Figure F57. Total alkalis (Na₂O + K₂O) vs. SiO₂ content showing shipboard Leg 197 data for lavas recovered from basement penetrations of Detroit, Nintoku, and Koko Seamounts. All samples are basalt except for the two hawaiite clasts from Nintoku Seamount that occur in a conglomerate overlying the basement. The alkalic-tholeiitic dividing line for Hawaiian basalt is from Macdonald and Katsura (1964). Fields for alkalic and tholeiitic basalt recovered from the shield of Mauna Kea volcano by the Hawaiian Scientific Drilling Project (data from Rhodes, 1996; Vollinger and Rhodes, unpubl. data) are shown for comparison. All data are given on a volatile-free basis with 90% of the iron as Fe²⁺.