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

A suite of tholeiitic picrites from eight of the younger (<2 Ma) Hawaiian s hield volcanoes provides new information about the compositions of primitiv e 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 similar to 10% Al2O3 The picrites have bulk compositions ran ging 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 closel y related to the melt fraction of these lavas and are not accidental xenocr ysts. Diverse isotopic compositions (Pb, Os, Sr, Nd) of these picrites requ ire multiple sources in the Hawaiian plume, but key trace element character istics (La, Nh abundances normalized to 16% MgO, Sm/Nd, Lu/Hf, La/Yb, Zr/Nb ) are consistent with variable degrees of melting of a common, garnet-beari ng source for all of the volcanoes except Koolau. The trace element composi tion of this Hawaiian pyrolite plume source can be modelled as an incipient ly depleted, nearly primitive mantle that has lost a very small melt fracti on, but a more complex origin may be more realistic. The Koolau picrites ar e 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, indi cating a distinctive source component that is also expressed in major eleme nt and isotopic compositions. The nearly constant Sr/Pb, Sr/Y, and Ba/Th ra tios of these isotopically variable picrites are inconsistent with formatio n of the plume source either by bulk recycling of oceanic crust into the ma ntle, or by addition of dacitic melts from entrained eclogite to plume-deri ved basaltic magmas. Alternatively, the Hawaiian plume may consist of varia bly depleted mantle that was enriched by small-degree melts, possibly durin g subduction or entrainment of lithospheric mantle. Radiogenic Os-186/Os-18 8 isotopic compositions of these picrites are consistent with transport of this material to the deep lower mantle and addition of a small amount of ou ter core to the plume source. (C) 1999 Elsevier Science B.V. All rights res erved.