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Six basaltic whole-rock samples from DSDP Leg 59 in the Philippine Sea (Fig. 1) have been dated by a combination of K/Ar and Ar/Ar techniques. Two samples were collected from Hole 447A of the West Philippine Basin, three from Holes 448 and 448A on the Palau-Kyushu Ridge (Fig. 2), and one from Hole 450 in the Parece Vela Basin. We followed a general scheme first to identify samples that are potentially closed K/Ar systems and then to refine further the established age of these closed-system samples. Each sample of basalt that showed little or no alteration of K-bearing phases in thin section was prepared and then irradiated with fast neutrons to produce Ar from K (see Dalrymple and Lanphere, 1971, 1974; and Lanphere and Dalrymple, 1971 for details of this analytical technique). After irradiation, gases (including argon) were extracted from each sample in six successively higher temperature steps, the last of which was above the melting point of the sample (called the FUSE step). The argon contained in each temperature step was purified and isotopically analyzed in order to produce six sets of argon isotopic data versus temperature for each of the samples analyzed. These data were then used to generate an age spectrum and isochron plot for each sample (see Figs. 3-5). Each rectangular box in the age spectrum plot represents the argon data for one temperature step. The width of each box is the percentage of the K-derived Ar released at that temperature compared to the total K-derived Ar released in all six temperature steps. The height of each box represents the analytical precision of the argon isotopic data released at that temperature. The analytical precision represents a standard deviation calculated at the 67% confidence level. If all temperature steps, each containing more than 5°7o of the total K-derived Ar released, have the same apparent age (using the critical value test of Dalrymple and Lanphere, 1969), then the age spectrum is said to be "concordant" (Fleck et al., 1977). The spectrum is "discordant" if these criteria are not met. The spectrum contains a "plateau" if adjacent temperature steps, each containing more than 5% of the total K-derived Ar, are the same age (using the critical value test just cited) and together constitute more than 50% of the total K-derived Ar. The terms "disturbed spectrum" and "undisturbed spectrum" are only used when a geologic interpretation is made. For instance, an age spectrum would be termed "undisturbed" if the plateau portion defined an age that, together with other geologic information, could be interpreted as the crys120° 135= 150"

K/Ar and 40Ar/39Ar Dating of Basaltic Rocks from Deep Sea Drilling Project Leg 59