Hawaiian plume dynamics

Lassiter, John

The Hawaiian Islands have long shaped geologists' views about Earth's interior. The apparently fixed position of the Hawaiian 'hot spot' led to the theory that deep-seated plumes of hot, buoyant mantle were responsible for ocean island volcanism at Hawaii and many other ocean island chains (1). Chemical and isotopic differences between ocean island basalts and mid-ocean ridge basalts have long been used by geochemists to constrain models of mantle convection and the chemical evolution of Earth (2-4). In this issue, Blichert-Toft et al. (5) present evidence from hafnium isotopes suggesting that ancient deep ocean (pelagic) sediments are present in the source of some Hawaiian lavas. Important in its own right, this result also suggests that combined geochemical and seismologic study of the Hawaiian 'plume' may help resolve one of the most important and long-standing questions in earth science: whether convection of Earth's mantle is layered. The mounting evidence for ancient recycled crust and sediments in the Hawaiian plume suggests how seismologists and geochemists can combine forces to constrain the mechanism of mantle convection, at present and far back in Earth history. Depending on the depth at which the Hawaiian plume originates, the presence of old recycled crust in the plume suggests hvo different models of mantle convection. If the Hawaiian plume derives from the 660-km discontinuity, the presence of ancient recycled crust in the plume would suggest that this region of the mantle has acted as a slab 'graveyard' for much of Earth history. This would strengthen suggestions that whole-mantle convection began recently (4). In contrast, if the Hawaiian plume derives from the core-mantle boundary, the presence of ancient crust in this plume would require such recycled material to be present in the lower mantle. This would in turn require that slabs penetrated the 660-km discontinuity at I to 3 billion years ago, much as they do today. How can we determine the depth from which the Hawaiian plume originates? Geochemistry can provide only indirect evidence. Brandon et al. (14) recently argued that anomalous abundances of an osmium isotope, 186Os, in some Hawaiian lavas reflect incorporation of a small quantity of core material. Blichert-Toft et al. argue that the Hf-Nd isotope trend in Hawaiian lavas precludes melting of depleted upper mantle. The lack of an upper-mantle signature in Hawaiian basalts may indirectly suggest that the Hawaiian plume originated at greater depth. Seismological studies can potentially provide a stronger constraint on the Hawaiian plume's depth of origin. Recent seismological studies have shown that the Iceland plume extends through the upper/lower mantle transition zone (15) to the core/mantle boundary (16), providing the most direct evidence to date for some mantle plumes deriving from the core/mantle boundary. No comparable study has yet been performed for Hawaii. An extensive array of ocean-bottom seismometers will be necessary for detecting a hot, narrow plume in the deep mantle beneath Hawaii, if such a plume exists. Such a study, combined with the increasingly robust evidence for ancient recycled crust in the Hawaiian plume, may finally answer the question whether mantle convection is, or has been, substantially layered.

Hawaiian plume dynamics

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