Ca diffusion in K-Feldspar

Abstract

Diffusion of Ca has been characterized in natural K-feldspar under dry, 1 atm conditions using a 44Ca tracer. Polished sections of feldspar were surrounded by source powders in Pt capsules and annealed in air, or with solid buffers to buffer at NNO or IW. One hydrous experiment was conducted in a piston-cylinder device at 800 °C and 1 GPa. In all cases, the source of diffusant was pre-synthesized anorthitic powder doped with 44Ca. Prepared sample capsules were held at run conditions for times ranging from a few hours to a few months, at temperatures from 750 to 1050 °C. The 44Ca distributions in the feldspars were profiled by Rutherford Backscattering Spectrometry (RBS). The following Arrhenius relation is obtained for Ca diffusion in K-feldspar for diffusion normal to (001):D = 3.91 × 10−9 exp.(−276 ± 41 kJ/mol/RT) m2 sec−1.Diffusivities normal to (010) appear similar, suggesting little anisotropy for Ca diffusion. Diffusion coefficients for experiments buffered at NNO and IW agree within experimental uncertainty with those for experiments run in air, indicating little dependence of Ca diffusion on oxygen fugacity under the investigated range of conditions. On the basis of the single high-pressure experiment, neither pressure nor the presence of H2O appear to affect Ca diffusion.Ca diffusion in K-feldspar is slower than the diffusivities of the larger divalent cations Sr and Pb, and also slower than Ca diffusion in plagioclase, which may be attributable to the large size mismatch of Ca in the K-feldspar structure.Ca diffusion is 7–8 orders of magnitude slower than Ar diffusion in K-feldspar, indicating that K-feldspars will be much more retentive of Ca chemical signatures than of Ar. As an example, 1 mm diameter feldspar grains will experience ∼50% Ar loss in 1500 years at 500 °C, while Ca loss of only 1% would require 100 million years at this temperature.