| Abstrakt: |
Diffusion rates of18O tracer in quartz (? c, 1 Kb H2O) and Amelia albite (? 001, 2 Kb H2O) have been measured, using Secondary Ion Mass Spectrometry (SIMS). A new technique involving hydrothermal deposition of labelled materials has removed the possibility of pressure solution-reprecipitation processes adversely affecting the experiments. Reported diffusion constants are:ß-quartz (? c), $$D_0 = 3.4\left( {\begin{array}{*{20}c} { + 4.8} \\ { - 2.0} \\ \end{array} } \right)x 10^{ - {\text{13}}} {\text{m}}^{\text{2}} {\text{s}}^{ - {\text{1}}} $$ ,Q=98±7 KJ mol-1 (600–825° C, 1 Kb); Amelia albite (? 001), $$D_0 = 1.0\left( {\begin{array}{*{20}c} { + 2.0} \\ { - 0.7} \\ \end{array} } \right)x 10^{ - {\text{13}}} {\text{m}}^{\text{2}} {\text{s}}^{ - {\text{1}}} $$ ,Q=85±7 KJ mol-1, (400–600° C, 2 Kb). Measured quartz18O diffusivities decrease discontinuously at thea-ß transition, reflecting strong structural influences. The reported albite data agree with previously recorded studies, butß-quartz data indicate significantly lower activation energies. Possible causes of this discrepancy, and some geological consequences, are noted. |
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