APPLICATION OF INTERDIFFUSION DATA IN CARBONATES TO DECIPHER KINETICALLY CONTROLLED PROCESSES OF TEXTURE FORMATION

Carbonates appear as very promising candidates to study heat and mass transport during metamorphism or texture formation in general as they are major constituents of the Earth’s crust and even planetary bodies such as Mars. Well defined element and isotope partitioning between various carbonate phases can be used for geothermometry assuming that equilibrium was reached at the scale of interest. However, natural carbonate crystals often exhibit distinct compositional zoning indicating the occurrence of kinetically controlled non-equilibrium processes, such as diffusive element exchange.

Recently presented experimental data for interdiffusion of divalent cations in carbonates revealed surprisingly low activation energies (< 100 kJ/mol) at temperatures below 500°C (Mueller et al. 2010). As a result, temperatures calculated using the well known Dodson equation for diffusive closure of grain with a radius of 100µm and a monotonic cooling rate of 10°C/Ma can be as low as 230°C for the Fe-Mg exchange and about 200°C for the exchange of Mn-Mg, respectively. These low closure temperatures together with relatively slow diffusivities make compositional profiles of Mn and Fe suitable to record processes of texture formation at relatively low temperatures. Moreover, knowledge of exact diffusion parameters in carbonates allows constraining rates and durations of metamorphic events if additional information on the time-temperature path is available.

Taken together, solid solutions of carbonates are not only useful to determine temperatures using phase equilibria. Their compositional zoning pattern potentially record information of kinetically controlled processes such as mineral growth, time-temperature-path or estimates on the duration of texture forming processes 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.