High-precision measurement and standard calibration of triple oxygen isotopic compositions (δ¹⁸O, Δ^′17O) of sulfate by F₂ laser fluorination

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• 作者：Benjamin R. Cowie ; David T. Johnston ; ^{johnston@eps.harvard.edu" class="auth_mail" title="E-mail the corresponding author}
• 关键词：Triple oxygen isotopes ; Sulfate isotope standards ; Fluorination methods
• 刊名：Chemical Geology
• 出版年：2016
• 出版时间：15 November 2016
• 年：2016
• 卷：440
• 期：Complete
• 页码：50-59
• 全文大小：790 K

文摘

A new approach for measuring the triple oxygen isotope composition of sulfate minerals was developed using fluorine gas and an infrared laser to generate O₂. A correction for the mass-dependent isotope effect observed during barite fluorination was rigorously calibrated. Analyte gas purification was performed via numerous cryofocus steps and the introduction of an in-line gas chromatograph. As with previous methods, our fluorination technique still requires pairing with independent δ¹⁸O measurements made by TC/EA conversion to carbon monoxide, but leads to higher yields than BrF₅-based methods. We first calibrated our method against known silicate standards (UWG-2, SCO, NBS-28). Following from this, we report high-precision triple oxygen isotope ratios for international sulfate standards (IAEA-SO-5, IAEA-S O-6, NBS-127), and an internal laboratory standard, JMG. Replicate analyses of JMG yielded a Δ^′17O value of  −0.057 ± 0.004‰ (standard error), demonstrating per-meg level precision that approaches instrumental limits. We provide δ¹⁷O and δ¹⁸O values that can be translated into any preferred reference frame for comparison with gases, water, or rocks/minerals. Quantification of differences in triple oxygen isotope composition of the standard reference materials further enables application of this measure and approach on environmental and geological materials.