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K-Ar dates were obtained for three pillow basalt samples recovered from Site 608 (Samples 608-58-1, 103-109 cm; 608-59-1, 3-7 cm; 608-59-1, 48-53 cm; see Table 1). Reliable K-Ar dates cannot be routinely obtained for deep-sea igneous rocks, because they may be subject to inaccuracies related to seawater alteration (Seidemann, 1977) and/or the presence of excess radiogenic ^Ar (Dalrymple and Moore, 1968; Dymond, 1970). Thus, the possibility that the samples dated in this study were subject to these sources of inaccuracy must be evaluated. Submarine pillow basalts, such as those analyzed in this study, cool rapidly, sometimes preventing the escape of gases associated with the magma. The presence of excess radiogenic Ar trapped in the rapidly cooled glassy margins of submarine pillows at the time of their formation results in anomalously high K-Ar dates. The interiors of basalt pillows, which cool less rapidly than the margins, are frequently devoid of excess radiogenic ^Ar. The abundance of vesicles increases inward from the pillow margin, indicating a larger degree of degassing in the interior. There is also a systematic decrease in the excess radiogenic ^Ar content of submarine basalt pillow margins with decreasing water depth of extrusion. This decrease is accompanied by an increase in the size and abundance of vesicles. The decrease in hydrostatic pressure with decreasing depth apparently promotes exsolution of dissolved gases (Dalrymple and Moore, 1968; Dymond, 1970). The presence of abundant vesicles in the pillow basalts dated in this study indicates that some degassing has occurred, making it less likely that the presence of excess radiogenic ^Ar is source of inaccuracy in the K-Ar dates obtained.

K-Ar Dates of Deep Sea Drilling Project Site 608 Basalts