GEOCHEMICAL HETEROGENEITIES IN THE HAWAIIAN PLUME: THE ORIGIN OF DIFFERENCES BETWEEN LOA- AND KEA-TREND SHIELD LAVAS

Geochemical differences between shield lavas erupted along the spatially defined Loa- and Kea-trends of recent Hawaiian volcanoes can be described as arising from a systematically spatial distribution in the proportions of Loa- and Kea-type components. The origin of these geochemical types is commonly attributed to subduction and recycling of oceanic plates with different proportions of basaltic upper crust, gabbroic lower crust and mantle lithosphere in the sources of Loa-and Kea- trend lavas. We use variations in abundance ratios involving highly incompatible elements (Th, Nb, La) and moderately incompatible elements (Sr, Zr) to determine the process that created the Loa-type component. An important result is that La/Nb, La/Th, Zr/Nb, Sr/Nd and Sr/Nb are negatively correlated with ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb with the Loa component having the highest abundance ratios and lowest isotopic ratios. The most effective process for changing ratios of highly incompatible elements is fractional melting which creates residues that are very depleted in incompatible elements with abundance ratios markedly different from the source ratios. However this process affects all ratios; therefore the relatively constant ratios of Nb/Th, Th/U, Nb/Ta and Zr/Sm in Loa- and Kea-trend lavas is not explained. We reject this explanation. High La/Nb, La/Th, Sr/Nb and Sr/Nd are typical of plagioclase/melt partition coefficients. We infer that the Loa-component has the geochemical characteristics of plagioclase. Zr/Nb is also correlated with isotopic ratios and garnet which has very large DZr/DNb can control Zr/Nb. We conclude that the Loa component has the geochemical characteristics of garnet granulite. Incompatible element abundance ratios of garnet-clinopyroxene granulites from Tanzania are similar to those of the Loa component. Granulites commonly form in the lower continental crust as cumulates, and their depths of equilibration, greater than 50 km, are achieved in lower continental crust. In oceanic crust such depths are only reached in oceanic plateaus with thick lithosphere; for example, granulite xenoliths are found in lavas of the Kerguelen Archipelago which formed on the Kerguelen Plateau.