璃中Fe3+/Fe2+的可靠微量分析来说，特定成分的校准和复杂的多变量分析技术是必要的。因此，这项提议的目标是为玻璃提供广泛的独立分析的Fe3+/&Fe标准，涵盖广泛的地质可行条件。研究人员提议在严格控制的fO2条件下进行一系列广泛的实验，结果产物经过多种分析技术来量化Fe3+。这些实验的结果将为广泛的地质相关成分和冷却历史提供可靠的铁XANES技术校准。

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.