Methane Clumped Isotopes: Progress and Potential for a New Isotopic Tracer

The isotopic composition of methane is of longstanding geochemical interest, with important implications for understanding petroleum systems, atmospheric greenhouse gas concentrations, the global carbon cycle, and life in extreme environments. Recent analytical developments focusing on multiply substituted isotopologues (‘clumped isotopes’) are opening a valuable new window into methane geochemistry. When methane forms in internal isotopic equilibrium, clumped isotopes can provide a direct record of formation temperature, making this property particularly valuable for identifying different methane origins. However, it has also become clear that in certain settings methane clumped isotope measurements record kinetic rather than equilibrium isotope effects. Here we present a substantially expanded dataset of methane clumped isotope analyses, and provide a synthesis of the current interpretive framework for this parameter. In general, clumped isotope measurements indicate plausible formation temperatures for abiotic, thermogenic, and microbial methane in many geological environments, which is encouraging for the further development of this measurement as a geothermometer, and as a tracer for the source of natural gas reservoirs and emissions. We also highlight, however, instances where clumped isotope derived temperatures are higher than expected, and discuss possible factors that could distort equilibrium formation temperature signals. In microbial methane from freshwater ecosystems, in particular, clumped isotope values appear to be controlled by kinetic effects, and may ultimately be useful to study methanogen metabolism

InterCarb: a community effort to improve interlaboratory standardization of the carbonate clumped isotope thermometer using carbonate standards

Increased use and improved methodology of carbonate clumped isotope thermometry has greatly enhanced our ability to interrogate a suite of Earth-system processes. However, interlaboratory discrepancies in quantifying carbonate clumped isotope (Δ47) measurements persist, and their specific sources remain unclear. To address interlaboratory differences, we first provide consensus values from the clumped isotope community for four carbonate standards relative to heated and equilibrated gases with 1,819 individual analyses from 10 laboratories. Then we analyzed the four carbonate standards along with three additional standards, spanning a broad range of δ47 and Δ47 values, for a total of 5,329 analyses on 25 individual mass spectrometers from 22 different laboratories. Treating three of the materials as known standards and the other four as unknowns, we find that the use of carbonate reference materials is a robust method for standardization that yields interlaboratory discrepancies entirely consistent with intralaboratory analytical uncertainties. Carbonate reference materials, along with measurement and data processing practices described herein, provide the carbonate clumped isotope community with a robust approach to achieve interlaboratory agreement as we continue to use and improve this powerful geochemical tool. We propose that carbonate clumped isotope data normalized to the carbonate reference materials described in this publication should be reported as Δ47 (I-CDES) values for Intercarb-Carbon Dioxide Equilibrium Scale

Implications of Pt Crucibles-H2O Vapour Interaction on Past DeltaD Measurements in Silicate Glasses and Minerals

The extraction of water from igneous rocks and minerals is classically achieved by induction heating of a platinum alloy crucible where the sample has been deposited. Here, we show that chemical interaction between water and Pt-10%Rh crucibles occurs at high temperature. Known amounts of water were reacted with a Pt crucible held at high temperatures (900-1300oC) for 5 to 10 minutes and then recovered. The experiments show that in average 18% of the water was lost to the crucible during the reaction, and that the isotopic composition of the remaining water was shifted by up to 25‰ . From 20 to 50% of the lost water was recoverable by re-heating the crucible at 1300oC. Repeated experiments using the same standard water on the crucible showed a decrease of the isotopic shift to only 2‰ . This is compatible with a memory effect of the Pt-10%Rh crucible. We propose that a large amount (at least several tens of {μ }mol) of water remains trapped in or at the surface of the crucible and that isotopic exchange between trapped and introduced water affects subsequent isotopic composition of injected water. We conclude that the use of Pt alloys, as crucibles or foils, to extract water from rocks or minerals should be avoided. The interaction highlighted in this study shed light on previously inconsistent observations made on several mantle-derived samples. This effect could potentially explain the very low δ D ( ∼-110‰ ) measured by Bell and Ihinger (2000) in anhydrous mantle minerals with low water concentrations. Moreover, 14 basaltic glasses previously measured using Pt or Pt alloys crucibles were re-analysed using externally heated silica tubes, yielding δ D heavier by about 15‰ and suggesting a δ D for the source of N-MORB closer to ∼-60‰ rather than -80‰

An isotopically distinct hydrogen reservoir in the South Pacific mantle

No abstract available