MAGMAS that erupt onto oceanic islands differ in composition from those produced at mid-ocean ridges. The differences are subtle, and largely restricted to trace elements and radiogenic-element isotope ratios, but they are important because they reflect differences in the composition of the mantle that molts to produce these magmas. Such traces provide persuasive evidence for the idea that most oceanic islands are produced by mantle plumes — columns of hot rock rising from deep inside the Earth. Incontrast, magmas erupted at mid-ocean ridges are thought to be produced by melting of shallow upper mantle, welling up only because the crust above is moving apart. But are there also differences in the abundances of the major oxides? That would tell usa lot about the physics and origin of mantle plumes, but it is a notoriously difficult question to answer. On page 415 of this issue, Hauri reports perhaps the best evidence yet that the major- element chemistry of the Hawaiian mantle plume does indeed differ from that of the upper-mantle source of midocean-ridge basalts (MORB).
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