Other
Scientific paper
Jun 1980
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980e%26psl..48..185r&link_type=abstract
Earth and Planetary Science Letters, Volume 48, Issue 1, p. 185-208.
Other
40
Scientific paper
Alpine biotites containing excess 40Ar have been analysed by step-heating argon analysis of both neutron irradiated and unirradiated samples. In addition to age spectra the data are discussed in terms of the thermal release of 40Ar, 39Ar, 37Ar and 36Ar and also displayed on a correlation plot of 36Ar/40Ar vs. 39Ar/40Ar which is used to interpret the data and present a model of isotopic evolution during metamorphic cooling. This diagram overcomes misleading complications of isochron plots. The samples exhibit the following argon systematics: (1) flat age spectra for 80-90% 39Ar release with anomalously old ages but early gas fractions that approximate the accepted cooling ages; (2) each sample shows decreasing 36Ar/40Ar with increasing temperature of heating step with three samples having a negative correlation of 36Ar/40Ar vs. 39Ar/40Ar and one a positive correlation; (3) there appear to be two 36Ar components, one released at high temperatures and correlated with radiogenic 40Ar and one released at low temperatures which is not correlated with radiogenic 40Ar; and (4) there is no significant effect of neutron irradiation on the release of 40Ar and 36Ar.
Interpretation suggests that these biotites contain a record of the evolution and isotopic composition of ambient argon retained within the metamorphic host rocks during cooling. After incorporation of argon of high 40Ar/36Ar another argon component, of atmospheric composition, was retained at lower temperature and argon partial pressures.
Cliff Robert A.
Rex D. C.
Roddick J. C.
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