Geochemical characteristics of West Molokai shield‐ and postshield‐stage lavas: Constraints on Hawaiian plume models

Abstract

There are systematic geochemical differences between the <2 Myr Hawaiian shields forming the subparallel spatial trends, known as Loa and Kea. These spatial and temporal geochemical changes provide insight into the spatial distribution of geochemical heterogeneities within the source of Hawaiian lavas, and the processes that create the Hawaiian plume. Lavas forming the ∼1.9 Ma West Molokai volcano are important for evaluating alternative models proposed for the spatial distribution of geochemical heterogeneities because (1) the geochemical distinction between Loa and Kea trends may end at the Molokai Fracture Zone and (2) West Molokai is a Loa‐trend volcano that has exposures of shield and postshield lavas. This geochemical study (major and trace element abundances and isotopic ratios of Sr, Nd, Hf, and Pb) shows that the West Molokai shield includes lavas with Loa‐ and Kea‐like geochemical characteristics; a mixed Loa‐Kea source is required. In contrast, West Molokai postshield lavas are exclusively Kea‐like. This change in source geochemistry can be explained by the observed change in strike of the Pacific plate near Molokai Island so that as West Molokai volcano moved away from a mixed Loa‐Kea source it sampled only the Kea side of a bilaterally zoned plume.