SILICON SELF-DIFFUSION IN SINGLE-CRYSTAL NATURAL QUARTZ AND FELDSPAR

Abstract

Silicon diffusion was measured in natural quartz and anorthitic feldspar under dry, low-pressure (0.1 MPa) conditions using a 30Si tracer. Sources of diffusant consisted of 30Si-enriched silica powder for experiments on quartz and microcrystalline 30Si-doped synthetic feldspar of composition comparable to the feldspar specimens. Distributions of 30Si were measured with Rutherford backscattering spectrometry and nuclear reaction analysis, using the reaction 30Si (p,γ)31P. The following Arrhenius relations were obtained for anneals at 1 atm in air. For quartz: transport normal to c: Dqtz,#c=7.97x10-6 exp (-447+/-31 kJ mol-1/RT) m2 s-1; transport parallel to c: Dqtz,#c=6.40x10-6 exp (-443+/-22 kJ mol-1/RT) m2 s-1. For anorthitic feldspar (An93): DAn=3.79x10-7 exp (-465+/-50 kJ mol-1/RT) m2 s-1. The few successful experiments on diffusion in plagioclase of more albitic compositions (An67 and An23) reveal Si diffusivities a few orders of magnitude faster than that in the anorthite. The results for these feldspars bracket the determination of CaAl-NaSi interdiffusion under dry conditions by Grove et al. [Geochim. Cosmochim. Acta 48 (1984) 2113-2121], suggesting that the rate-limiting process is indeed Si diffusion. Si diffusion in quartz under more reducing conditions (NNO) is slightly slower (by about half an order of magnitude) than diffusion in samples annealed in air. This is consistent with observations made in studies of synthetic quartz [Bejina and Jaoul, Phys. Earth Planet. Inter. 50 (1988) 240-250].