Primitive magmas and source characteristics of the Hawaiian plume: petrology and geochemistry of shield picrites

Abstract

A suite of tholeiitic picrites from eight of the younger (<2 Ma) Hawaiian shield volcanoes provides new information about the compositions of primitive magmas and source components in the Hawaiian plume. Olivine and bulk rock compositions show that parental melts at Hawaiian volcanoes have at least 13–17% MgO and ∼10% Al2O3. The picrites have bulk compositions ranging from 14 to 30% MgO, and although most of these lavas have accumulated olivine + spinel, several have compositions that may approximate primitive melts. Olivine and spinel compositions show that the phenocrysts are closely related to the melt fraction of these lavas and are not accidental xenocrysts. Diverse isotopic compositions (Pb, Os, Sr, Nd) of these picrites require multiple sources in the Hawaiian plume, but key trace element characteristics (La, Nb abundances normalized to 16% MgO, Sm/Nd, Lu/Hf, La/Yb, Zr/Nb) are consistent with variable degrees of melting of a common, garnet-bearing source for all of the volcanoes except Koolau. The trace element composition of this Hawaiian pyrolite plume source can be modelled as an incipiently depleted, nearly primitive mantle that has lost a very small melt fraction, but a more complex origin may be more realistic. The Koolau picrites are exceptional in having anomalously low Nb and Ti contents, and high Zr/Nb ratios that fall off the melting arrays defined by the other picrites, indicating a distinctive source component that is also expressed in major element and isotopic compositions. The nearly constant Sr/Pb, Sr/Y, and Ba/Th ratios of these isotopically variable picrites are inconsistent with formation of the plume source either by bulk recycling of oceanic crust into the mantle, or by addition of dacitic melts from entrained eclogite to plume-derived basaltic magmas. Alternatively, the Hawaiian plume may consist of variably depleted mantle that was enriched by small-degree melts, possibly during subduction or entrainment of lithospheric mantle. Radiogenic 186Os/188Os isotopic compositions of these picrites are consistent with transport of this material to the deep lower mantle and addition of a small amount of outer core to the plume source.