Diffusion compensation in natural silicates |
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| Authors: | SR Hart |
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| Institution: | Center for Geoalchemy, Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. |
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| Abstract: | The temperature dependence of diffusion is usually found to follow the Arrhenius law: D = D0e?E/RT Winchell (1969) showed that there is commonly an inter-dependence between D0 and E (for diffusion in silicate glasses), such that diffusion of different species show a positive correlation on a log D0 vs E plot. A similar effect was noted by Hofmann (1980) for cation diffusion in basalt. This implies that diffusion rates of different species tend to converge at a particular temperature; this effect is known as the ‘compensation effect’. I will show that this effect is also present for diffusion in feldspars and olivines. The equations for the compensation lines (with E given in kcal/mol) are: basalt—E = 50 + 7.5 log D0 feldspar—E = 50.7 + 3.4 log D0 olivine—E = 78.0 + 7.5 log D0 The convergence, or crossover, temperatures for diffusion in various materials are: obsidian—3400°C basalt—1370°C olivine—1360°C feldspar—460°C Compensation plots are useful for evaluating and comparing experimental diffusion data (though of limited usefulness in a predictive sense) and for understanding ‘closure temperatures’ for diffusion in petrogenetic processes (since closure temperature, the temperature at which natural diffusion processes are frozen in, is dependent on E, log d0, and cooling rate). I show that most diffusing species in feldspar have a closure-temperature close to the crossover or convergence temperature, implying that all species in feldspars can be expected to ‘freeze-in’ simultaneously at temperatures in the range 400–600°C (for cooling rates in the range 101–105°C/myr). Closure temperatures of various species in olivine, on the other hand, span a much larger range (800°C) for a similar range in cooling rates, implying that different elements in olivine will record different time-temperature stages in petrogenetic processes. |
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