MSGID: 1:317/3 645486c4   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    New clues about the rise of Earth's continents    
    One popular explanation for properties that result in dry land is   
   unlikely according to new experiments    
      
     Date:   
         May 4, 2023   
     Source:   
         Smithsonian   
     Summary:   
         New research deepens the understanding of Earth's crust by testing   
         and ultimately eliminating one popular hypothesis about why   
         continental crust is lower in iron and more oxidized compared to   
         oceanic crust. The iron- poor composition of continental crust is   
         a major reason why vast portions of the Earth's surface stand above   
         sea level as dry land, making terrestrial life possible today. The   
         study uses laboratory experiments to show that the iron-depleted,   
         oxidized chemistry typical of Earth's continental crust likely did   
         not come from crystallization of the mineral garnet, as a popular   
         explanation proposed in 2018.   
      
      
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   FULL STORY   
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   Continents are part of what makes Earth uniquely habitable for life among   
   the planets of the solar system, yet surprisingly little is understood   
   about what gave rise to these huge pieces of the planet's crust and   
   their special properties. New research from Elizabeth Cottrell, research   
   geologist and curator of rocks at the Smithsonian's National Museum of   
   Natural History, and lead study author Megan Holycross, formerly a Peter   
   Buck Fellow and National Science Foundation Fellow at the museum and now   
   an assistant professor at Cornell University, deepens the understanding of   
   Earth's crust by testing and ultimately eliminating one popular hypothesis   
   about why continental crust is lower in iron and more oxidized compared   
   to oceanic crust. The iron-poor composition of continental crust is a   
   major reason why vast portions of the Earth's surface stand above sea   
   level as dry land, making terrestrial life possible today.   
      
   The study, published today in Science, uses laboratory experiments to show   
   that the iron-depleted, oxidized chemistry typical of Earth's continental   
   crust likely did not come from crystallization of the mineral garnet,   
   as a popular explanation proposed in 2018.   
      
   The building blocks of new continental crust issue forth from the   
   depths of the Earth at what are known as continental arc volcanoes,   
   which are found at subduction zones where an oceanic plate dives beneath   
   a continental plate. In the garnet explanation for continental crust's   
   iron-depleted and oxidized state, the crystallization of garnet in the   
   magmas beneath these continental arc volcanoes removes non-oxidized   
   (reduced or ferrous, as it is known among scientists) iron from the   
   terrestrial plates, simultaneously depleting the molten magma of iron   
   and leaving it more oxidized.   
      
   One of the key consequences of Earth's continental crust's low iron   
   content relative to oceanic crust is that it makes the continents less   
   dense and more buoyant, causing the continental plates to sit higher atop   
   the planet's mantle than oceanic plates. This discrepancy in density and   
   buoyancy is a major reason that the continents feature dry land while   
   oceanic crusts are underwater, as well as why continental plates always   
   come out on top when they meet oceanic plates at subduction zones.   
      
   The garnet explanation for the iron depletion and oxidation in continental   
   arc magmas was compelling, but Cottrell said one aspect of it did not   
   sit right with her.   
      
   "You need high pressures to make garnet stable, and you find this low-iron   
   magma at places where crust isn't that thick and so the pressure isn't   
   super high," she said.   
      
   In 2018, Cottrell and her colleagues set about finding a way to test   
   whether the crystallization of garnet deep beneath these arc volcanoes   
   is indeed essential to the process of creating continental crust as is   
   understood. To accomplish this, Cottrell and Holycross had to find ways to   
   replicate the intense heat and pressure of the Earth's crust in the lab,   
   and then develop techniques sensitive enough to measure not just how much   
   iron was present, but to differentiate whether that iron was oxidized.   
      
   To recreate the massive pressure and heat found beneath continental   
   arc volcanoes, the team used what are called piston-cylinder presses   
   in the museum's High-Pressure Laboratory and at Cornell. A hydraulic   
   piston-cylinder press is about the size of a mini fridge and is mostly   
   made of incredibly thick and strong steel and tungsten carbide. Force   
   applied by a large hydraulic ram results in very high pressures on tiny   
   rock samples, about a cubic millimeter in size. The assembly consists   
   of electrical and thermal insulators surrounding the rock sample, as   
   well as a cylindrical furnace. The combination of the piston-cylinder   
   press and heating assembly allows for experiments that can attain the   
   very high pressures and temperatures found under volcanoes.   
      
