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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
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    Iron-rich rocks unlock new insights into Earth's planetary history   
    Study suggests ancient microorganisms helped cause massive volcanic   
   events    
      
     Date:   
         May 25, 2023   
     Source:   
         Rice University   
     Summary:   
         A new study suggests iron-rich ancient sediments may have helped   
         cause some of the largest volcanic events in the planet's history.   
      
      
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   Visually striking layers of burnt orange, yellow, silver, brown   
   and blue-tinged black are characteristic of banded iron formations,   
   sedimentary rocks that may have prompted some of the largest volcanic   
   eruptions in Earth's history, according to new research from Rice   
   University.   
      
   The rocks contain iron oxides that sank to the bottom of oceans long ago,   
   forming dense layers that eventually turned to stone. The study published   
   this week in Nature Geoscience suggests the iron-rich layers could connect   
   ancient changes at Earth's surface -- like the emergence of photosynthetic   
   life -- to planetary processes like volcanism and plate tectonics.   
      
   In addition to linking planetary processes that were generally thought   
   to be unconnected, the study could reframe scientists' understanding   
   of Earth's early history and provide insight into processes that could   
   produce habitable exoplanets far from our solar system.   
      
   "These rocks tell -- quite literally -- the story of a changing   
   planetary environment," said Duncan Keller, the study's lead author and   
   a postdoctoral researcher in Rice's Department of Earth, Environmental   
   and Planetary Sciences.   
      
   "They embody a change in the atmospheric and ocean chemistry."   
   Banded iron formations are chemical sediments precipitated directly   
   from ancient seawater rich in dissolved iron. Metabolic actions of   
   microorganisms, including photosynthesis, are thought to have facilitated   
   the precipitation of the minerals, which formed layer upon layer over   
   time along with chert (microcrystalline silicon dioxide). The largest   
   deposits formed as oxygen accumulated in Earth's atmosphere about 2.5   
   billion years ago.   
      
   "These rocks formed in the ancient oceans, and we know that those   
   oceans were later closed up laterally by plate tectonic processes,"   
   Keller explained.   
      
   The mantle, though solid, flows like a fluid at about the rate that   
   fingernails grow. Tectonic plates -- continent-sized sections of the   
   crust and uppermost mantle -- are constantly on the move, largely as a   
   result of thermal convection currents in the mantle. Earth's tectonic   
   processes control the life cycles of oceans.   
      
   "Just like the Pacific Ocean is being closed today -- it's subducting   
   under Japan and under South America -- ancient ocean basins were destroyed   
   tectonically," he said. "These rocks either had to get pushed up onto   
   continents and be preserved -- and we do see some preserved, that's   
   where the ones we're looking at today come from -- or subducted into   
   the mantle."  Because of their high iron content, banded iron formations   
   are denser than the mantle, which made Keller wonder whether subducted   
   chunks of the formations sank all the way down and settled in the lowest   
   region of the mantle near the top of Earth's core. There, under immense   
   temperature and pressure, they would have undergone profound changes as   
   their minerals took on different structures.   
      
   "There's some very interesting work on the properties of iron oxides at   
   those conditions," Keller said. "They can become highly thermally and   
   electrically conductive. Some of them transfer heat as easily as metals   
   do. So it's possible that, once in the lower mantle, these rocks would   
   turn into extremely conductive lumps like hot plates."  Keller and his   
   co-workers posit that regions enriched in subducted iron formations   
   might aid the formation of mantle plumes, rising conduits of hot rock   
   above thermal anomalies in the lower mantle that can produce enormous   
   volcanoes like the ones that formed the Hawaiian Islands. "Underneath   
   Hawaii, seismological data show us a hot conduit of upwelling mantle,"   
   Keller said.   
      
