Combined ¹³C-D and D-D clumping in methane: Methods and preliminary results

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• 作者：D.A. Stolper ; A.L. Sessions ; A.A. Ferreira ; E.V. Santos Neto ; A. Schimmelmann ; S.S. Shusta ; D.L. Valentine ; J.M. Eiler
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：1 February, 2014
• 年：2014
• 卷：126
• 期：Complete
• 页码：169-191
• 全文大小：1568 K

文摘

The stable isotopic composition of methane (e.g., 未D and 未¹³C values) is often used as a tracer for its sources and sinks. Conventional 未D and 未¹³C measurements represent the average isotope ratios of all ten isotopologues of methane, though they are effectively controlled by the relative abundances of the three most abundant species: ¹²CH₄, ¹³CH₄, and ¹²CH₃D. The precise relative abundances of the other seven isotopologues remains largely unexplored because these species contain multiple rare isotopes and are thus rare themselves. These multiply substituted (or 鈥榗lumped鈥? isotopologues each have their own distinctive chemical and physical properties, which could provide additional constraints on the geochemistry of methane. This work focuses on quantifying the abundances of two rare isotopologues, ¹³CH₃D and ¹²CH₂D₂, of methane in order to assess their utility as a window into methane鈥檚 geochemistry. Specifically, we seek to assess whether clumped isotope distributions might be useful to quantify the temperature at which methane formed and/or equilibrated. To this end, we report the first highly precise combined measurements of the relative abundances of ¹³CH₃D and ¹²CH₂D₂ at natural abundances (i.e., unlabeled) via the high-resolution magnetic-sector mass spectrometry of intact methane. We calibrate the use of these measurements as a geothermometer using both theory and experiment, and apply this geothermometer to representative natural samples. The method yields accurate average (i.e., bulk) isotopic ratios based on comparison with conventional techniques. We demonstrate the accuracy and precision of measurements of ¹³CH₃D and ¹²CH₂D₂ through analyses of methane driven to high temperature (>200 掳C) equilibrium in the laboratory. Application of this thermometer to natural samples yields apparent temperatures consistent with their known formation environments and appears to distinguish between biogenic and thermogenic methane.