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

岩浆是地球物理条件和化学的探测器？的内部。探索这些喷发熔体的矿物学可以深入了解地球的持续地质演化。除了基本的地球化学之外，温度和压力等参数对于量化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（fO 2），这是岩浆结晶时氧化环境的量度。在岩浆中的所有元素中，铁是唯一以多价态存在的主要造岩元素。因此，准确的价态（Fe 3 +/Fe 2+）的知识提供了最好的代理的量的氧，目前当矿物结晶，并允许估计岩浆氧逸度。氧逸度的岩浆是一个主要参数之一，沿着，随着温度，压力和水含量，决定了岩浆？的结晶路径，以及所产生的矿物的组成。为了破译任何火成岩的起源，必须首先了解在给定fO 2的封闭系统中由岩浆过程引起的成分差异、相变和结晶顺序变化。已经开发了氧气压计来使用矿物内的多价元素（例如铁）的化学或分配行为来量化岩浆系统的fO 2（例如，FeTi氧化物、辉石）或玻璃。评估Fe 3 +/Fe 2+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量样品来混淆小尺度的变化。Fe XANES允许在玻璃中原位测量Fe 3+，但存在主要与组合物和冷却历史引起的熔体结构变化相关的重要分析问题。因此，组合物特定的校准和复杂的多元分析技术是必要的强大的微量分析的Fe 3 +/Fe 2+的玻璃。因此，本提案的目标是为跨越广泛的地质可行条件的玻璃提供广泛的独立分析的Fe 3 +/Fe标准。研究人员提出了一系列广泛的实验，在仔细控制的fO 2条件下，所得产品进行多种分析技术，以量化Fe 3+。这些实验的结果将提供一个强大的校准的Fe XANES技术的广泛的地质相关的组合物和冷却的历史。

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.