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.

对地球材料的高质量化学分析主要在固定地点实验室进行，并涉及冗长的样品制备程序。 然而，仪器仪表方面的最新技术进步有望使地球化学实践发生根本性转变。可靠、高精度、现场便携式分析仪器的出现正在开启一个新时代，越来越多的来自火山和其他岩石的数据将在现场实时或接近实时地获得（例如，可以在野外大本营建立功能齐全的实验室），并且可以更便宜、更快速地获得高质量的岩石分析。 该项目将部署便携式光谱仪来解决火山学中的一个突出问题，该问题由于需要大量数据点而无法进行传统地球化学分析，即超级喷发期间岩浆的行为。 自 20 世纪 80 年代以来，人们建立了许多大型火山喷发的模型和模拟，但它们的预测价值在很大程度上未经检验，因为过去超级喷发产物的数据严重不足。 在传统实验室中分析足够数量的样品不仅需要很长时间而且成本也太高。 该项目将成为技术、方法和实用性的测试案例，将单次超级喷发产物的化学分析数量比当前实践增加一个数量级或更多（数千个而不是数十到数百个样本） 。 由于缺乏地球化学数据，了解分区硅质岩浆系统的成分结构受到阻碍。 问题不在于确定喷发产物中存在的成分范围，也不在于调查岩浆的最终起源；而是在于确定喷发产物中存在的成分范围。现有的数据集可能已经足以解决这些问题。 相反，问题在于分析多个地点的足够样本，以获得凝灰岩成分结构的统计上有效的图像。 所需的分析数量太大 (1,000)，无法使用波长色散 X 射线荧光等常规方法来完成，并且在任何情况下，只要存在的成分范围满足，就不需要对每个样品进行完整分析。是通过传统方法在较少数量的样本（~100）上建立的。 便携式 X 射线荧光 (PXRF) 技术已发展到非常适合此类研究的程度，因为对于某些元素，尤其是关键微量元素 Rb、Sr、Y、Zr、Nb 和其他一些元素，通常表现出由于分区流纹岩凝灰岩变化较大，精度和准确度接近全尺寸波长色散 XRF。 使用 PXRF，可以在从现场工作开始的两年研究项目过程中以非常低的成本获得数千次分析。 该项目将购买一台 PXRF 仪器，并通过对新墨西哥州巴莱斯火山口班德利尔凝灰岩 1.61 Ma Otowi 岩层进行案例研究，开发分区火凝灰岩的方法。 在现有数据的基础上，充分理解了该单元的整体成分变化，使得一些元素能够用作整个成分的代理。 该研究将重点关注不相容元素 Zn、Rb、Y 和 Nb，这些元素从早期喷发凝灰岩到晚期喷发凝灰岩表现出 3 倍到 4 倍的变化，并且存在足够的浓度，以便能够通过 PXRF 进行高精度测定。 有了足够大的数据集，凝灰岩“样本”（例如，在单个位置 1 m 垂直范围内收集的许多浮石碎屑）可以通过其中的成分分布来描述，而不是通过单个数据点来描述，如下所示目前情况就是如此。 这将为根据喷发和沉积过程解释凝灰岩的成分模式提供基础，目标是“将岩浆放回岩浆室”，以得出比使用岩浆岩浆更受定量限制的分区模型。常规数据集。