Ion microprobe studies of water in silicate melts. Concentration-dependent water diffusion in obsidian |
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| Authors: | JR Delaney JL Karsten |
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| Institution: | Department of Oceanography, WB-10, University of Washington, Seattle, WA 98195 U.S.A. |
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| Abstract: | The ion microprobe at Johnson Space Center has been calibrated for in situ water determinations on a 10-μm scale over the range 0.2 wt.% H2O to 1.8, 6.8, and 3.7 wt.%, for basaltic, albitic, and rhyolitic glasses, respectively. The basalt glass calibration curve differs substantially from those of albite and rhyolite glasses, indicating a need to carefully match composition and/or melt structure between H2O standards and unknowns.A value for the diffusivity of water as a function of concentration and time has been calculated from water diffusion profiles measured in rhyolite glasses prepared at 850°C and 700 barsPt(H2O) 1]. Transient diffusion into a semi-infinite medium is described by the equation:?(φ/2)?¸/?φ=?(Dw?¸/?φ)/?φ #x003B8;=1, φ=0, θ→ 0, θ→∞, wherex =distance from the cylinder edge,t =time,C0 =initial concentration,Cs =concentration at the edge,C =concentration at x,θ = C ? C0/Cs ? C0,φ = x/t1/2, andDw =diffusivity of water. An iterative technique has been used to calculate solutions to the diffusion equation as a function ofDw 2]. Comparison of these solutions with the ion probe data indicate that, for0.2wt.% ≤ C ≤ 3.7wt.%H2O,Dw can be described by an exponential function of θ, of the formDw = D0exp(bθ), withD0 (i.e.,Dw at 0.2%) = (0.8?2.2) × 10?8 cm2/s and2 ≤ b ≤ 4. |
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