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NanoSIMS determination of the water content of staurolite

Abstract : Staurolite is an important mineral that can reveal much about metamorphic processes. For instance, it dominates the Fe-Mg exchange reactions in amphibolite-facies rocks between about 550 and 700 °C, and can be also found at suprasolidus conditions. Staurolite contains a variable amount of OH in its structure, whose determination is a key petrological parameter. However, staurolite is often compositionally zoned, fine-grained, and may contain abundant inclusions. This makes conventional water analysis (e.g. FTIR or by chemical titration) unsuitable. With its high sensitivity at high spatial resolution, NanoSIMS is potentially a valuable tool for determining water contents in staurolite. However; a calibration with relevant standards covering a large range of water content is required to obtain accurate and reliable analyses, because matrix effects typically prevent direct quantification of water content by SIMS techniques. METHODS In this study, a calibration for NanoSIMS analyses of water content by using minerals with crystallographic structures comparable to that of staurolite (i.e. amphibole and kyanite, an inosilicate and a nesosilicate respectively) has been developed. RESULTS Water measurements in an inclusion-free crystal from Pizzo Forno, Ticino, Switzerland, by FTIR (1.56±0.14 wt% H2O) and by ERDA (1.58±0.15 wt% H2O) are consistent with NanoSIMS results (1.56±0.04 wt% H2O). CONCLUSIONS This implies that our approach can accurately account for NanoSIMS matrix effects in the case of staurolite. With this calibration, it is now possible to investigate variation of water content at microscale in metamorphic minerals exhibiting high spatial variability and/or very small size (few micrometers).
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https://hal-cea.archives-ouvertes.fr/cea-03693183
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Submitted on : Friday, June 10, 2022 - 12:11:34 PM
Last modification on : Saturday, June 25, 2022 - 3:34:03 AM

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Samantha Azevedo-Vannson, Laurent Remusat, Hélène Bureau, Keevin Béneut, Bernardo Cesare, et al.. NanoSIMS determination of the water content of staurolite. Rapid Communications in Mass Spectrometry, Wiley, 2022, pp.e9331. ⟨10.1002/rcm.9331⟩. ⟨cea-03693183⟩

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