40Ar [sbnd]39Ar stepwise degassing experiments on some submarine rocks

Abstract

40Ar 39Ar stepwise degassing experiments were made on six submarine rocks. The 40Ar 39Ar ages thus obtained were compared with the conventional K-Ar ages. Samples with high K-content generally give a linear correlation on a ( 40Ar*/ 36Ar*) - ( 39Ar*/ 36Ar*) diagram, showing insignificant excess 40Ar. The excess 40Ar is shown to reside in higher temperature fractions, though the amount is small (< 10 −11 mol/g) and becomes significant only for low K-content samples. Basalt drilled from the Experimental Mohole gives a 40Ar 39Ar age of 19 my which agrees with magnetic anomaly lineation isochron. The K-Ar age of 42 my reported for this sample by Kulp is shown to be in error owing to excess 40Ar. The geophysical implications of the 40Ar 39Ar ages of other samples are also discussed.