Other
Scientific paper
Dec 1970
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1970gecoa..34.1327z&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 34, Issue 12, pp.1327-1350
Other
3
Scientific paper
The dehydration and release of argon from three different kinds of mica was investigated as a function of temperature, chiefly using a mass spectrometer. Step-heating and continuous heating results show that both water and argon are released over three temperature intervals, which, in the case of water are identified respectively as superficial adsorbed water, as water in cracks and inclusions, and as water of constitution. Release of combined water profoundly affects the crystal lattice which in turn largely determines the release of argon. In the case of biotite, the large amount of combined iron widens the structure so that the release of water and other gases becomes easier. These conclusions are supported by measurements of loss of weight, release of hydrogen and lattice dimensions. The term Q / Q ° as a function of 1/ T for the dehydration and for the release of argon by continuous heating is represented by several straight lines. The slopes for the dehydration and for the release of argon are nearly the same when the temperature is high enough and the release of gases is important. The results of step-heating and those of continuous heating with different rates of heating permit an approximative determination of activation energies for the diffusion of argon. The first method gives, between 500 and 1000°C, only one value of activation energy, 25 ± 5 kc/mole for the biotites, 30 ± 5 kc/mole for the phlogopites and 55 ± 5 kc/mole for the muscovites. The second method gives several energies (2 to 3) between 20 kc/mole and 80 kc/mole; the energy which corresponds to the most important release of argon is always in the neighbourhood of that calculated by isothermal heating. On a practical level, this knowledge of dehydration should lead to a better understanding of metamorphic processes, while for argon it leads to a better extraction method for dating.
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