PRELIMINARY EXPERIMENTAL RESULTS FOR OXYGEN DIFFUSION IN TITANITE UNDER ANHYDROUS CONDITIONS

     Oxygen diffusion in natural titanite was characterized under dry conditions at 1 atm pressure over the temperature range of 850-1050° C at QFM-buffered conditions. The oxygen-diffusion source was crystalline Si¹⁸O₂ powder synthesized by reacting silicon metal powder with ¹⁸O enriched water. 3-mm diameter Ag-Pd capsules were partitioned into two chambers by strips of Ag-Pd. The bottom chamber contained the QFM buffer and in the upper chamber a pre-annealed, gem-quality natural titanite slab (cut perpendicular to c axis) packed in the diffusant source. The capsules were welded on both ends and sealed under vacuum in a silica glass tube. The diffusion annealing times ranged from 1 hour to 8 weeks. The run products were cleaned in distilled water and ethanol in an ultrasonic cleaner. The ¹⁸O diffusive profiles were measured using the nuclear reaction ¹⁸O(p,a )¹⁵N.

     Data obtained thus far give the following Arrehenius relation for diffusion parallel to c:

     D _Oxygen=7.2 X 10 ^–2 exp (-369KJmol^-1/RT) m²/sec

     From a comparison of our data with the wet diffusion data of Morishita et al. (1996) at 700-900° C and 100MPa water pressure, it suggests  that oxygen  may diffuse faster under dry conditions which is unusual (the reverse is true for all silicates investigated to date). The reason for this behavior is not yet clear, but ongoing and future experiments should shed light on this interesting phenomena.