Application of Diffusion Data in Carbonates to Estimate Timescales and Conditions of Texture Forming Processes

Diffusive exchange of elements between minerals is undoubtedly one of the major processes controlling mass transport in the Earth’s interior, and contributes to a wide range of phenomena, including inter-mineral reactions, crystal growth and texture formation. Chemical zoning within minerals is a commonly observed feature indicating that a crystal was not able to homogenize its composition as a function of external conditions such as pressure and temperature as dictated by thermodynamics. Detailed knowledge of the rates and mechanism of diffusive processes can be used to determine timescales and P-T conditions of geological events, even if compositional equilibrium was not reached at the scale of interest. Carbonates are very promising candidates to study these kinetically controlled processes as they are major constituents of terrestrial and planetary bodies. Recently presented data for diffusion of divalent cations in carbonates (Mueller et al., 2010) revealed surprisingly low activation energies for temperatures below 500°C (< 100 kJ/mol) and suggest low temperatures for diffusive closure of carbonates. For example, calculated closure temperatures for a grain with a radius of 100µm and a monotonic cooling rate of 10°C/Ma can be as low as 230°C for Fe-Mg exchange and about 200°C for the exchange of Mn-Mg. Hence, the combination of low activation energies together with slow diffusivities potentially makes Fe and Mn profiles suitable tracer of texture formation processes and allows constraining timescales of metamorphic events even at relatively low temperatures. Our data may also bear on retention of heavy stable isotope biosignatures and indicators of paleo-redox conditions. We present examples of natural compositional zoning and numerical modeling to show that compositional zoning in carbonates can be used to decipher information of kinetically controlled processes such as mineral growth, time-temperature-paths or estimates on the duration of texture-forming processes and isotope retention in carbonate crystals on terrestrial and planetary bodies. Mueller, T.; Cherniak, D.; Watson, E.B. (2010): Experimentally determined interdiffusion data of divalent cations in carbonates. Supplement to Geochimica Cosmochimica Acta, v.74, p.733.