Friday Morning: Looking High and Low for Missing Lead

	Enlarge ImageThe actual drawing Stan Hart used to explain the first lead paradox to me. This is the isotope geochemist's version of having Paul McCartney sign your copy of Hard Day's Night (photo by Hugh Powell)
	Enlarge ImageLast May, Stan Hart was part of a research expedition that discovered hundreds of eels living in a volcanic crater in waters off Samoa (photo courtesy Hawaii Undersea Research Laboratory)
	Related Links
	» Conduits into Earth's Inaccessible Interior Hot plumes from deep within the Earth bring up telltale chemical clues about the mantle, from Oceanus magazine. » Nafanua, Eel City, and the Crater of Death More details on the May find near Samoa, from Oceanus magazine » Voyage to Vailulu'u Scientists discover a 14,300-foot active volcano off Samoa, from Woods Hole Currents. » Ocean Floor Covered in Volcanic Vents USA Today covers Chris German and the hydrothermal vents story I mentioned on Tuesday » Hitting the Hotspots How rock moves through the earth's mantle and up to the seafloor, from Oceanus magazine

A scientific poster session can look very much like a large high-school science fair project, except with more equations and fewer hand-drawn graphs. Among today’s 55 sessions (with 30 posters each), I set my sights on “Frontiers in Isotope Geochemistry IV,” a session organized by Nobu Shimizu and Albrecht Hofmann in honor of one of the field’s pioneers, Stan Hart.

Almost immediately I bumped into Hart himself, and he began to explain the first of the Great Lead Paradoxes. Sketching a quick graph on my notepad (see photo), Hart explained a problem that has been vexing geologists for decades: They have calculated how much of two lead isotopes (called lead-206 and lead-207) ought to be found in the Earth. The paradox: In all parts of the Earth examined so far, most of the lead-207 is missing. Where did it go?

Hart said one possible location is deep in the Earth’s mantle, whose depth and intense heat and pressure make it off limits to human exploration. Hart thinks that dense sulfides, whose crystal structure can store lots of lead atoms, carry the lead downward. A nearby poster showed the first few experimental results supporting this explanation. But still, Hart said, the inaccessibility of the mantle means the explanation is hard to test and therefore not terribly convincing at the moment.

As a working hypothesis, though, the idea spurs scientists to look for evidence of the Earth’s mantle in more accessible places. Shimizu’s graduate student Jessica Warren went looking in a kind of rock called peridotite that they found on the Southwest Indian Ridge between Africa and Antarctica.

This region of seafloor is spreading apart extremely slowly, bringing up rock that normally lies much deeper. Dredging peridotite from this ocean ridge is about as close as geologists can get to visiting the center of the Earth with a collecting jar. Perhaps sulfides scattered through the peridotite will lead geologists to discover a massive lead-207 hideout deep in Earth’s mantle.