Diffusive relaxation of carbon and nitrogen isotope heterogeneity in diamond: a new thermochronometer

The spatial distribution of carbon and nitrogen isotopes in diamonds provides information on mantle residence time. Diamonds with long residence at high temperature will gradually lose their initial zoning patterns due to diffusion. Using experimentally determined carbon self-diffusion coefficients and nitrogen diffusion coefficients derived from aggregation experiments, we have modeled the diffusive relaxation of zoning profiles with a spectrum of wavelengths. Carbon isotope heterogeneity will be preserved on wavelengths greater than ∼1 μm after 1 billion years’ residence at 1400 K, and on wavelengths greater than ∼200 μm after 1 million years’ residence at 2000 K. Nitrogen isotope zoning is relaxed much more slowly, with 0.1 μm zoning preserved over the age of the Earth at 1400 K and 1 μm zoning preserved after 1 million years at 2000 K. The large difference in diffusive relaxation times between carbon and nitrogen isotopes means that initially correlated carbon and nitrogen profiles will lose their correlation after sufficient diffusion of carbon has taken place. This could be used as a thermochronometer. Carbon isotope heterogeneity in diamonds associated with lower mantle mineral assemblages has significantly smaller amplitude than nitrogen isotope heterogeneity, consistent with diffusive relaxation at high temperatures in the lower mantle.