So, by analyzing the elements contained in one of these meteorites, we should be able to back out the composition of our planet. This is one of the ways we've guessed at the composition of the core. Meteorites tell us that the Earth should have a lot more iron and nickel than what we have observed in the crust. It can't go in the mantle because the seismic wave speeds aren't fast enough and also we don't observe iron and nickel coming out of volcanoes that apparently have a deep mantle source.
Therefore, the missing iron and nickel must be in the core. When you read them, think about the following questions for discussion:. You will be graded on the quality of your participation. See the grading rubric for specifics on how this assignment will be graded. Skip to main content. Eliza at about 3. Diamond anvil cell for conducting high-pressure mineralogic experiments assembled left and disassembled with a US quarter for scale right.
Allende, Mexico, carbonaceous chondrite. This chondrite contains chondrules note the round cavity left by the removal of a large chondrule. Allende, like many carbonaceous chondrites, contains calcium-aluminum-rich inclusions CAIs. Unlike chondrules, which are round and composed mostly of silicate minerals like olivine and pyroxene, CAIs are predominantly white to light gray in color, irregularly shaped, and rich in refractory high- temperature minerals like melilite and spinel.
They are believed to predate chondrules by at least 2 million years. Schmandt and Jacobsen's findings build on a discovery reported in March in the journal Nature in which scientists discovered a piece of the mineral ringwoodite inside a diamond brought up from a depth of miles by a volcano in Brazil. That tiny piece of ringwoodite — the only sample in existence from within the Earth — contained a surprising amount of water bound in solid form in the mineral.
For years, Jacobsen has been synthesizing ringwoodite, colored sapphire-like blue, in his Northwestern lab by reacting the green mineral olivine with water at high-pressure conditions. The Earth's upper mantle is rich in olivine.
He found that more than one percent of the weight of the ringwoodite's crystal structure can consist of water — roughly the same amount of water as was found in the sample reported in the Nature paper. This mineral can contain a lot of water under conditions of the deep mantle. For the study reported in Science, Jacobsen subjected his synthesized ringwoodite to conditions around miles below the Earth's surface and found it forms small amounts of partial melt when pushed to these conditions.
Jacobsen uses small gem diamonds as hard anvils to compress minerals to deep-Earth conditions. Jacobsen's findings produced the same evidence of partial melt, or magma, that Schmandt detected beneath North America using seismic waves. Because the deep mantle is beyond the direct observation of scientists, they use seismic waves — sound waves at different speeds — to image the interior of the Earth.
The melting the researchers have detected is called dehydration melting. Rocks in the transition zone can hold a lot of H2O, but rocks in the top of the lower mantle can hold almost none. The water contained within ringwoodite in the transition zone is forced out when it goes deeper into the lower mantle and forms a higher-pressure mineral called silicate perovskite, which cannot absorb the water.
This causes the rock at the boundary between the transition zone and lower mantle to partially melt. Just a little bit of melt, about one percent, is detectible with the new array of seismometers aimed at this region of the mantle because the melt slows the speed of seismic waves, Schmandt said. The USArray is part of EarthScope , a program of the National Science Foundation that deploys thousands of seismic, GPS and other geophysical instruments to study the structure and evolution of the North American continent and the processes the cause earthquakes and volcanic eruptions.
The paper is titled " Dehydration melting at the top of the lower mantle. Dueker, the University of Wyoming. A photo of rock from one of these locations is shown on this page. Find Other Topics on Geology. Maps Volcanoes World Maps. Igneous and Volcanic Features. Geology Dictionary. Roadside Geology Guides. Angel Falls. Land Below Sea Level. Divisions of Geologic Time. Rocks: Galleries of igneous, sedimentary and metamorphic rock photos with descriptions. Minerals: Information about ore minerals, gem materials and rock-forming minerals.
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