• Earliest geochemical evidence of plate t

    From ScienceDaily@1:317/3 to All on Thu Apr 21 22:30:46 2022
    Earliest geochemical evidence of plate tectonics found in 3.8-billion- year-old crystal
    Tiny zircons found in South Africa point to an early start for the active global process that shapes Earth's surface and climate

    Date:
    April 21, 2022
    Source:
    American Geophysical Union
    Summary:
    Plate tectonics may be unique to Earth and may be an essential
    characteristic of habitable planets. Estimates for its onset range
    from over 4 billion years ago to just 800 million years ago. A new
    study reports evidence of a transition in multiple locations around
    the world, 3.8-3.6 billion years ago, from stable 'protocrust'
    to pressures and processes that look a lot like modern subduction,
    suggesting a time when plates first got moving.



    FULL STORY ==========================================================================
    A handful of ancient zircon crystals found in South Africa hold the oldest evidence of subduction, a key element of plate tectonics, according to a
    new study published today in AGU Advances, AGU's journal for high-impact,
    open- access research and commentary across the Earth and space sciences.


    ========================================================================== These rare time capsules from Earth's youth point to a transition around
    3.8 billion years ago from a long-lived, stable rock surface to the
    active processes that shape our planet today, providing a new clue in
    a hot debate about when plate tectonics was set in motion.

    Earth's crust and the top layer of mantle just under it are broken
    up into rigid plates that move slowly on top of viscous but mobile
    lower layers of mantle rock. Heat from Earth's core drives this slow
    but inexorable motion, responsible for volcanoes, earthquakes, and the
    uplift of mountain ranges.

    Estimates for when this process revved up and modern crust formed range
    from over 4 billion years ago to just 800 million years ago. Uncertainty
    arises because the geologic record from Earth's youth is sparse, due to
    the surface recycling effect of plate tectonics itself. Almost nothing
    remains from the Hadean Eon, Earth's first 500 million years.

    "The Hadean Earth is this big mystery box," said Nadja Drabon, a geologist
    at Harvard University and the lead author of the new study.

    Tiny time capsules In an exciting step forward in solving this mystery,
    in 2018 Drabon and her colleagues unearthed a chronological series of 33 microscopic zircon crystals from a rare, ancient block of crust in the Barberton Greenstone Belt in South Africa, that formed at different times
    over a critical 800-million-year span from 4.15 to 3.3 billion years ago.



    ========================================================================== Zircon is a relatively common accessory mineral in Earth's crust, but
    ancient representatives from the Hadean Eon, 4 to 4.56 billion years ago,
    are exceedingly rare, found in only 12 places on Earth, and usually in
    numbers fewer than three at each location.

    Hafnium isotopes and trace elements preserved in the Greenstone Belt
    zircons told a story about the conditions on Earth at the time they crystalized.

    Zircons 3.8-billion-years-old and younger appeared to have formed in rock experiencing pressures and melting similar to modern subduction zones, suggesting the crust may have started moving.

    "When I say plate tectonics, I'm specifically referring to an arc setting,
    when one plate goes under another and you have all that volcanism --
    think of the Andes, for example, and the Ring of Fire," Drabon said,
    describing a classic example of subduction.

    "At 3.8 billion years there is a dramatic shift where the crust is destabilized, we have new rocks forming and we see geochemical signatures becoming more and more similar to what we see in modern plate tectonics," Drabon said.

    In contrast, the older zircons preserved evidence of a global cap of "protocrust" derived from remelting mantle rock that had remained stable
    for 600 million years, the study found.



    ========================================================================== Signs of global change The new study found a similar transition to
    conditions resembling modern subduction in zircons from other locations
    around the world, dating to within about 200 million years of the South
    African zircons.

    "We see evidence for a significant change on the Earth around 3.8 to
    3.6 billion years ago and evolution toward plate tectonics is one clear possibility." Drabon said.

    While not conclusive, the results suggest a global change may have begun, Drabon said, possibly starting and stopping in scattered locations before settling into the efficient global engine of constantly moving plates
    we see today.

    Plate tectonics shapes Earth's atmosphere as well as its surface. Release
    of volcanic gasses and production of new silicate rock, which consumes
    large amounts carbon dioxide from the atmosphere, temper large temperature swings from too much or too little greenhouse gas.

    "Without all of the recycling and new crust forming, we might be going
    back and forth between boiling hot and freezing cold," Drabon said. "It's
    kind of like a thermostat for the climate." Plate tectonics has,
    so far, only been observed on Earth, and may be essential to making a
    planet livable, Drabon said, which makes the origins of plate motions
    of interest in research into the early development of life.

    "The record we have for the earliest Earth is really limited, but just
    seeing a similar transition in so many different places makes it really feasible that it might have been a global change in crustal processes,"
    Drabon said. "Some kind of kind of reorganization was happening on Earth."

    ========================================================================== Story Source: Materials provided by American_Geophysical_Union. Note:
    Content may be edited for style and length.


    ========================================================================== Journal Reference:
    1. Nadja Drabon, Benjamin L. Byerly, Gary R. Byerly, Joseph L. Wooden,
    Michael Wiedenbeck, John W. Valley, Kouki Kitajima, Ann M. Bauer,
    Donald R. Lowe. Destabilization of Long‐Lived Hadean
    Protocrust and the Onset of Pervasive Hydrous Melting at 3.8
    Ga. AGU Advances, 2022; 3 (2) DOI: 10.1029/2021AV000520 ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/04/220421131008.htm

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