⁴⁰Ar^-39Ar age of rocks, and the development mode of the Philippine Sea

Abstract

THE origin and development of the Phillipine Sea have been central issues in tectonic studies of a marginal sea: the deep-sea drilling project (DSDP), Leg 31, was primarily intended to resolve the question¹. Unfortunately, at only two of the Leg 31 sites (292 and 296) were microfossils indicating the age of the basement recovered, so the age of the ocean basin, had to be deduced by dating the drilled basement rocks. There seems to be some confusion in the literature² in interpreting the fossil data with regard to the age of the ocean basin. We have dated several DSDP Leg 31 drilled rocks by the ⁴⁰Ar–³⁹Ar step-heating technique (Table 1 and Fig. 1) and re-evaluated the fossil ages and various hypotheses concerning the development of the western Philippine Sea and the Shikoku Basin. We applied two statistical criteria to judge the reliability of ⁴⁰Ar–³⁹Ar ages, (1) the experimental data with MSUM values (Table 1) less than 3.50 and (2) the apparent ages should agree with each other, within Iσ, for more than 65% of ⁴⁰Ar released. Although such criteria do not prove the reliability of this dating technique, they provide a measure of it³. Of the nine drilling sites, six reached the basement—site 290, 291, 292, 293, 294 and 296—their ages were determined by both the ⁴⁰Ar–³⁹Ar step-heating dating and micropalaeontological methods (Table 1).