Calibration of GA1550 biotite standard for K/Ar and 40 Ar/ 39 Ar dating

Abstract

The mineral separate GA1550 biotite has become an international standard for K/Ar and 40Ar/39Ar dating studies, although it was prepared as an intralaboratory standard at ANU to monitor tracer depletion from a gas pipette. It is one of a small number of samples that has been calibrated against 38Ar tracers, some of which had been mixed with known amounts of atmospheric argon, so that a so-called primary calibration has been performed. By measuring GA1550 biotite against additional tracers from the same batch we have determined the radiogenic argon content of this sample as 1.342 (±0.007)×10-9mol/g, and together with the measured K content of 7.645 (±0.050) weight percent, we derive a best estimate for the K/Ar age as 98.5±0.5Ma, where the error is derived from averaging the ages determined relative to the 38Ar tracer.