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

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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.