Alteration of basalt samples in the basement rocks of Site 647 produced a range of K-Ar ages from 35 to 71 Ma. Microprobe data indicate that an alteration phase contributes nearly 50% of the potassium in the samples. Thus, a component with uncertain argon-retention characteristics is considered responsible for the range of ages. While the older ages are consistent with the expected magnetic anomaly age and the fossil evidence, the presence of the alteration makes these ages unreliable for defining the age of the basaltic crust.

Supplement to: Roddick, J C (1989): K-Ar dating of basalts from Site 647, Ocean Drilling Program Leg 105. In: Srivastava, SP; Arthur, M; Clement, B; et al. (eds.), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program), 105, 885-887