Combined ^(13)C–D and D–D clumping in methane: Methods and preliminary results

Abstract

The stable isotopic composition of methane (e.g., δD and δ^(13)C values) is often used as a tracer for its sources and sinks. Conventional δD and δ^(13)C 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: (^12)CH_4, (^13)CH_4, and (^12)CH_3D. 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 'clumped') 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, (^13)CH_3D and (^12)CH_2D_2, of methane in order to assess their utility as a window into methane's 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 (^13)CH_3D and (^12)CH_2D_2 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 (^13)CH_3D and (^12)CH_2D_2 through analyses of methane driven to high temperature (> 200 °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.

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