Other
Scientific paper
Mar 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981gecoa..45..279h&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 45, Issue 3, pp.279-291
Other
20
Scientific paper
The temperature dependence of diffusion is usually found to follow the Arrhenius law: D = D 0 e - E / RT Winchell (1969) showed that there is commonly an inter-dependence between D 0 and E (for diffusion in silicate glasses), such that diffusion of different species show a positive correlation on a log D 0 vs E plot. A similar effect was noted by (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 D 0 feldspar-- E = 50.7 + 3.4 log D 0 olivine-- E = 78.0 + 7.5 log D 0 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 d 0 , 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 10 1 -10 5 °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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