Ti diffusion in zircon

Chemical diffusion of Ti under anhydrous conditions at I atm and under fluid-present elevated pressure (1.1-1.2 GPa) conditions has been measured in natural zircon. The source of diffusant for 1-atm experiments was a ZrO2-TiO2-zircon mixture, with experiments run in crimped Pt capsules. Diffusion experiments done in the presence of H2O-CO2 fluid were run in a piston-cylinder apparatus, using a source of ground TiO2, ZrSiO4 and SiO2, with oxalic acid added to produce H2O-CO2 vapor and partially melt the solid source material, yielding an assemblage of rutile + zircon + melt + vapor. Nuclear reaction analysis (NRA) with the resonant nuclear reaction Ti-48(p,gamma)V-49 was used to measure diffusion profiles for both sets of experiments. The following Arrhenius parameters were obtained for Ti diffusion nonnal to cover the temperature range 1350-1550 degrees C at 1 atm: D-Ti = 3.33 x 10(2)exp (-754 +/- 56 U mol(-1)/RT) m(2)S(-1). \Ti diffusivities were found to be similar for experiments run under fluid-present conditions. A fit to all of the data yields the Arrhenius relation D = 1.34 x 10(2) exp(- 741 +/- 46 kJ mol(-1)/RT) m(2) s(-1). These data suggest that zircon should be extremely retentive of Ti chemical signatures, indicating that the recently-developed Ti-in-zircon crystallization geothennometer [Watson, E.B., Harrison, T.M., 2005. Zircon thermometer reveals minimum melting conditions on earliest Earth. Science 308, 841-844] will be quite robust in preserving temperatures of zircon crystallization. Titanium diffuses somewhat faster in zircon than larger tetravalent cations U, Th, and Hf, but considerably more slowly than Ph, the REE, and oxygen; hence Ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost. (c) 2007 Elsevier B.V. All rights reserved.