Carbon isotope fractionation during diamond growth in depleted peridotite: Counterintuitive insights from modelling water-maximum CHO fluids as multi-component systems

Application of our model to a recently published set of in-situ carbon isotope analyses for peridotitic diamonds from Marange, Zimbabwe (Smit et al., 2016), which contain CH4 fluid inclusions, allows us to perfectly match the observed co-variations in δ13C, δ15N and N content and at the same time explain the previously counter-intuitive observation of progressive C13 enrichment in diamonds that appear to have grown from a fluid with methane as the dominant carbon species. Similarly, the almost complete absence in the published record of progressive C13 depletion trends within diamonds likely reflects ubiquitous precipitation from CH4- and CO2-bearing water-rich fluids, rather than diamond formation exclusively by carbonate-bearing and CH4-free oxidized fluids or melts.