What exactly is the nature of that hidden fluid deepbeneath the surface and what changes does it undergo as it seeks anever-deeper venue? Answer to these questions can be found in article ina recent issue of the journal Nature by Dr. Ronit Kessel of the HebrewUniversity's Institute of Earth Sciences and her collaborators Prof.Max Schmidt, Prof. Peter Ulmer and Dr. Thomas Pettke from the SwissFederal Institute of Technology, Zurich. In the article, theresearchers report on a unique study in which fluids released fromsubmerging earth plates at depths of 120-180 kilometers andtemperatures between 700 and 1200 degrees centigrade are characterizedfor the first time.
Although the earth is principally a solidplanet, water plays a major role in its evolution and differentiation.The presence of fluid affects the depth, temperature and composition ofthe melting that takes place deep within the bowels of the earth.
Thesurface of the earth is broken into large plates which move onerelative to the other. When two plates collide, one plate can push theother downwards towards the center, carrying water deep into themantle, which is the area between the earth's crust and its core. Aswater enters the interior of the earth, it passes through water-rich(hydrous) minerals in the rock.
These hydrous minerals break downat depths of 50 to 200 kilometers, facilitating melting and ultimatelyleading to the super-heated volcanic zone. It is volcanic eruptionswhich regenerate our continental lithosphere (crust). Such volcanoescreated, for example, the Aegean islands in southern Greece (the mostfamous of them is Santorini, which erupted in the 16th century B.C.E.),the "ring of fire" around the Pacific Ocean, and more.
Thesevolcanoes eject many gases, for example carbon dioxide, to theatmosphere. These gases lead to thickening of the clouds and rain. As aresult, water carried down to the interior of the earth influenceprecipitation on the surface. In addition, water at depth dissolvessignificant amounts of matter. The mobility of the water at depths alsoresults in recycling other elements (including elements of economicvalue, such as chromium, nickel and vanadium) from place to place.
Inorder to characterize the fluids participating in every stage of thedownward water cycle, the Hebrew University and Swiss researchersdeveloped a novel experimental and analytical laboratory technique bywhich the composition of a fluid phase can be directly analyzedfollowing high pressure and temperature experiments. Their work focusedon determining how much water is stored in the down-going earth plate,how much dissolved matter it contains, and when these fluids arereleased from the plate and transferred to the mantle.
Theresults indicate that up to 180 kilometers deep, two kinds of fluidsexist. One, at higher levels, is a fluid rich in water (70-90%) withonly a little dissolved matter in it. This fluid exists at relativelylow temperatures. The second component is a thick "hydrous melt," richin dissolved matter, which contains only 10-30% water. This componentis a result of melting of the hydrous rocks at high temperature.
Intheir studies, the researchers found that different minerals displayvarying "preferences" for solubility, depending on the temperature atvarious depth levels.
Beyond a depth of 180 kilometers, only onekind of fluid exists, which is called supercritical liquid. Asupercritical liquid is defined as a component which smoothly changesits character from fluid-like to a more solid, melt-like state, but isneither.
The researchers stress that it is essential todistinguish between melts, fluids and supercritical liquids in order toachieve a better understanding of the relationship between thedown-going plate and volcanic eruptions and how matter is transferredin the earth interior.
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