By Sid Perkins
Between our planet’s crust and its core lies the mantle, a realm where solid rock oozes under pressures millions of times as great as that exerted by the atmosphere. Although some scientists hold that the entire mantle gradually mixes, others suspect that the mantle’s deeper rock–and the trace elements it contains–typically doesn’t get too close to Earth’s crust, says geophysicist David Bercovici of Yale University. The limited-circulation scenario stems from the observation that the molten rock oozing from midocean ridges lacks much of the uranium, thorium, and other trace elements that spew from some aboveground volcanoes. The volcanoes are fueled by so-called hot-spot plumes that originate lower in the mantle than do areas feeding the ocean ridges.
Bercovici and his Yale colleague Shun-Ichiro Karato propose that when slow-rising mantle rock reaches within 400 kilometers of Earth’s surface, the material sheds much of its water and partially melts. During that process, the scientists surmise, most trace elements leach out of the rock. This occurs mostly within a 10-km-thick layer of the mantle, and the resulting liquid later circulates back toward the planet’s core.
Because mantle material in the hot-spot plumes rises quickly through the thin layer where leaching occurs, it doesn’t lose as many of its trace elements as slow-rising rock does. The researchers describe their model in the Sept. 4 Nature.
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