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The Nazca-Cocos plates separation zone is actually a crust-forming, sea floor spreading mid-ocean ridge itself. And if the Nazca-Cocos boundary continues moving west another 36 miles beyond Hess Deep, it will form what geologists call a "triple junction" with a major Pacific mid-ocean ridge, the East Pacific Rise.
What is happening already is interesting enough. A separate small and rotating wedge of oceanic crust, called the Galapagos Microplate, has formed on Hess Deep's south boundary. And a spreading zone between the Galapagos and the Nazca plates has already made a triple junction with the East Pacific Rise at a point more than 60 miles further south.
Much further east than we can see from the tallest deck of the research ship R/V Atlantis, a "hot spot" in the ocean crust has also raised a group of volcanoes high enough to reach the surface, forming the Galapagos Islands. That craggy and arid archipelago is home to rare and exceptionally long lived giant land tortoises. It was also a major place where naturalist Charles Darwin gathered evidence for his theory on the evolution of species during a voyage aboard a 19th century Atlantis, the HMS Beagle.
Meanwhile, about 30 miles northwest of Hess Deep, oceanographers and geologists have caught another mid-ocean ridge virtually in the act of formation. This "incipient" spreading center is creating another embryonic triple junction edged by the Galapagos Microplate, the East Pacific Rise and the Cocos plate.
"I know of no other place where we have a new incipient ridge trying to form," said Emily Klein of a co-principal investigator on Atlantis's geological expedition to Hess Deep.
Klein was speaking on the afternoon of Wednesday, March 24, when Atlantis had been repositioned from Hess Deep to directly over this "baby" of mid-ocean ridges.
During a 24-hour break in the Hess Deep study schedule, Klein, Duke doctoral student Michael Stewart, and James Brophy of arranged to visit a portion of that ridge to look for samples of freshly formed ocean crust. In the geological time scale, "fresh" means younger than about 20,000 years.
They would collect samples using the low-technology "wax coring" method, which Klein acknowledged to be "very rudimentary." Wax coring is a two stage process. The first stage is like using a battering ram to storm a castle. The second is like using cellophane tape to remove lint from a wool overcoat.
Klein's and Stewart's key working material was a sticky mixture of surfboard wax and petroleum jelly, a recipe created through "trial and error by lots of people," she said. Dipping into the ice cream containers where the concoction was stored, they scooped out enough to thoroughly coat the tops of five solid iron cylinders.
Stewart then wrote names or slogans on each cylinder. The first was called "Blue Devils" and the next "Lady Blue Devils," both acknowledging Duke's highly rated The others were called "Betty Boop," "Incip (for "incipient") This," and "Drop Wax, Not Bombs."
In a procedure that continued until after midnight, each treated cylinder in turn would be bolted, wax side down, to a 100 pound steel weight attached to a cable reel. The whole ensemble would then be dangled over Atlantis's starboard (right) side with a crane, with the hard-hatted and lifejacketed Klein, Stewart and Brophy helping position it.
Finally, the package would be dropped into the Pacific, and the heavy weight would begin pushing the corer towards the bottom. As the device plowed though the water, the weight would build up the corer's momentum enough to smash into the bottom with significant force.
Stepping back inside Atlantis, the trio would go to the computer lab to wait, occasionally monitoring a video camera aimed at the falling cable. They would be watching the video screen for the relaxed cable tension that would tell them the corer had crash landed. If the researchers were lucky, the corer would bash the kind of rock they wanted: black-colored basaltic glass. And the glass fragments would immediately embed themselves in the sticky wax for hoisting back to the ship.
All corers except "Incip This" emerged with black fragments on their noses. And, by Thursday afternoon the pieces had all been stored in water-filled vials for transport to Duke in several weeks.
Once in Durham, N.C., they will be analyzed on the division of special inductively coupled plasma mass spectrometer. That device, which Klein bought with a
grant, uses a powerful laser to strip electrons from atoms, then pass the resulting charged ions through a powerful magnetic field to separate them by weight.
"It's a very bright light," said Brophy, Stewart's former advisor at Indiana.
When erupting 1,800 91社区福利 Fahrenheit molten basaltic lava encounters the near-freezing seawater on the ocean floor, its outer surface cools too rapidly for atoms to organize into crystals, said Klein, a geochemist.
What forms instead is a shell of black glass that can be easily "spalled" (flaked) off the underlying basalt. One layer can effortlessly separate from the other because the inner basalt, insulated by its surrounding glassy shell, does have time to crystallize normally and is thus physically different.
The flakes of glass are like time machines, because their sudden cooling (also called "quenching") preserves a record of the lava's chemistry at the moment it emerged from underground.
"My real interest is understanding the physical conditions under which melt is generated by looking at the products of mantle melting," Klein explained. "I have to make inferences based on the chemistry."
Scientists think the various forms of basalt, the basic construction materials of oceanic crust, originate in the inaccessible mantle, a semi-solid, hot and highly pressurized intermediate layer underneath Earth's outer skin.
Basalts are thought to form whenever mantle materials melt. Once melting begins, the liquid basalt must then rise, simply because it is then less dense than its more-solid surroundings.
By studying the chemistries of melts that reach the surface, researchers have already deduced that the mantle's composition varies from region to region, Klein said. "People use terms like a plum pudding mantle, or a paisley mantle."
They make these deductions by analyzing how isotopes of the same chemical elements, especially lead and strontium, vary among basalt samples collected from different places, she added.
Isotopes are forms of the same elements that differ only in the number of neutrons in their atoms. And "isotope compositions in the Indian Ocean differ from those in the Atlantic and Pacific," Klein said. "On a smaller scale, lavas two miles away from each other along a mid-ocean ridge may have different isotopic composition."
Klein also uses chemical sleuthing to study melting conditions down in the 1,380-mile-thick mantle, which is thought to begin slightly more than four miles below the ocean floor with melting occurring at deeper depths. "After years of study, it turns out that we can tell a lot about melting conditions by looking at major and trace elements in the mantle," she said.
Major elements are ones most ubiquitous in Earth's crust, while trace elements are those less plentiful. The major elements especially helpful for Klein's chemical investigations are sodium and iron, while important trace elements include barium, various rare earth elements, and uranium.
By testing a basalt for its sodium concentrations, for example, she can deduce whether it was formed at a site where there was much melt or only a little. That's because sodium "prefers" being in a melt more than it does in the hot semi-solid mantle. So the initial melt, which is also the smallest, will be most concentrated in sodium. Sodium concentrations in a larger melt, by contrast, will be more diluted by other major and trace elements.
Sodium concentrations can thus tell her how hot the mantle was where the basalt formed, because the hotter the temperatures the larger the melt. Likewise, the concentration of iron is a good benchmark for the pressure, and hence the depth, at which a basalt sample was created. That's because iron "prefers" to be in a melt at higher pressure.
A third process she studies is how the chemistry of a melt changes as it rises toward Earth's surface. She can do that by testing for the ratios of three different kinds of minerals that are present in basalt. Those mineral types are olivine, pyroxene and plagioclase feldspar.
Rising magma "will begin to cool, because it is encountering cooler and cooler surrounding rocks," she said. And because the three types of minerals crystallize at different rates depending on the temperature, the relative proportions of olivine, pyroxene and feldspar in basalt will thus vary.
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