Figure F12. A. Total alkali content (Na₂O + K₂O) vs. SiO₂ classification plot (from Le Bas et al., 1986) for volcanic rocks showing lava compositions from Detroit Seamount. The dashed line is the alkalic-tholeiitic dividing line for Hawaiian basalt. Lavas from Site 884 are tholeiitic basalt, whereas lavas from Site 883 are alkalic basalt, although Keller et al. (1995) inferred that prior to alteration these lavas were transitional between alkalic and tholeiitic basalt. Data for these two Leg 145 sites in this and all subsequent figures are from Keller et al. (2000) and M. Regelous et al. (unpubl. data). Lavas from Site 1203 range from tholeiitic, overlapping with Site 884 lavas, to alkalic, overlapping with Site 883 lavas. Most of the alkalic Site 1203 lavas have loss on ignition (LOI) >2 wt%. The volcaniclastite sample has high total alkalis, which is interpreted as a result of alkali gain during alteration. The two Site 1203 basalt samples in the tholeiitic field at <45% SiO₂ are picritic as a result of olivine accumulation. B. Total alkali content (Na₂O + K₂O) vs. SiO₂ comparing Detroit Seamount lava compositions with those from Mauna Kea Volcano (shield and postshield stage lavas-lower and upper shaded areas, respectively) on the island of Hawaii. Site 884 and some Site 1203 lavas overlap with the shield-stage tholeiitic basalt, whereas Site 883 and some Site 1203 lavas overlap with Hawaiian postshield alkalic basalt. The irregular line encloses postshield-stage lava erupted at Mauna Kea Volcano, Hawaii.