40Ar/39Ar analysis of volcanic rocks recovered from the Japan Sea floor: constraints on the age of formation of the Japan Sea

Abstract

Volcanic rocks recovered from the Japan Sea during ODP Legs 127 and 128 were analyzed by 40Ar-39Ar whole-rock stepwise-heating experiments. All three experiments on samples from Site 795 in the Japan Basin revealed disturbed age spectra, but they are consistent with crystallization ages of 15 to 25 Ma for the samples. At Site 797 in the Yamato Basin, three of the five samples showed plateau ages of 18-19 Ma. At Site 794 in the northern Yamato Basin, three of the five samples revealed concordant age spectra of 20-21 Ma. The radiometric ages are concordant with previously inferred ages for the formation of the Japan Sea floor. The present results indicate that formation of the Japan Sea floor started at least 19-20 Ma ago and give more precise age constraints. -from Authors