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Numerous large igneous provinces formed in the Pacific Ocean during Early Cretaceous time, but their origins and relations are poorly understood. We present new geochronological and geochemical data on rocks from the Manihiki Plateau and compare these results to those for other Cretaceous Pacific plateaus. A dredged Manihiki basalt gives an [sup.40]Ar-[.sup.39]Ar age of 117.9 [+ or -] 3.5 Ma (2[sigma]), essentially contemporaneous with the Ontong Java Plateau ~2500 km to the west, and the possibly related Hikurangi Plateau ~3000 km to the south. Drilled Manihiki lavas are tholeiitic with incompatible trace element abundances similar to those of Ontong Java basalts. These lavas may result from high degrees of partial melting during the main eruptive phase of plateau formation. There are two categories of dredged lavas from the Danger Islands Troughs, which bisect the plateau. The first is alkalic lavas having strong enrichments in light rare earth and largeion lithophile elements; these lavas may represent late-stage activity, as one sample yields an [sup.40]Ar-[.sup.39]Ar age of 99.5 [+ or -] 0.7 Ma. The second category consists of tholeiitic basalts with U-shaped incompatible element patterns and unusually low abundances of several elements; these basaits record a mantle component not previously observed in Manihiki, Ontong Java, or Hikurangi lavas. Their trace element characteristics may result from extensive melting of depleted mantle wedge material mixed with small amounts of volcaniclastic sediment. We are unaware of comparable basalts elsewhere. Keywords: large igneous provinces, Early Cretaceous, Manihiki Plateau, Pacific Ocean, geochemistry, geochronology.

Author(s): Zhong-Yuan Ren (corresponding author) [1]; Stephanie Ingle [1, 3]; Eiichi Takahashi [1, 2]; Naoto Hirano [1]; Takafumi Hirata [1] The Hawaiian-Emperor volcanic island and seamount chain is usually attributed [...]

At Elan Bank of the Kerguelen Plateau in the southeast Indian Ocean, Leg 183 of the Ocean Drilling Program recovered clasts of garnet-biotite gneiss in a fluvial conglomerate intercalated with basalt flows. U-Pb and Pb-Pb dates of zircons and monazites in these clasts and an overlying sandstone range from 534 to 2547 Ma, which is much older than the surrounding Indian Ocean seafloor. These dates show that old continental crust resides in the shallow crust of the oceanic Kerguelen Plateau and that the breakup of Gondwana dispersed continental fragments into the nascent Indian Ocean lithosphere. Keywords: Kerguelen Plateau, geochronology, continental crust, Proterozoic, Indian Ocean.

Basalts forming the floor of the Indian Ocean are geochemically distinct from those of the Atlantic and Pacific Oceans. These differences have been attributed to a deeply recycled continental component or to widespread dispersion of plume-type mantle within the Indian Ocean asthenosphere. The discovery of Proterozoic continental crustal rocks within the uppermost basaltic basement of Elan Bank, part of the Kerguelen Plateau, shows instead that direct shallow-level contamination of basaltic magmas by isolated continental crust fragments may have produced the anomalous isotopic ratios of some Indian Ocean basalts. Keywords: Indian Ocean, Kerguelen plume, continental contamination, large igneous province, Kerguelen Plateau.

The Hawaiian–Emperor volcanic island and seamount chain is usually attributed to a hot mantle plume, located beneath the Pacific lithosphere, that delivers material sourced from deep in the mantle to the surface. The shield volcanoes of the Hawaiian islands are distributed in two curvilinear, parallel trends (termed ‘Kea’ and ‘Loa’), whose rocks are characterized by general geochemical differences. This has led to the proposition that Hawaiian volcanoes sample compositionally distinct, concentrically zoned, regions of the underlying mantle plume. Melt inclusions, or samples of local magma ‘frozen’ in olivine phenocrysts during crystallization, may record complexities of mantle sources, thereby providing better insight into the chemical structure of plumes. Here we report the discovery of both Kea- and Loa-like major and trace element compositions in olivine-hosted melt inclusions in individual, shield-stage Hawaiian volcanoes—even within single rock samples. We infer from these data that one mantle source component may dominate a single lava flow, but that the two mantle source components are consistently represented to some extent in all lavas, regardless of the specific geographic location of the volcano. We therefore suggest that the Hawaiian mantle plume is unlikely to be compositionally concentrically zoned. Instead, the observed chemical variation is probably controlled by the thermal structure of the plume. [ABSTRACT FROM AUTHOR]