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

对地球材料的高质量化学分析主要在固定地点的实验室进行，涉及冗长的样品准备程序。然而，最近仪器技术的进步预示着地球化学实践将发生根本性转变。可靠、高精度、现场便携的分析仪器的出现开启了一个新时代，在这个时代，越来越多的火山岩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一个功能齐全的实验室)，而且可以更便宜、更快地获得高质量的岩石分析。该项目将部署一台便携式光谱仪，以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，即超级喷发期间岩浆的行为，而违反了传统的地球化学分析。自20世纪80年代以来，已经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品的数据严重不足。在传统实验室中对足够数量的样本进行分析简直是耗时太长，成本太高。该项目将是一项技术、方法和效用的测试案例，可以在目前的做法基础上，将一次超级喷发产物的化学分析数量增加一个数量级或更多(数千个样本，而不是几十到数百个样本)。由于缺乏地球化学数据，对分带硅质岩浆系统的成分结构的了解受到阻碍。问题不是确定喷发产物中存在的成分范围，也不是调查岩浆的最终来源；现有的数据集可能已经足以解决这些问题。相反，问题是要在众多地点分析足够的样本，以获得凝灰岩-S构成建筑的统计有效图片。所需的分析次数太多(1,000次)，无法使用波长色散X射线荧光等常规方法完成，而且无论如何，只要通过传统方法在较少数量的样本(~100)上建立了现有的组成范围，就不需要对每个样本进行完整的分析。便携式X射线荧光(PXRF)技术已经发展到非常适合这种研究的程度，因为对于某些元素，特别是关键微量元素Rb、Sr、Y、Zr、Nb和其他几个典型地在流纹岩凝灰岩中表现出很大变化的元素，其精密度和准确度接近全尺寸波长色散XRF。使用pXRF，在为期两年的研究项目期间，从实地考察开始，可以以非常低的成本获得数千种分析。该项目将获得一台pXRF仪器，并通过对新墨西哥州Valles Caldera的Bandelier凝灰岩1.61 Ma Oowi段进行案例研究，开发分带火辉石的方法。在现有数据的基础上，充分了解了该单元中的总体成分变化，以便能够将少数元素用作整个组合物的替代物。这项研究将集中于不相容元素Zn、Rb、Y和Nb，它们从早喷发到晚喷发的凝灰岩表现出3到4倍的变化，并且存在足够的浓度，使pXRF能够进行高精度的测定。有了足够大的数据集，凝灰岩(例如，在单个地点从1米垂直范围内收集的许多浮石碎屑)的“样本”可以通过其内部成分的分布来描述，而不是像目前的情况那样通过单个数据点来描述。这将为根据喷发和沉积过程解释凝灰岩中的成分模式提供基础，目标是将岩浆重新放入洞穴中，以得出一个比传统数据集更受数量限制的分带模型。

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.