GEOCHEMICAL CHARACTERISTICS OF KOOLAU VOLCANO - IMPLICATIONS OF INTERSHIELD GEOCHEMICAL DIFFERENCES AMONG HAWAIIAN VOLCANOS

The voluminous shields of Hawaiian volcanoes are dominantly composed o f tholeiitic basalts, but there are important intershield geochemical differences. The subaerial lavas forming the approximately 2-3 Ma Kool au shield have several extreme characteristics: relatively high abunda nces Of SiO2, low abundances of total iron and CaO, and high ratios of La/Nb and Sr/Nb. In addition, they range to near bulk-earth strontium , neodymium, and lead isotopic ratios. Although postmagmatic alteratio n has significantly affected the compositions of some Koolau lavas (de creases in SiO2, K2O, and rubidium contents, increases in total iron a nd in unusual cases, increases in yttrium and REE abundances), the geo chemical characteristics of unaltered Koolau lavas reflect a distincti ve primary magma composition. Within a stratigraphic sequence of lavas , Koolau lavas vary significantly in incompatible element abundance an d isotopic ratios, but these variations are not systematic with erupti on age, and they are smaller than the differences between Hawaiian shi elds. Intershield differences in some incompatible element abundance r atios, La/Nb and Sr/Nb, are correlated with intershield differences in isotopic ratios, thereby indicating that each shield formed from a co mpositionally distinct source. However, other intershield compositiona l differences are not correlated with differences in radiogenic isotop e ratios. Some of these compositional differences probably reflect var iations in the melting process; e.g., inverse correlations between SiO 2 and total iron contents may reflect differences in the pressure of m elt segregation, differences in abundances of incompatible elements ma y reflect variations in mean degree of melting, and variations in rati os like Sm/Nd may reflect the presence of residual garnet. Each shield appears to reflect a unique combination of source components and vari ables, such as extent of melting and pressure of melt segregation. Con sequently, the intershield geochemical differences have important impl ications for plume structure. Either a relatively large plume has a sp atially systematic distribution of geochemical heterogeneities which a re sampled by the overlying shields, or each shield is derived from a small radius (<20 km) conduit composed of geochemically distinct diapi rs or solitary waves.