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Pacific volcanoes share split personality

Dual chemistry of island chains reflects variations in their deep source

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11:07am, September 19, 2011
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Hawaii’s scenic volcanoes come in two chemical flavors, and now scientists think the igneous peaks on several other Pacific island chains do, too.

Two parallel lines of volcanoes stretch from the Big Island of Hawaii in the southeast to Molokai in the northwest.  Volcanoes on the Samoan and Marquesas islands are similarly paired. A new study finds that, as in Hawaii, one row is richer than the other in versions of elements such as lead and neodymium.

“This might be a common feature for all the Pacific hotspots,” says Shichun Huang, a geochemist at Harvard University and lead author of a paper appearing online September 18 in Nature Geoscience.

If so, these island chains may tap the same source deep in Earth’s mantle. Molten rock rises toward the surface in two chemically distinct streams, one stream feeding each row of volcanoes.

Geologists think Hawaii, Samoa, and the Marquesas each formed as a plate of Earth’s crust moved across a “hotspot,” the top of a plume carrying molten material from the planet’s deep interior. Like a welding torch passing across a piece of metal, the hotspot punched out island after island as the plate moved over it.

Recent studies have shown that the hotspots are more complex than once thought, says isotope geochemist Dominique Weis of the University of British Columbia. The mantle plume rising below Hawaii, for instance, feeds individual streams of chemically distinct magmas into Mauna Loa and Mauna Kea, both on the Big Island. Mauna Loa has a higher ratio of the most abundant form of lead on Earth, lead-208, compared with lead-206, which has two fewer neutrons in its nucleus.

By analyzing published data on lava samples, Huang and his colleagues have now shown that this chemical difference also exists in Samoa and the Marquesas.

The plumes feeding these island chains (as well as Hawaii’s) apparently tap a single massive reservoir that underlies much of the central and southern Pacific. This reservoir contains chemical signatures of ancient surface rock that plowed into the interior eons ago through plate tectonics. As a plume rises, it carries part of this material with it.

The new work shows how surface volcanoes can be linked to deep sources of magma, says geochemist Albrecht Hofmann of the Max Planck Institute for Chemistry in Mainz, Germany. A few other scientists have questioned the existence of mantle plumes, but the new work “strongly suggests that at least these particular hotspots are actually mantle plumes that ascend from the lowermost mantle,” Hofmann says.

Huang’s group is now checking other Pacific island chains to see if they too show this same two-faced nature.

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