Generation of Large Geochemical Data Sets for Single Units of Volcanic Rock: Application of Portable XRF Spectrometry to Zoned Ignimbrites

生成单个火山岩单元的大型地球化学数据集：便携式 XRF 光谱测量在分区熔凝灰岩中的应用

• 批准号: 1145127
• 负责人: John Wolff
• 金额: $ 21.73万
• 依托单位: Washington State 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: 2012-01-01 至 2014-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1145127&HistoricalAwards=false
• 关键词: Generation; Large; Geochemical; Data; Sets

High-quality chemical analyses of Earth materials are dominantly conducted in fixed-site laboratories and involve lengthy sample preparation procedures. However, recent technical advances in instrumentation hold the promise of a fundamental shift in geochemical practice. The advent of reliable, high-precision, field-portable analytical instruments is opening a new era in which an increasing proportion of data from volcanic and other rocks will be obtained in the field in real time or near-real time (for example, a fully functional lab could be set up at a field base camp), and high-quality rock analyses can be obtained ever more cheaply and quickly. This project will deploy a portable spectrometer to address an outstanding problem in volcanology that has defied conventional geochemical analysis due to the large number of data points needed, namely the behavior of magma during super-eruptions. Since the 1980s, numerous models and simulations of large volcanic eruptions have been constructed, but their predictive value is largely untested because there is a critical shortfall of data from the products of past super-eruptions. Analysis of a sufficient number of samples in conventional labs simply takes too long and is too expensive. This project will be a test case for the technology, methodology and utility of increasing the number of chemical analyses from the products of a single super-eruption by an order of magnitude or more (thousands rather than tens to hundreds of samples) over current practice. Understanding the compositional structure of zoned silicic magma systems is hampered by a lack of geochemical data. The problem is not that of establishing the range of compositions that are present among eruptive products, nor of investigating the ultimate origins of the magmas; existing data sets are probably already sufficient to address those questions. Rather, the issue is one of analyzing enough samples at numerous locations in order to obtain a statistically valid picture of the tuff?s compositional architecture. The number of analyses required is too large (1,000) to feasibly accomplish using conventional methods such as wavelength-dispersive X-ray fluoresence, and in any case, a complete analysis of each sample is not needed as long as the range of compositions present has been established by conventional methods on a smaller number of samples (~100). Portable X-ray fluorescence (PXRF) technology has advanced to the point where it is ideally suited to such an investigation, because for some elements, especially the critical trace elements Rb, Sr, Y, Zr, Nb and a few others which typically exhibit large variations in zoned rhyolitic tuffs, the precision and accuracy approaches that of full-size wavelength-dispersive XRF. Using PXRF, thousands of analyses can be obtained at very low cost during the course of a two-year research project, starting during fieldwork. This project will acquire a PXRF instrument and develop methods for zoned ignimbrites by carrying out a case study on the 1.61 Ma Otowi Member of the Bandelier Tuff, Valles caldera, NM. On the basis of existing data, overall compositional variations in this unit are sufficiently understood to enable a few elements to be used as proxies for the whole composition. The research will focus on the incompatible elements Zn, Rb, Y and Nb, which exhibit 3-fold to 4-fold variations from early-erupted to late-erupted tuff, and are present at sufficient concentrations to enable high-precision determinations by PXRF. With a large enough data set, a 'sample' of tuff (e.g. many pumice clasts collected from within a 1 m vertical range at a single location) can be described by the distribution of compositions within it, rather than by a single data point as is currently the case. This will then provide a basis for interpreting compositional patterns in the tuff in terms of eruptive and depositional processes, with the goal of 'putting the magma back into the chamber' to arrive at a model of zoning that is more quantitatively constrained than is possible with a conventional data set.

地球材料的高质量化学分析主要是在固定位置实验室中进行的，并涉及长时间的样品制备程序。 但是，仪器的最新技术进步具有地球化学实践基本转变的希望。可靠，高精度，可携带的分析仪器的出现正在开放一个新时代，在该时代中，来自火山和其他岩石的数据的比例越来越多，可以实时或接近现场的时间获得（例如，可以在野外基地营地设置一个功能齐全的实验室），并且可以获得高质量的岩石分析，可以得到更便宜和更便宜的。 该项目将部署便携式光谱仪，以解决火山学上的一个鲜明问题，该问题由于所需的数据点数量大量而违反了常规的地球化学分析，即岩浆在超级爆发过程中的行为。 自1980年代以来，已经构建了大型火山喷发的许多模型和模拟，但是它们的预测价值在很大程度上未经测试，因为从过去的超级爆发的产物中存在严重的数据短缺。 分析传统实验室中足够数量的样本的时间太长，而且太贵了。 该项目将是用于增加单个超级爆发产物的技术，方法和实用性的测试案例。 缺乏地球化学数据，了解分区硅质岩浆系统的组成结构受到阻碍。 问题不是要建立喷发产品中存在的一系列成分，也不是要研究岩浆的最终起源。现有的数据集可能已经足够解决这些问题。 相反，问题是在众多位置分析足够的样本，以获取凝灰岩构图架构的统计有效图片。 所需的分析数量太大（1,000），无法使用常规方法（例如波长 - 分散性X射线荧光）可行地完成，无论如何，只要通过常规方法建立了对较小的样品（〜100）建立对每个样品的完整分析，只要已经通过常规方法确定了存在的组成范围。 便携式X射线荧光（PXRF）技术已提高到理想情况下适合进行此类研究的地步，因为对于某些元素，尤其是关键的微量元素RB，SR，SR，Y，ZR，NB和其他一些通常表现出很大的变化，通常在划分的节奏性节奏tuffs和精确度中，精确的精度和精度是完整的wavelffff。 使用PXRF，在两年的研究项目过程中，从实地调查开始，可以以非常低的成本来获得数千个分析。 该项目将通过对新墨西哥州Valles Caldera的Bandelier Tuff成员进行1.61 Ma Otowi成员进行案例研究，从而获取PXRF仪器并开发针对Ignimbrites划分的方法。 根据现有数据，对本单元中的总体组成变化进行了充分的理解，以使一些元素可以用作整个组成的代理。 该研究将重点放在不兼容的元素Zn，RB，Y和NB上，它们表现出从早期喷发到晚期爆发的Tuff的3至4倍变化，并以足够的浓度存在以实现PXRF的高精度测定。 有了足够大的数据集，可以通过其中的组合物的分布来描述凝灰岩的“样本”（例如，从单个位置的垂直范围内收集的许多Pumice碎屑）可以描述，而不是像目前的情况一样通过单个数据点来描述。 然后，这将提供一个根据喷发和沉积过程来解释凝灰岩中的组成模式的基础，其目的是“将岩浆重新放回室内”，以达到一个分区模型，该模型比常规数据集更具定量约束。