Argon Isotopic Ratios in Historic Lavas:

Importance of Correction for the Initial Argon in K-Ar Dating of Young Volcanic Rocks

Abstract

Precise determination of argon isotopic ratios was made on twenty-seven historic lavas from Japan. Their isotopic ratios were mostly mass fractionated from that of the atmospheric argon toward the enrichment in lighter isotopes. This suggests that the correction for the mass fractionated initial ⁴⁰Ar/³⁶Ar ratio is very important for the K-Ar dating of volcanic rocks younger than 0.1 Ma. The limit of measurable K-Ar age is estimated to be 0.03 Ma for a normal andesitic lava having a fractionated initial argon.