BA DIFFUSION IN FELDSPAR

Abstract

Diffusion of Ba in natural sanidine and plagioclase has been characterized under dry 1 atm conditions. Polished or cleaved sections of the feldspars were surrounded by source powders in Pt capsules and annealed in air. Sources of diffusant consisted of BaO, SiO2 and Al2O3 powders mixed with ground feldspar and prereacted in Pt crucibles at 975°C. Prepared sample capsules were annealed for times ranging from 1 h to a few months at temperatures from 775 to 1124°C. The Ba distributions in the feldspars were profiled by Rutherford Backscattering Spectrometry (RBS).The following Arrhenius relation is obtained for Ba diffusion in sanidine (Or61) for diffusion normal to the (001) cleavage face: Dsan=2.9x10-1 exp(-455 +/- 20 kJ mol-1/RT) m2sec-1. Diffusion normal to (010) appears to be slightly slower than diffusion normal to (001).For diffusion normal to (001) and normal to (010) in labradorite (An67), the Arrhenius relations are, respectively, Dlab#(001)=2.3x10-7 exp(-323+/-20 kJ mol-1/RT) m2sec-1 Dlab#(010)=1.1x10-6 exp(-341+/-23 kJ mol-1/RT) m2sec-1Diffusion in oligoclase (An23) can be described by the following Arrhenius relations: Dolig#(001)=1.7x10-7 exp(-303+/-32 kJ mol-1/RT) m2sec-1 Dolig#(010)=1.1x10-4 exp(-377+/-28 kJ mol-1/RT) m2sec-1Ba diffusion in sanidine is more than an order of magnitude slower than Sr diffusion measured on specimens of the same composition (Cherniak, 1996), a not unexpected result given the significantly larger ionic radius of Ba. Similarly, in both of the plagioclase compositions, Ba diffusion is slower than Sr or Pb diffusion by about two orders of magnitude.These slower diffusivities indicate that Ba zoning will be preserved under more extreme thermal conditions than those under which Sr zoning is retained. For example, at 700°C, zones on the scale of 100-μm width will retain Ba chemical signatures for times > 10 Ma. Sr zoning in regions of comparable width will be preserved for time periods approximately an order of magnitude shorter at this temperature.