   "Imagine a hot spot on your stove burner. As the water in your pot is   
   boiling, you'll see more bubbles over a column of rising water in that   
   area. Mantle plumes are sort of a giant version of that."  "We looked at   
   the depositional ages of banded iron formations and the ages of large   
   basaltic eruption events called large igneous provinces, and we found   
   that there's a correlation," Keller said. "Many of the igneous events --   
   which were so massive that the 10 or 15 largest may have been enough to   
   resurface the entire planet -- were preceded by banded iron formation   
   deposition at intervals of roughly 241 million years, give or take 15   
   million. It's a strong correlation with a mechanism that makes sense."   
   The study showed that there was a plausible length of time for banded   
   iron formations to first be drawn deep into the lower mantle and to then   
   influence heat flow to drive a plume toward Earth's surface thousands   
   of kilometers above.   
      
   In his effort to trace the journey of banded iron formations, Keller   
   crossed disciplinary boundaries and ran into unexpected insights.   
      
   "If what's happening in the early oceans, after microorganisms chemically   
   change surface environments, ultimately creates an enormous outpouring   
   of lava somewhere else on Earth 250 million years later, that means these   
   processes are related and 'talking' to each other," Keller said. "It also   
   means it's possible for related processes to have length scales that are   
   far greater than people expected. To be able to infer this, we've had to   
   draw on data from many different fields across mineralogy, geochemistry,   
   geophysics and sedimentology."  Keller hopes the study will spur further   
   research. "I hope this motivates people in the different fields that it   
   touches," he said. "I think it would be really cool if this got people   
   talking to each other in renewed ways about how different parts of the   
   Earth system are connected."  Keller is part of the CLEVER Planets:   
   Cycles of Life-Essential Volatile Elements in Rocky Planets program,   
   an interdisciplinary, multi-institutional group of scientists led by   
   Rajdeep Dasgupta, Rice's W. Maurice Ewing Professor of Earth Systems   
   Science in the Department of Earth, Environmental and Planetary Sciences.   
      
   "This is an extremely interdisciplinary collaboration that's looking at   
   how volatile elements that are important for biology -- carbon, hydrogen,   
   nitrogen, oxygen, phosphorus and sulfur -- behave in planets, at how   
   planets acquire these elements and the role they play in potentially   
   making planets habitable," Keller said.   
      
   "We're using Earth as the best example that we have, but we're trying to   
   figure out what the presence or absence of one or some of these elements   
   might mean for planets more generally," he added.   
      
   Cin-Ty Lee, Rice's Harry Carothers Wiess Professor of Geology, Earth,   
   Environmental and Planetary Sciences, and Dasgupta are co-authors on   
   the study.   
      
   Other co-authors are Santiago Tassara, an assistant professor at Bernardo   
   O'Higgins University in Chile, and Leslie Robbins, an assistant professor   
   at the University of Regina in Canada, who both did postdoctoral work   
   at Yale University, and Yale Professor of Earth and Planetary Sciences   
   Jay Ague, Keller's doctoral adviser.   
      
   NASA (80NSSC18K0828) and the Natural Sciences and Engineering Research   
   Council of Canada (RGPIN-2021-02523) supported the research.   
      
       * RELATED_TOPICS   
             o Plants_&_Animals   
                   # Nature # Fish # Marine_Biology   
             o Earth_&_Climate   
                   # Geology # Earth_Science # Earthquakes   
             o Fossils_&_Ruins   
                   # Origin_of_Life # Fossils # Early_Climate   
       * RELATED_TERMS   
             o Mesopotamia o Decade_Volcanoes o Earthquake_liquefaction o   
             Toba_catastrophe_theory o Timeline_of_environmental_events o   
             Iron_Age o Krakatoa o Caldera   
      
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   Story Source: Materials provided by Rice_University. Original written   
   by Silvia Cernea Clark.   
      
   Note: Content may be edited for style and length.   
      
      
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   Journal Reference:   
      1. Duncan S. Keller, Santiago Tassara, Leslie J. Robbins, Cin-Ty   
      A. Lee, Jay   
         J. Ague, Rajdeep Dasgupta. Links between large igneous province   
         volcanism and subducted iron formations. Nature Geoscience, 2023;   
         DOI: 10.1038/ s41561-023-01188-1   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/05/230525140951.htm   
      
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