TEMPERATURE AND PRESSURE EFFECTS ON 40AR-39AR SYSTEMATICS

The effects of thermal and compressional treatment on 40Ar-39Ar systematics have been investigated on three artificially heated biotite samples (heated for 1 hour at 700°C and 860°C in air and 700°C in vacuum respectively) and uniaxially compressed granite (p = 1400 bar) and basalt samples (p = 1650 bar). The 40Ar-39Ar results for the disturbed samples are compared with those for undisturbed samples. Except for the vacuum-heating case, the effects of the disturbances may be interpreted as the combined effect of a partial loss of radiogenic 40Ar from the sample and an incorporation of air Ar into the sample. Common diagnostic effects are (1) reduction of the total fusion age, (2) distortion of the age spectrum and, if the degree of the partial Ar loss is small, (3) approximate preservation of the isochron age, and (4) reduction of the intercept value (40Ar/36Ar) in the isochron plot. The features observed in the age spectra of artificially disturbed samples are rather common in geologically disturbed samples, suggesting that the artificial disturbances simulate the effects of geological disturbances on 40Ar-39Ar systematics. © 1979.