Ice-VII: Deep Ice, Tectonic Slabs, and Diamond Inclusions

      Ice is found deep within the hot interior of the earth. Let that statement sink in. In research published today, a form of super-compact ice, found embedded in diamonds, offers the first direct clue that there is abundant water more than 610 kilometers deep in the mantle.

      This ice, identified by its crystal structure and called ice-VII, doesn’t exist at Earth’s surface. It forms only at pressures greater than about 24 gigapascals — corresponding to depths between 610 and 800 kilometers, researchers report today in Science. The structure of ice-VII comprises a hydrogen bond framework in the form of two interpenetrating (but non-bonded) sublattices. Hydrogen bonds pass through the center of the water hexamers (as shown above, and, to some degree below) and thus do not connect the two lattices. 

  

      Ice-VII's presence in diamonds suggests that there is water-rich fluid in the transition zone between the upper and lower mantle, and even into the top of the lower mantle.

     “This is really the first time that we see water at such depths,” says Dr. Oded Navon, a mantle petrologist at the Hebrew University of Jerusalem.

      When slabs of earth’s crust sink into the mantle layer below, they drag ocean water with them. How deep the slabs sink has been a long-standing question. Researchers have suspected that abundant aqueous fluid exists in the deep mantle, carried there by slabs bearing water-rich minerals that shed their water when they reach the transition zone. But scientists have not previously found direct evidence of that water.

     That is where diamonds come in. Diamonds form at high temperatures and pressures, crystallizing in pockets rich in the mineral carbonate before being carried to the surface with erupting magma. As the diamond crystals form, they can enclose tiny amounts of fluid or rock from their surroundings. These impurities represent tiny capsules of mantle. Diamond inclusions are the only direct window scientists have into the fabric of earth more than a kilometer beneath the surface.

     Dr. Oliver Tschauner, a mineralogist at the University of Nevada, Las Vegas, and his colleagues set out to study diamond inclusions, but they weren’t looking for ice. They were hunting for signs of a molecular form of carbon dioxide that might help reveal clues to the cycling of carbon from slabs into the mantle. The researchers used a variety of techniques, including X-ray diffraction, infrared spectroscopy and X-ray fluorescence, to try to identify the composition of the inclusions within three diamonds, one from China and two from southern Africa.

      Instead of carbon dioxide, the team saw a telltale pattern in how some of the X-rays scattered as they passed through the diamond. That pattern pointed to ice-VII. The presence of that extremely high-pressure form of ice was a powerful clue to the depth at which the diamond must have formed. The diamonds also contained separate inclusions of fluids rich in certain salts, such as magnesium calcite and halite, and of carbon-rich fluids. 

     Water-rich fluids deep in the mantle could be important for driving the circulation that fuels the movements of tectonic plates and the eruptions of volcanoes. The presence of water can make it easier for rocks to melt, Dr.  Navon says, by lowering the melting point of hot rock under pressure. Additionally, fluids can help redistribute heat within the mantle.

     In addition, some large, heat-producing radioactive elements such as potassium, thorium and uranium don’t fit easily into the rigid crystalline structures of minerals, so scientists prefer melted rock when it’s available. “You just need a little bit of fluid, and they are moving into the melt,” Dr. Navon adds.

     The study also raised another mystery. Fluid inclusions within diamonds originating at shallower depths, perhaps 150 to 200 kilometers below the surface, contain a mélange of water, salt, and carbonates. But Dr. Tschauner and his colleagues found that in their deep diamonds, the inclusions are sequestered individually: ice in one inclusion, carbonates in another, salts in yet a third. “We were surprised that they were all separate rather than occurring together,” Dr. Tschauner said.  

Any ideas about this mysterious separation in the materials in inclusions? 

And, how cool to have remnants of tectonic slabs as inclusions in your jewelry!
Steph

Two Hundredth Post: More Continental Crust on South America's Andean Plateau

       This is our 200th blog post and our 48th month of publishing Partial Ellipsis of the Sun! Thanks for your support over the past four years!

      "Seismologists investigating how earth forms new continental crust have compiled more than 20 years of seismic data from a wide swath of South America's Andean Plateau and determined that processes there have produced far more continental rock than previously believed.


     "When crust from an oceanic tectonic plate plunges beneath a continental tectonic plate, as it does beneath the Andean Plateau, it brings water with it and partially melts the mantle, the layer below earth's crust," said Rice University's Dr. Jonathan Delph, co-author of the new study published this week. "The less dense melt rises, and one of two things happens: It either stalls in the crust to crystallize in formations called plutons or reaches the surface through volcanic eruptions."

     Dr. Delph said the findings suggest that mountain-forming regions like the Andean Plateau, which geologists refer to as "orogenic plateaus," could produce much larger volumes of continental rock in less time than previously believed.

     Co-author Dr. Kevin Ward, a researcher at the University of Utah, said, "When we compared the amount of trapped plutonic rock beneath the plateau with the amount of erupted volcanic rock at the surface, we found the ratio was almost 30:1. That means 30 times more melt gets stuck in the crust than is erupted, which is about six times higher than what's generally believed to be the average. That's a tremendous amount of new material that has been added to the crust over a relatively short time period."

     The Andean Plateau covers much of Bolivia and parts of Peru, Chile, and Argentina. Its average height is more than 12,000 feet, and though it is smaller than Asia's Tibetan Plateau, different geologic processes created the Andean Plateau. The mountain-building forces at work in the Andean plateau are believed to be similar to those that worked along the western coast of the U.S. some 50 million years ago. Dr. Delph said it's possible that similar forces were at work along the coastlines of continents throughout Earth's history.


     Most of the rocks that form Earth's crust initially came from partial melts of the mantle. If the melt erupts quickly, it forms basalt, which makes up the crust beneath the oceans on Earth; but there are still questions about how continental crust, which is more buoyant than oceanic crust, is formed. Drs. Delph and Ward spent several months combining public datasets from seismic experiments. Seismic energy travels through different types of rock at different speeds, and by combining datasets that covered a 500-mile-wide swath of the Andean Plateau, Ward and Delph were able to resolve large plutonic volumes that had previously been seen only in pieces.



     Over the past 11 million years, volcanoes have erupted thousands of cubic miles' worth of material over much of the Andean Plateau. Ward and Delph calculated their plutonic-to-volcanic ratio by comparing the volume of regions where seismic waves travel extremely slowly beneath volcanically active regions, indicating some melt is present, with the volume of rock deposited on the surface by volcanoes.

     "Orogenic oceanic-continental subduction zones have been common as long as modern plate tectonics have been active," Dr. Delph said. "Our findings suggest that processes similar to those we observe in the Andes, along with the formation of supercontinents, could have been a significant contributor to the episodic formation of buoyant continental crust."


Happy 200th!
Steph