2017 03 Temperature Earth Mantle | Mantle (Geology) | Plate Tectonics

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2017 03 Temperature Earth Mantle
    Taking earth's inner temperature: Surprisingnew study finds that the mantle is hotterthan we thought 2 March 2017  Each of the tiny rocks in this circular mount is about halfof a synthetic mantle sample -- after it has been heatedand crushed in the piston-cylinder apparatus, then cutopen and polished. Sarafian puts her samples in thismount in order to analyze them for their water contentusing secondary ion mass spectrometry (SIMS). Credit:Photo by Jayne Doucette, Woods Hole OceanographicInstitution The temperature of Earth's interior affectseverything from the movement of tectonic plates tothe formation of the planet. A new study led by Woods Hole OceanographicInstitution (WHOI) suggests the mantle—the mostlysolid, rocky part of Earth's interior that lies betweenits super-heated core and its outer crustallayer—may be hotter than previously believed. Thenew finding, published March 3 in the journal Science  , could change how scientists think aboutmany issues in Earth science including how oceanbasins form. At mid-ocean ridges, the tectonic plates that formthe seafloor gradually spread apart, said thestudy's lead author Emily Sarafian, a graduatestudent in the MIT-WHOI Joint Program. Rockfrom the upper mantle slowly rises to fill the voidbetween the plates, melting as the pressuredecreases, then cooling and re-solidifying to formnew crust along the ocean bottom. We wanted tobe able to model this process, so we needed toknow the temperature at which rising mantle rockstarts to melt. But determining that temperature isn't easy. Sinceit's not possible to measure the mantle'stemperature directly, geologists have to estimate itthrough laboratory experiments that simulate thehigh pressures and temperatures inside the Earth.Water is a critical component of the equation: themore water (or hydrogen) in rock, the lower thetemperature at which it will melt. The peridotite rockthat makes up the upper mantle is known to containa small amount of water. But we don't knowspecifically how the addition of water changes thismelting point, said Sarafian's advisor, WHOIgeochemist Glenn Gaetani. So there's still a lot ofuncertainty.   Image of one of the team's lab mimicry experiments,which was conducted in a capsule made of gold-palladium alloy. The black boxes highlight the locations ofolivine grains, and the dark pits in the olivines are actualmeasurements for the water content of the olivine. The  1 / 3    peridotite is the super fine-grained matrix. Credit: EmilySarafian. To figure out how the water content of mantle rockaffects its melting point, Sarafian conducted aseries of lab experiments using a piston-cylinderapparatus , a machine that uses electrical current,heavy metal plates, and stacks of pistons in orderto magnify force to recreate the high temperaturesand pressures found deep inside the Earth.Following standard experimental methodology,Sarafian created a synthetic mantle sample. Sheused a known, standardized mineral compositionand dried it out in an oven to remove as muchwater as possible.Until now, in experiments like these, scientistsstudying the composition of rocks have had toassume their starting material was completely dry,because the mineral grains they're working with aretoo small to analyze for water. After running theirexperiments, they correct their experimentallydetermined melting point to account for the amountof water known to be in the mantle rock. The problem is, the starting materials are powders,and they adsorb atmospheric water, Sarafian said. So, whether you added water or not, there's waterin your experiment. Sarafian took a different approach. She modifiedher starting sample by adding spheres of a mineralcalled olivine, which occurs naturally in the mantle.The spheres were still tiny—about 300 micrometersin diameter, or the size of fine sand grains—but theywere large enough for Sarafian to analyze theirwater content using secondary ion massspectrometry (SIMS). From there, she was able tocalculate the water content of her entire startingsample. To her surprise, she found it containedapproximately the same amount of water known tobe in the mantle.Based on her results, Sarafian concluded thatmantle melting had to be starting at a shallowerdepth under the seafloor than previously expected.  In her laboratory experiments, Sarafian used a piston-cylinder apparatus--the red machine behind her--tosimulate the high pressures and temperature of theEarth's mantle. The heavy stainless steel plates visibleon the table are stacked on the apparatus, with the tinysynthetic mantle sample inside a 'pressure vessel'underneath them. Once the machine is turned on, pistonsapply massive pressure from above and below thesample, which is simultaneously heated with electricalcurrent. Credit: Photo by Veronique LaCapra, WoodsHole Oceanographic Institution To verify her results, Sarafian turnedmagnetotellurics—a technique that analyzes theelectrical conductivity of the crust and mantle underthe seafloor. Molten rock conducts electricity muchmore than solid rock, and using magnetotelluricdata, geophysicists can produce an image showingwhere melting is occurring in the mantle.But a magnetotelluric analysis published in Nature  in 2013 by researchers at the Scripps Institution ofOceanography in San Diego showed that mantlerock was melting at a deeper depth under the seafloor than Sarafian's experimental data hadsuggested.At first, Sarafian's experimental results and themagnetotelluric observations seemed to conflict,but she knew both had to be correct. Reconcilingthe temperatures and pressures Sarafian measuredin her experiments with the melting depth from theScripps study led her to a startling conclusion: Theoceanic upper mantle must be 60°C (~110°F)hotter than current estimates, Sarafian said.A 60-degree increase may not sound like a lot  2 / 3    compared to a molten mantle temperature of morethan 1,400°C. But Sarafian and Gaetani say theresult is significant. For example, a hotter mantlewould be more fluid, helping to explain themovement of rigid tectonic plates. More information:   Experimental constraints onthe damp peridotite solidus and oceanic mantlepotential temperature, Science  , science.sciencemag.org/cgi/doi/10.1126/science.aa j2165 Provided by Woods Hole OceanographicInstitutionAPA citation: Taking earth's inner temperature: Surprising new study finds that the mantle is hotter thanwe thought (2017, March 2) retrieved 3 March 2017 from https://phys.org/news/2017-03-temperature-earth-mantle.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only. Pwred byTCPD wwwtcpdf.org)  3 / 3
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