Solving real-world analytical challenges: Developing SIMS reference materials for quantifying hydrogen contents of orthopyroxene

氢，俗称水，是一种重要的微量物质，在所谓的名义上无水矿物的晶体结构中以羟基的形式存在。地球地幔矿物（如辉石和橄榄石）中水的存在对地球地幔的物理性质有不成比例的巨大影响。因此，对地幔流变学的全面了解是建立在对地幔内水的含量和分布的详细了解的基础上的。因此，需要所有可能的地幔环境的定量数据，但这些数据几乎完全局限于从次大陆地幔中恢复的样品。这项提案的首席研究员和她的同事们只提供了一个非常有限的海底橄榄岩数据集。这种明显的取样偏差可以直接归因于样品质量：次大陆捕虏体通常产生高质量的物质，而海底和高山型地幔岩石通常性质非常复杂。Erlangen小组已经成功地利用红外（IR）光谱（标准方法）获得了海洋橄榄岩的第一个水数据，但是该数据集仅限于相对较少的样本。由于强烈的压裂和蚀变，大多数样品的晶体质量较差，有效地阻止了通过红外光谱进行定量测量。这里提出的项目旨在克服红外方法固有的局限性，通过开发二次离子质谱（SIMS）作为一种替代方法来定量复杂，细颗粒材料中的水分含量。我们打算把重点放在正辉石上，它在过去已被证明是大块地幔水的理想替代品。我们需要克服的关键挑战是SIMS需要矩阵匹配的参考材料用于校准。因此，本研究的主要任务是鉴定天然正硅氧烷，这些正硅氧烷是SIMS定量氢的合适校准剂。一旦确定和表征，由此产生的一套参考材料将用于对从海洋和高山型地幔标本中回收的正辉石进行原理验证的SIMS测量。

Hydrogen, colloquially water, is a significant trace substance that is incorporated as hydroxyl groups in the crystal structure of so-called nominally anhydrous minerals. The presence of water in Earth mantle minerals such as pyroxene and olivine has a disproportionally large effect on the physical properties of the Earth mantle. A complete understanding of mantle rheology is therefore based upon a detailed knowledge of the content and distribution of water within the mantle. Quantitative data from all possible mantle environments are therefore needed but such data are almost exclusively confined to samples recovered from the sub-continental mantle. Only a very modest data set, produced by this proposal's principal investigator and her co-workers, exists for sub-oceanic peridotites. This obvious sampling bias can be directly attributed to sample quality: Sub-continental xenoliths generally yield high-quality material, whereas sub-oceanic and alpine-type mantle rocks are often highly complex in nature. The Erlangen group has successfully obtained the first water data for oceanic peridotites using infrared (IR) spectroscopy (being the standard method), however this dataset is confined to a relatively small number of samples. The majority of such samples show poor crystal quality due to intense fracturing and alteration, effectively prohibiting quantitative measurements via IR spectroscopy. The project proposed here is intended to overcome the limitations inherent to the IR method by developing secondary ion mass spectrometry (SIMS) as an alternative approach for quantifying water contents in complex, fine grained materials. Our intended focus is on orthopyroxene, which in the past has proven to be an ideal proxy for bulk mantle water. The key challenge we will need to overcome it that SIMS requires that a matrix-matched reference material be available for calibration. Hence, the main task of the proposed study is to identify natural orthopyroxenes that are well-suited calibrants for hydrogen quantification by SIMS. Once identified and characterized, the resulting suite of reference materials will be used for proof-of-principle SIMS measurements that will be undertaken on orthopyroxene recovered from oceanic and alpine-type mantle specimens.