On the significance of argon release from biotite and amphibole during 40 Ar/ 39 Ar vacuum heating

Details On the significance of argon release from biotite and amphibole during 40 Ar/ 39 Ar vacuum heating On the significance of argon release from biotite and amphibole during 40 Ar/ 39 Ar vacuum heating

Oct 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988gecoa..52.2457g&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 52, Issue 10, pp.2457-2465

Mathematics

Logic

16

Scientific paper

The nature of Ar release from biotite and amphibole is addressed via vacuum-heating experiments followed by measurements of 40 Ar loss and unit cell parameters and scanning electron microscope observations of the heated specimens. 40 Ar diffusion during hydrothermal heating was previously reported for the studied biotite and amphibole specimens allowing direct comparison to the Ar loss kinetics from vacuum heating. The results demonstrate conclusively that volume diffusion is not the mechanism controlling Ar loss during normal 40 Ar/ 39 Ar measurements. These hydrous minerals also suffer weight loss and irreversible structural changes in the range 600 to 800°C. Moreover, biotite suffers morphological modifications dramatically expressed by splitting along the basal cleavage. A few experiments on muscovite suggest similar weight loss and splitting effects. The behavior of these minerals has important implications for the interpretation of 40 Ar/ 39 Ar step-heating data. The Ar-loss kinetics of 40 Ar/ 39 Ar experiments cannot be used to calculate diffusion coefficients and it is inappropriate to estimate closure temperatures from such data. The morphological changes will not permit the meaningful resolution of "fossil" Ar gradients which may have been present in the initial grains.

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