Carbon isotope heterogeneity in metamorphic diamond from the Kokchetav UHP dolomite marble, northern Kazakhstan

We analysed isotopic compositions of metamorphic microdiamond secondary ion mass spectrometry. Typical microdiamonds in this dolomite marble show star-shaped morphologies (S-type) consisting of single-crystal cores and polycrystalline rims. Four S-type microdiamonds and two R-type microdiamonds (single crystals with rugged surfaces) were analysed using a 5 μm diameter ion beam. S-type microdiamonds have heterogeneous carbon isotopic compositions even in a single grain. Analysis of a typical S-type microdiamond (no. xx01-1-13) revealed clear difference in δ¹³C between core and rim. The rim shows lighter isotopic compositions ranging from -17.2% to -26.9%, whereas the core is much heavier, with δ¹³C ranging from -9.3% to -13.0%. The δ¹³C values of R-type microdiamonds fall into narrow ranges from -8.3% to -14.9% for no. xx01-1-10 and from -8.3% to -15.3% for no. xx01-1-16. These δ¹³C values are similar to those of the S-type microdiamond cores. The R-type probably formed at the same stage as the core of the S-type, whereas rim growth at a second stage did not occur or occurred very weakly in R-type microdiamonds. These carbon isotopic data support the two-stage growth of microdiamonds in the Kokchetav ultrahigh-pressure host rock. To explain the second stage growth of S-type microdiamonds, we postulate a simple fluid infiltration of light carbon from neighbouring gneisses into the dolomite marble.