The geochemistry of redox variable elements

该项目旨在利用同步加速器光确定高温下岩浆中金属的氧化态（或电荷）。这项工作的动机是通过正确解释微量元素特征的地球化学起源来了解火成岩过程。火成岩岩石学是一门成熟的学科，最近由于技术的出现（如激光消融ICP-MS），使其能够准确测定微量元素浓度而得到了显著的发展。然而，对特征的解释往往落后于数据的获取，因此，元素之间的经验相关性归因于过程（例如，源中的石榴石、斜长石的结晶、岛弧），而对负责该行为的基本化学控制的了解很少。下一个量子飞跃将是了解这些特征的化学来源。要做到这一点，我们需要在一系列地质变量中确定微量元素的氧化态。氧化态决定了微量元素的电荷和大小，因此决定了它在矿物相中占据晶格位置的适宜性。这种适用性控制部分熔化和分步结晶过程中元素的晶体-熔体分配，从而产生痕量元素特征。氧化态比的影响，特别是，氧逸度，但也温度和压力的熔体，因此是一个强大的潜在指标的岩浆过程。然而，氧化态的测定并不容易。人们对确定Fe（最丰富的氧化还原可变元素）的氧化态比有着巨大的兴趣和努力。由于Fe在大多数分析技术（包括湿化学和光谱）中的干扰作用，其他元素几乎不可能进行研究。文献中的陈述强调了这一点，例如“没有已知的方法来测量含Fe熔体中Cr的氧化态”。确定氧化态的一种相对较新的方法是在同步加速器光源下进行的X射线吸收近边结构（XANES）光谱。XANES光谱技术是一种元素特异性技术，具有亚微米空间分辨能力，适用于硅酸盐熔体的原位研究。这是很重要的，因为氧化还原状态，存在于熔体中的高温下，并控制分区和地球化学行为的元素，不一定保留在冷却由于电荷转移反应与大的氧化还原可变水库的铁。例如，在1400 C的洋中脊玄武岩（MORB）中，大约一半的Cr是Cr（II），尽管这种氧化态从未在陆地材料中被确定（Cr（II）在Fe（III）的存在下是“不可淬火的”）。为了这项研究，已经设计了一个炉，允许XANES光谱被记录为几乎任何元素在熔体中的温度高达1500 ℃作为氧逸度的函数。该熔炉允许在原位研究熔体。这项工作将允许几乎任何元素的氧化态比的天然组合物的熔体中首次确定。

The project aims to use synchrotron light to determine the oxidation state (or charge) on metals in magmas at high temperature. The work is motivated by a desire to understand igneous processes through correctly interpreting the geochemical origins of trace element signatures. Igneous petrology is a mature subject which has recently been significantly advanced by the advent of technologies (e.g. laser-ablation ICP-MS) enabling the accurate determination of trace element concentrations. However, the interpretation of the signatures has tended to lag behind the acquisition of the data, such that empirical correlations between elements are attributed to processes (e.g. garnet in the source, crystallisation of plagioclase, island-arcs) with minimal understanding of the fundamental chemical controls responsible for the behaviour. The next quantum leap will be to understand the chemical origin of these signatures. To do this, we need to determine the oxidation state of trace elements over a range of geological variables. The oxidation state defines the charge and size of a trace element and hence its suitability for occupying a lattice site in mineral phases. This suitability controls the crystal-melt partitioning of the element during partial melting and fractional crystallisation, leading to trace element signatures. The oxidation state ratio is influenced by, in particular, the oxygen fugacity, but also the temperature and pressure of the melt, and is thus a powerful potential indicator of magmatic processes. However, the determination of oxidation states is not easy. There has been huge interest in, and effort devoted to, determining the oxidation state ratio of Fe (the most abundant redox variable element). Other elements have been almost impossible to study due to the interfering effect of Fe in most analytical (both wet chemical and spectroscopic) techniques. This is emphasised by statements in the literature such as 'There is no known method to measure the oxidation state of Cr in melts containing Fe'. A relatively new method for determining oxidation states is X-ray Absorption Near Edge Structure (XANES) spectroscopy, undertaken at a synchrotron light source. XANES spectroscopy is an element specific technique, with the capability of sub-micron spatial resolution, and which is suitable to in situ studies of silicate melts. This is important since the redox states that exist in melts at high temperature, and which control the partitioning and geochemical behaviour of an element, are not necessarily retained on cooling due to charge transfer reactions with the large redox variable reservoir of Fe. For example, approximately half the Cr in a mid-ocean ridge basalt (MORB) at 1400 C is Cr(II) even though this oxidation state has never been identified in a terrestrial material (Cr(II) in the presence of Fe(III) is 'unquenchable'). For this research a furnace has been designed that allows XANES spectra to be recorded for almost any element in a melt at temperatures up to 1500 C as a function of the oxygen fugacity. The furnace allows melts to be studied in situ. This work will allow the oxidation state ratio of almost any element in a melt of natural composition compositions to be determined for the first time.