page_title: Methane clumped isotopes
section_title: Equilibrium thermodynamics
hierarchical_section_title: Methane clumped isotopes / Physical chemistry / Equilibrium thermodynamics
caption_attribution_description: English: The equilibrium distribution of Δ12CH2D2 as a monotonic function of temperature. Redrawn from Young et al., 2017.
context_page_description: Methane clumped isotopes are methane molecules that contain two or more rare isotopes. Methane contains two elements, carbon and hydrogen, each of which has two stable isotopes. For carbon, 98.9% are in the form of carbon-12 and 1.1% are carbon-13; while for hydrogen, 99.99% are in the form of protium and 0.01% are deuterium. Carbon-13 and deuterium are rare isotopes in methane molecules. The abundance of the clumped isotopes provides information independent from the traditional carbon or hydrogen isotope composition of methane molecules.
context_section_description: When formed or re-equilibrated in reversible reactions, methane molecules can exchange isotopes with each other or with other substances present, such as H₂O, H₂ and CO₂, and reach internal isotopic equilibrium. As a result, clumped isotopologues are enriched relative to the stochastic distribution.  and  values of methane in internal isotopic equilibrium are predicted and verified to vary as monotonic functions of temperature of equilibration as follows:
  Δ values are in permil (‰).
  Similar relationship also applies to :
  Based on these correlations, ,  and  can be used as a geothermometer to indicate the formation temperature of methane (,  and ). And the correlation of  and can help to determine whether methane is formed in internal isotopic equilibrium.
caption: The equilibrium distribution of 
    
      
        
          
            Δ
            
              
                
                
                  12
                
              
              C
              
                H
                
                  2
                
              
              
                D
                
                  2
                
              
            
          
        
      
      {\displaystyle \Delta _{^{12}CH_{2}D_{2}}}
    
   as a monotonic function of temperature. Redrawn from Young et al., 2017.
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