The geochemistry of redox variable elements

该项目旨在使用同步加速器光来确定高温下岩浆中金属的氧化状态(或电荷)。这项工作的动机是希望通过正确解释微量元素特征的地球化学来源来了解火成岩过程。火成岩岩石学是一门成熟的学科，最近随着能够准确测定微量元素浓度的技术(如激光消融-电感耦合等离子体质谱)的出现，这一学科得到了显着的发展。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关联被归因于过程(例如，来源中的石榴石、斜长石的结晶、岛弧)，而对导致行为的基本化学控制知之甚少。下一个重大飞跃将是理解这些签名的化学来源。要做到这一点，我们需要确定一系列地质变量中微量元素的氧化状态。氧化态定义了微量元素的电荷和大小，从而决定了它在矿物相中占据晶格位置的适宜性。这种适宜性在部分熔化和分步结晶过程中控制元素的晶体熔体分配，导致痕量元素特征。氧化状态比不仅受氧逸度的影响，而且还受熔体的温度和压力的影响，因此是岩浆作用的一个强有力的潜在指示器。然而，氧化态的确定并不容易。人们对确定Fe(最丰富的氧化还原可变元素)的氧化态比率一直非常感兴趣，也投入了大量的努力。由于铁在大多数分析(包括湿化学和光谱)技术中的干扰作用，其他元素几乎不可能研究。文献中的陈述强调了这一点，例如“目前还没有已知的方法来测量含铁熔体中铬的氧化态”。一种相对较新的确定氧化态的方法是在同步加速器光源下进行的X射线吸收近边缘结构(XANES)光谱。XANES光谱是一种元素专属技术，具有亚微米级的空间分辨率，适合于硅酸盐熔体的现场研究。这一点很重要，因为高温熔体中存在的、控制元素分配和地球化学行为的氧化还原状态在冷却时不一定会由于与大型氧化还原可变铁库的电荷转移反应而保留。例如，在1400℃的大洋中脊玄武岩(MORB)中，大约一半的铬是铬(II)，尽管这种氧化状态从未在陆地物质中被发现(在存在铁(III)的情况下，铬(II)是‘不可熄灭的’)。为了进行这项研究，设计了一种熔炉，它可以在温度高达1500摄氏度的情况下记录熔体中几乎任何元素的XANES光谱，作为氧逸度的函数。熔炉允许对熔体进行现场研究。这项工作将首次确定天然组合物熔体中几乎所有元素的氧化态比率。

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.