   In 13 different experiments, Cottrell and Holycross grew samples of   
   garnet from molten rock inside the piston-cylinder press under pressures   
   and temperatures designed to simulate conditions inside magma chambers   
   deep in Earth's crust.   
      
   The pressures used in the experiments ranged from 1.5 to 3 gigapascals --   
   that is roughly 15,000 to 30,000 Earth atmospheres of pressure or 8,000   
   times more pressure than inside a can of soda. Temperatures ranged from   
   950 to 1,230 degrees Celsius, which is hot enough to melt rock.   
      
   Next, the team collected garnets from Smithsonian's National Rock   
   Collection and from other researchers around the world. Crucially, this   
   group of garnets had already been analyzed so their concentrations of   
   oxidized and unoxidized iron were known.   
      
   Finally, the study authors took the materials from their experiments   
   and those gathered from collections to the Advanced Photon Source at the   
   U.S. Department of Energy's Argonne National Laboratory in Illinois. There   
   the team used high- energy X-ray beams to conduct X-ray absorption   
   spectroscopy, a technique that can tell scientists about the structure   
   and composition of materials based on how they absorb X-rays. In this   
   case, the researchers were looking into the concentrations of oxidized   
   and unoxidized iron.   
      
   The samples with known ratios of oxidized and unoxidized iron provided   
   a way to check and calibrate the team's X-ray absorption spectroscopy   
   measurements and facilitated a comparison with the materials from their   
   experiments.   
      
   The results of these tests revealed that the garnets had not incorporated   
   enough unoxidized iron from the rock samples to account for the levels   
   of iron- depletion and oxidation present in the magmas that are the   
   building blocks of Earth's continental crust.   
      
   "These results make the garnet crystallization model an extremely unlikely   
   explanation for why magmas from continental arc volcanoes are oxidized   
   and iron depleted," Cottrell said. "It's more likely that conditions   
   in Earth's mantle below continental crust are setting these oxidized   
   conditions."  Like so many results in science, the findings lead to more   
   questions: "What is doing the oxidizing or iron depleting?" Cottrell   
   asked. "If it's not garnet crystallization in the crust and it's something   
   about how the magmas arrive from the mantle, then what is happening in   
   the mantle? How did their compositions get modified?"  Cottrell said   
   that these questions are hard to answer but that now the leading theory   
   is that oxidized sulfur could be oxidizing the iron, something a current   
   Peter Buck Fellow is investigating under her mentorship at the museum.   
      
   This study is an example of the kind of research that museum scientists   
   will tackle under the museum's new Our Unique Planet initiative,   
   a public-private partnership, which supports research into some of   
   the most enduring and significant questions about what makes Earth   
   special. Other research will investigate the source of Earth's liquid   
   oceans and how minerals may have served as templates for life.   
      
   This research was supported by funding from the Smithsonian, the National   
   Science Foundation, the Department of Energy and the Lyda Hill Foundation.   
      
       * RELATED_TOPICS   
             o Earth_&_Climate   
                   # Geology # Earth_Science # Earthquakes # Volcanoes   
             o Fossils_&_Ruins   
                   # Early_Climate # Origin_of_Life # Anthropology #   
                   Charles_Darwin   
       * RELATED_TERMS   
             o Continental_crust o Crust_(geology) o Structure_of_the_Earth   
             o Lithosphere o Ocean o Volcano o Quartz o Earth   
      
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   Story Source: Materials provided by Smithsonian. Note: Content may be   
   edited for style and length.   
      
      
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   Journal Reference:   
      1. Megan Holycross, Elizabeth Cottrell. Garnet crystallization does not   
         drive oxidation at arcs. Science, 2023; 380 (6644): 506 DOI:   
         10.1126/ science.ade3418   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/05/230504155638.htm   
      
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