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Physical age measurements on samples from the ocean basins can provide useful information on the time scale of evolution of the basins. Such data are particularly desirable to supplement and validate extrapolations made to the physical time scale from marine magnetic anomaly profiles and paleontological studies. Isotopic ages may be determined in favorable cases on igneous material such as volcanic ash in the sedimentary sequences or from igneous rocks, most commonly basaltic, forming the basement or interbedded with sediments. The K-Ar dating method is perhaps the most useful means of physical dating of samples from the ocean basins, but considerable difficulties are encountered in applying the method. These difficulties arise because of the nature of the samples and because of the environment in which many of the rocks have crystallized. Doubt commonly exists as to whether all preexisting radiogenic argon was lost at the time of crystallization, particularly if this occurred under high hydrostatic pressure. Glass, a common component of oceanic volcanics, often retains radiogenic argon poorly, particularly if alteration or devitrification of the glass has occurred. Reliable age measurements can rarely be obtained on weathered or altered rocks because radiogenic argon commonly leaks from such samples. For the present study, nine core samples of basalt from near the bottom of the holes drilled at Sites 211,214, 215, and 216 were made available for possible dating. Each sample was examined in thin section; five were found to be quite unsuitable for K-Ar dating because of extensive low temperature alteration. The remaining four samples were considered worth attempting to date, although none was regarded as ideal. Two samples were measured as whole rocks by the conventional K-Ar method employing isotope dilution argon determination and potassium measurement by flame photometry using techniques described previously (McDougall et al., 1969). The other two samples were determined by the Ar/Ar method by the total fusion technique (Merrihue, 1965; Mitchell, 1968). This method involves irradiation of the sample in a fast neutron flux (in the present case in the core facility of the HIFAR reactor of the Australian Atomic Energy Commission). A proportion of the Ar is converted to Ar by the fast neutrons. Argon extracted from the samples is measured isotopically in a mass spectrometer and compared with ratios obtained from a standard sample of known age irradiated at the same time. These data enable an age to be calculated on a single aliquot of sample, thus overcoming difficulties of obtaining representative aliquots of inhomogeneous samples. A full description of the techniques employed will be given elsewhere. Results of the age determinations are listed in Tables 1 and 2. Geological information was obtained primarily from the preliminary report on Leg 22 (von der Borch, Sclater, et al., 1972).

Potassium-Argon Ages on Basaltic Rocks Recovered from DSDP, Leg 22, Indian Ocean