Petrogenesis of Hawaiian postshield lavas: Evidence from Nintoku Seamount, Emperor Seamount Chain

Abstract

We report major and trace element and Sr and Nd isotope data for a 283 m thick sequence of postshield lavas drilled during Ocean Drilling Project Leg 197 from Site 1205, Nintoku Seamount, an approximately 56 m.y. old volcano from the Emperor Seamount Chain. At least 25 subaerially erupted lava flow units were sampled, of which all but one are alkali basalts. Clasts of hawaiite and mugearite are present in a conglomerate overlying the basement sequence. Similar rock types characterize the postshield stage of many young Hawaiian volcanoes. Major and trace element, and age‐corrected Sr and Nd isotope compositions of the Nintoku lavas are similar to those of young postshield lavas from the Hawaiian Islands. Concentrations of highly incompatible elements tend to increase, La/Yb and Nb/Zr ratios increase, and 87Sr/86Sr decreases with decreasing depth in the drill core. The more evolved rock types can be related to the underlying alkali basalts by fractional crystallization of olivine, clinopyroxene, plagioclase, and Fe‐Ti oxides, all of which occur as phenocryst or significant groundmass phases in these lavas. However, variations in the trace element and isotopic compositions of the Nintoku lavas indicate derivation from a heterogeneous mantle source. The isotopic and trace element compositions of the lavas can be modeled using a modified version of the Chen and Frey (1985) mixing model for Hawaiian lavas, in which small‐degree melts of the ∼60 m.y. old oceanic lithosphere beneath Nintoku, previously fertilized by small‐degree melts soon after its formation at a ridge axis, are mixed with melts from the Hawaiian plume. The contribution from the oceanic lithosphere became more important with time as Nintoku Seamount moved away from the Hawaiian plume axis. Together with other data from Leg 197, our results show that the 87Sr/86Sr ratios of not only shield but also postshield lavas from the Emperor Seamounts increased with decreasing age between ∼80 and 50 m.y. ago. For alkalic postshield lavas, this could result from either increased melting of the oceanic lithosphere beneath volcanoes situated on younger oceanic crust or lower degrees of incompatible trace element enrichment in the lowermost portions of younger oceanic lithosphere.