Contact: Robert Irion (408/459-2495)

PLUMBING UNDER MIDOCEAN RIDGES CALLED INTO QUESTION BY NEW STUDY

* This release is embargoed until 5 p.m. EST Thursday, December 21, 1995. The study will appear in the December 22 issue of the journal Science.

SANTA CRUZ, CA--It's a common image in geology textbooks: Molten magma wells up from the planet's bowels to the seafloor at midocean ridges, giving birth to new slabs of the earth's crust. The magma, a fiery orange or red, ascends in teardrop-shaped blobs through what look like inverted funnels or gigantic pipes.

There's a problem with that image, say scientists at the University of California, Santa Cruz. It's probably wrong.

Instead of a pipe, the plumbing under a midocean ridge acts like a sponge, and a very bad sponge at that. A typical chunk of the earth's mantle in one of these zones, the scientists believe, holds just 1 or 2 parts magma per 1,000 parts unmelted rock--a far smaller ratio than predicted by the reigning model of seafloor spreading.

What's more, as the magma oozes upward through tiny channels, it appears to stay in chemical contact with the mantle rocks around it. Other researchers have assumed that once magma forms deep under the ocean floor, it rises quickly and no longer reacts with the surrounding mantle.

"Midocean ridges are the biggest magmatic systems on the planet," says Quentin Williams, associate professor of earth sciences at UCSC. "The previous picture of how these systems work may not be accurate. The chemistry of what comes out at the top seems to tell us that the magma percolates upward instead of flowing through conduits, and that it continuously bathes the mantle in a very small amount of melt."

A research team at UCSC led by graduate student Craig Lundstrom published its results in the December 22 issue of the journal Science. Coauthors are professor of earth sciences James Gill, a geochemist, and Williams, a mineral physicist.

The team analyzed rocks collected by other scientists with the submersible vessel ALVIN off the Pacific Northwest coast. The rocks had erupted recently at the Juan de Fuca Ridge, where new oceanic crust forms at the rate of a few centimeters per year. Juan de Fuca is but a minor part of a globe-encircling network of ocean ridges, such as the Mid-Atlantic Ridge and the East Pacific Rise. The ridges stretch some 60,000 kilometers and churn out about 80 percent of all magma on the planet each year.

Oceanic crust emerges in this way: Magma, known as basalt, begins to melt from the solid mantle about 100 kilometers under the ocean. When this buoyant material rises to within about 20 kilometers of the midocean ridge, it enters a chamber of magma-- the source material for future eruptions. Once it erupts, the basalt hardens into slabs of crust. The crust spreads slowly away from the ridge atop a thicker layer of mantle, depleted of its basalt.

In the most popular model of this process, the magma makes a clean break once it melts out of the mantle. It ascends rapidly (on a geologic timescale) via some sort of direct plumbing to the magma chamber. Along the way, the model dictates, the basalt does not interact chemically with the rest of the mantle. This picture seemed consistent with the few observations of midocean ridges, shrouded as they are by the sea's chilly depths.

Lundstrom and his coworkers applied a potent and relatively new tool to test the model. The tool uses the radioactive decay of uranium, a heavy element in basalt, as a geochemical tracer. On its train ride of decay to the stable element lead, uranium stops at several other elemental stations, including protactinium, thorium, and radium. These elements make a series of "parent" and "daughter" isotopes, each with well-known rates of decay. The ratios of the isotopes yield intriguing clues about the chemical and physical travels of the magma.

All three elements indicate that the basalt stays in constant chemical equilibrium with the solid mantle as it rises. Only at the last possible moment, when the melt separates from the rest of the mantle and joins the magma chamber, does it acquire a unique chemical signature. This implies that the melt seeps through tiny cracks, much as groundwater leaches through porous rocks. To satisfy the team's data, the melt must squeeze through spaces that compose a minuscule fraction of the mantle--just one-tenth to two-tenths of a percent.

"The amount of melt at any spot beneath the ridge is vanishingly small," says Lundstrom. "Even so, it can move at high rates." Indeed, the team's model predicts that magma wends its way through those tight spaces at one to several meters per year. Meanwhile, the solid mantle around it creeps upward relentlessly toward the ridge axis at about five centimeters per year. Says Williams: "It's like a fast conveyor belt on top of a slow one."

To make the model work, the team had to invoke veins of basalt-rich mantle far beneath the spreading centers, where the melting begins. Such veins could persist in the mantle, says Williams, if old slabs of oceanic crust don't mix completely into the rest of the mantle after they plunge back into the earth at subduction zones.

Editor's notes: You may reach the researchers as follows: Craig Lundstrom: (408) 459-5228 or cclund@bagnold.ucsc.edu James Gill: (408) 459-2425 or jgill@earthsci.ucsc.edu Quentin Williams: (408) 459-3132 or quentw@rupture.ucsc.edu

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