Petrogenesis of lavas from Detroit Seamount: Geochemical differences between Emperor Chain and Hawaiian volcanoes

The Hawaiian Ridge and Emperor Seamount Chain define a hot spot track that provides an 80 Myr record of Hawaiian magmatism. Detroit Seamount (∼76 to 81 Ma) is one of the oldest Emperor Seamounts. Volcanic rocks forming this seamount have been cored by the Ocean Drilling Program at six locations. Only tholeiitic basalt occurs at Site 884 on the eastern flank and only alkalic basalt, probably postshield lavas, occurs at Sites 883 and 1204 on the summit plateau. However, at Site 1203 the basement core (453 m penetration) includes four thick flows of pahoehoe alkalic basalt underlying ∼300 m of volcaniclastic rocks interbedded with submarine erupted tholeiitic basalt. The geochemical characteristics of these alkalic lavas indicate that phlogopite was important in their petrogenesis; they may represent preshield stage volcanism. The surprising upward transition from subaerial to submarine eruptives implies rapid subsidence of the volcano, which can be explained by the inferred near‐ridge axis setting of the seamount at ∼80 Ma. A near‐ridge axis setting with thin lithosphere is also consistent with a shallow depth of melt segregation for Detroit Seamount magmas relative to Hawaiian magmas, and the significant role for plagioclase fractionation as the Detroit Seamount magmas evolved in the crust. An important long‐term trend along the hot spot track is that 87 Sr/ 86 Sr decreases in lavas erupted from ∼40 to 80 Ma. Tholeiitic basalt at Site 884 on Detroit Seamount is the extreme and overlaps with the 87 Sr/ 86 Sr‐ 143 Nd/ 144 Nd field of Pacific mid‐ocean ridge basalts (MORB). Complementary evidence for a depleted component in Detroit Seamount lavas is that relative to Hawaiian basalt, Detroit Seamount lavas have lower abundances of incompatible elements at a given MgO content. These lavas, especially from Sites 883 and 884, trend to extremely unradiogenic Pb isotopic ratios which are unlike MORB erupted at the East Pacific Rise. A component with relatively low 87 Sr/ 86 Sr and 206 Pb/ 204 Pb is required. Lavas erupted from a spreading center in the Garrett transform fault, 13°28′S on the East Pacific Rise, have this characteristic. A plausible hypothesis is mixing of a plume‐related component with a component similar to that expressed in lavas from the Garrett transform fault. However, basaltic glasses from Detroit Seamount also have relatively high Ba/Th, a distinctive characteristic of Hawaiian lavas. We argue that all Detroit Seamount lavas, including those from Site 884, are related to the Hawaiian hot spot. Rejuvenated stage Hawaiian lavas also have high Ba/Th and define a trend to low 87 Sr/ 86 Sr and 206 Pb/ 204 Pb. We speculate that rejuvenated stage lavas and Detroit Seamount lavas sample a depleted mantle component, intrinsic to the plume, over the past 80 Myr.